Foreword

The “Illustrated Guide to Soil Taxonomy” is intended for use by multiple 

audiences. First, it is designed to help college students who have some

background in soil science, and especially those participating on collegiate 

soil judging teams, to learn the fundamental concepts of soil classification. 

Second, it is for professionals who use soil survey information in their work,

such as soil conservationists, agronomists, rangeland specialists, foresters, 

and engineers. The narrative descriptions for the soil orders, suborders, and 

great groups can help them to understand the broad concepts of soil

classification. In addition, definitions of many of the diagnostic horizons and 

features are accompanied by photos and background information. This can 

help the professional to understand the importance of these horizons and 

features and how they relate to their work. Third, the guide is for soil scientists 

beginning a career in soil survey. Its presentation of basic concepts will be 

helpful in their work, especially as they encounter new kinds of soil taxa.

This document is not intended to replace the full version of the “Keys to Soil 

Taxonomy” for the professional soil classifier. Although the authors estimate 

that the use of this version will result in the proper great group classification in 

approximately 90 percent of the cases in which it is used, the guide is not fully 

sufficient for correct placement in all cases. In this guide, some of the more 

technical and complicated criteria have either been omitted entirely or simply 

alluded to in notes which alert the user that exceptions or more complete 

definitions are only provided in the full version of the keys. To ensure proper 

application of the criteria for classifying soils, the latest version of the “Keys to 

Soil Taxonomy” is needed. In addition, this guide omits all information needed 

to classify soils to the subgroup and family levels, which are essential

categories in soil survey work.

The following individuals, listed alphabetically by last name, contributed to the 

development of this guide:

Janis Boettinger, Utah State University

Joe Chiaretti, NRCS, National Soil Survey Center (retired)

Craig Ditzler, NRCS, National Soil Survey Center (retired)

John Galbraith, Virginia Polytechnic Institute and State University

Kim Kerschen, Kansas State University (Master’s student)

Cam Loerch, NRCS, National Soil Survey Center (retired)

Paul McDaniel, University of Idaho

Shawn McVey, NRCS, National Soil Survey Center

Curtis Monger, New Mexico State University

Phillip Owens, Purdue University

Mickey Ransom, Kansas State University

i
Kenneth Scheffe, NRCS, National Soil Survey Center

Joey Shaw, Auburn University

Mark Stolt, University of Rhode Island 

David Weindorf, Texas Tech University

Recommended citation: 

Soil Survey Staff. 2014. Illustrated guide to soil taxonomy. U.S. Department of 

Agriculture, Natural Resources Conservation Service, National Soil Survey 

Center, Lincoln, Nebraska.

ii
Nondiscrimination Statement

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Department prohibits discrimination in employment or in any program or 

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If you wish to file an employment complaint, you must contact your agency’s 

EEO Counselor (http://directives.sc.egov.usda.gov/33081.wba) within 45 days 

of the date of the alleged discriminatory act, event, or personnel action. 

Additional information can be found online at 

http://www.ascr.usda.gov/complaint_filing_file.html.

If you wish to file a Civil Rights program complaint of discrimination, complete 

the USDA Program Discrimination Complaint Form, found online at 

http://www.ascr.usda.gov/complaint_filing_cust.html or at any USDA office, or 

call (866) 632-9992 to request the form. You may also write a letter containing 

all of the information requested in the form. Send your completed complaint 

form or letter by email to program.intake@usda.gov or by mail to:

USDA

Office of the Assistant Secretary for Civil Rights

1400 Independence Avenue, S.W.

Washington, D.C. 20250-9410

If you are deaf, are hard of hearing, or have speech disabilities and you wish 

to file either an EEO or program complaint, please contact USDA through the 

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If you have other disabilities and wish to file a program complaint, please see 

the contact information above. If you require alternative means of 

communication for program information (e.g., Braille, large print, audiotape, 

etc.), please contact USDA’s TARGET Center at (202) 720-2600 (voice and 

TDD).

iii

Contents

Foreword .....................................................................................................i

Nondiscrimination Statement..................................................................... iii

Part 1—How to Use This Version of the Keys

Introduction............................................................................................. 1-2 

General Steps to Follow When Classifying a Soil................................... 1-4

A Few Things to Know About Classifying Soils....................................... 1-7

Soil Moisture and Temperature Regimes.............................................. 1-10

Important Sources of Information Supporting the Use of the 

 Illustrated Guide to Soil Taxonomy .................................................... 1-17

Abbreviations and Acronyms........................................................ 1-17

Part 2—Horizon Nomenclature Used for Describing Soil 

 Profiles

Background Information ......................................................................... 2-2

Conventions for the Use of Horizon Nomenclature................................. 2-2

Table 1.................................................................................................... 2-5

Table 2A ................................................................................................. 2-6

Table 2B ................................................................................................. 2-7

Part 3—Diagnostic Horizons and Characteristics

Introduction............................................................................................. 3-2

Epipedons .............................................................................................. 3-4

Brief Descriptions of the Epipedons ............................................... 3-4

 Anthropic Epipedon................................................................... 3-7

 Folistic Epipedon....................................................................... 3-9

 Histic Epipedon ....................................................................... 3-10

 Melanic Epipedon.................................................................... 3-12

 Mollic Epipedon....................................................................... 3-14

 Ochric Epipedon...................................................................... 3-16

 Plaggen Epipedon................................................................... 3-18

 Umbric Epipedon..................................................................... 3-20

Diagnostic Subsurface Horizons and Characteristics (including 

 moisture and temperature regimes)................................................... 3-23

Abrupt Textural Change ............................................................... 3-27

Agric Horizon................................................................................ 3-29

Albic Horizon................................................................................ 3-30

Albic Materials.............................................................................. 3-32

Andic Soil Properties.................................................................... 3-34

Anhydritic Horizon........................................................................ 3-36

Anhydrous Conditions .................................................................. 3-38

Aquic Conditions .......................................................................... 3-40

Aquic Moisture Regime ................................................................ 3-43

v
Argillic Horizon ............................................................................. 3-45

Aridic (Torric) Moisture Regime.................................................... 3-47 

Calcic Horizon.............................................................................. 3-49 

Cambic Horizon............................................................................ 3-51 

Coefficient of Linear Extensibility (COLE) .................................... 3-54 

Cryoturbation................................................................................ 3-56 

Densic Contact............................................................................. 3-58 

Densic Materials........................................................................... 3-59

Duripan ........................................................................................ 3-61 

Fragipan....................................................................................... 3-63 

Fibric Soil Materials...................................................................... 3-66 

Gelic Materials ............................................................................. 3-68 

Glacic Layer................................................................................. 3-70 

Glossic Horizon............................................................................ 3-71 

Gypsic Horizon............................................................................. 3-73 

Hemic Soil Materials .................................................................... 3-75 

Human-Altered Material ............................................................... 3-77 

Human-Transported Material ....................................................... 3-79 

Identifiable Secondary Carbonates .............................................. 3-81

Interfingering of Albic Materials .................................................... 3-84

Kandic Horizon............................................................................. 3-86

Lamellae ...................................................................................... 3-89

Lithic Contact ............................................................................... 3-91

n Value ......................................................................................... 3-93

Natric Horizon .............................................................................. 3-95

Oxic Horizon................................................................................. 3-98

Paralithic Contact....................................................................... 3-100

Permafrost ................................................................................. 3-102

Perudic Moisture Regime........................................................... 3-104

Petrocalcic Horizon .................................................................... 3-105

Petroferric Contact ..................................................................... 3-109

Petrogypsic Horizon....................................................................3-111

Placic Horizon .............................................................................3-113

Plinthite .......................................................................................3-115

Redoximorphic Features .............................................................3-117

Salic Horizon...............................................................................3-118

Sapric Soil Materials .................................................................. 3-120

Slickensides ............................................................................... 3-122

Soil Temperature Regimes......................................................... 3-123

Sombric Horizon......................................................................... 3-124

Spodic Horizon........................................................................... 3-126

Spodic Materials......................................................................... 3-128 

Sulfidic Materials........................................................................ 3-130 

Sulfuric Horizon.......................................................................... 3-131 

vi
Udic Moisture Regime (Humid Areas)........................................ 3-133 

Udic Moisture Regime (Semiarid Areas) .................................... 3-134 

Xeric Moisture Regime............................................................... 3-135 

Moisture Regime Map ........................................................................ 3-136 

Temperature Regime Map .................................................................. 3-137 

Part 4—Keys to the Orders, Suborders, and Great Groups

Key to Soil Orders .................................................................................. 4-2

Introduction .................................................................................... 4-2

Brief Description of the Soil Orders................................................ 4-2

Key to Soil Orders.......................................................................... 4-3

Alfisols Order........................................................................................ 4-7

Alfisols Suborders................................................................................. 4-10

Classification of Alfisols ........................................................................ 4-10

Key to Suborders of Alfisols...................................................................4-11

Aqualfs Great Groups .................................................................. 4-12

 Key to Great Groups of Aqualfs............................................... 4-15

 Descriptions of Great Groups of Aqualfs ................................. 4-16

Cryalfs Great Groups ................................................................... 4-19

 Key to Great Groups of Cryalfs ............................................... 4-20

 Descriptions of Great Groups of Cryalfs.................................. 4-21

Ustalfs Great Groups ................................................................... 4-22

 Key to Great Groups of Ustalfs ............................................... 4-25

 Descriptions of Great Groups of Ustalfs.................................. 4-27

Xeralfs Great Groups ................................................................... 4-29

 Key to Great Groups of Xeralfs ............................................... 4-32

 Descriptions of Great Groups of Xeralfs.................................. 4-34 

Udalfs Great Groups .................................................................... 4-36 

 Key to Great Groups of Udalfs ................................................ 4-39 

 Descriptions of Great Groups of Udalfs................................... 4-41 

Andisols Order.................................................................................... 4-44 

Andisols Suborders .............................................................................. 4-47 

Classification of Andisols ...................................................................... 4-47 

Key to Suborders of Andisols................................................................ 4-49 

Aquands Great Groups ................................................................ 4-50 

 Key to Great Groups of Aquands ............................................ 4-51 

 Descriptions of Great Groups of Aquands............................... 4-52 

Gelands Great Groups ................................................................. 4-54

 Key to Great Groups of Gelands............................................. 4-55 

 Descriptions of the Great Group of Gelands ........................... 4-56 

Cryands Great Groups ................................................................. 4-57 

 Key to Great Groups of Cryands............................................. 4-58 

 Descriptions of Great Groups of Cryands ............................... 4-59 

Torrands Great Groups ................................................................ 4-61 

 Key to Great Groups of Torrands ............................................ 4-62 

 Descriptions of Great Groups of Torrands............................... 4-63 

vii
Xerands Great Groups ................................................................. 4-64 

 Key to Great Groups of Xerands............................................. 4-66 

 Descriptions of Great Groups of Xerands ............................... 4-67

Vitrands Great Groups ................................................................. 4-68

 Key to Great Groups of Vitrands ............................................. 4-70

 Descriptions of Great Groups of Vitrands................................ 4-71

Ustands Great Groups ................................................................. 4-72

 Key to Great Groups of Ustands ............................................. 4-74

 Descriptions of Great Groups of Ustands................................ 4-75

Udands Great Groups .................................................................. 4-76

 Key to Great Groups of Udands.............................................. 4-78

 Descriptions of Great Groups of Udands ................................ 4-79

Aridisols Order ................................................................................... 4-81

Aridisols Suborders .............................................................................. 4-84

Classification of Aridisols ...................................................................... 4-84

Key to the Suborders of Aridisols.......................................................... 4-85

Cryids Great Groups .................................................................... 4-86

 Key to Great Groups of Cryids ................................................ 4-87

 Descriptions of Great Groups of Cryids................................... 4-88

Salids Great Groups..................................................................... 4-89

 Key to Great Groups of Salids................................................. 4-91

 Descriptions of Great Groups of Salids ................................... 4-92

Durids Great Groups.................................................................... 4-93

 Key to Great Groups of Durids................................................ 4-95

 Descriptions of Great Groups of Durids .................................. 4-96

Gypsids Great Groups ................................................................. 4-97

 Key to Great Groups of Gypsids ............................................. 4-99

 Descriptions of Great Groups of Gypsids.............................. 4-100

Argids Great Groups .................................................................. 4-101

 Key to Great Groups of Argids .............................................. 4-103

 Descriptions of Great Groups of Argids................................. 4-104

Calcids Great Groups................................................................. 4-105

 Key to Great Groups of Calcids............................................. 4-107

 Descriptions of Great Groups of Calcids ............................... 4-108

Cambids Great Groups .............................................................. 4-109

 Key to Great Groups of Cambids ...........................................4-111

 Descriptions of Great Groups of Cambids..............................4-112

Entisols Order....................................................................................4-113

Entisols Suborders ..............................................................................4-117

Classification of Entisols......................................................................4-117

Key to Suborders of Entisols ...............................................................4-118

Wassents Great Groups..............................................................4-119

 Key to Great Groups of Wassents......................................... 4-122

 Descriptions of Great Groups of Wassents ........................... 4-123

viii
Aquents Great Groups ............................................................... 4-125

 Key to Great Groups of Aquents ........................................... 4-126

 Descriptions of Great Groups of Aquents.............................. 4-127

Psamments Great Groups ......................................................... 4-129

 Key to Great Groups of Psamments ..................................... 4-131

 Descriptions of Great Groups of Psamments........................ 4-132

Fluvents Great Groups............................................................... 4-134

 Key to Great Groups of Fluvents........................................... 4-136

 Descriptions of Great Groups of Fluvents ............................. 4-137

Orthents Great Groups............................................................... 4-139

 Key to Great Groups of Orthents........................................... 4-141

 Descriptions of Great Groups of Orthents ............................. 4-142

Geolisols Order................................................................................. 4-144

Gelisols Suborder............................................................................... 4-147

Classification of Gelisols..................................................................... 4-147

Key to Suborders of Gelisols .............................................................. 4-148

Histels Great Groups.................................................................. 4-149

 Key to Great Groups of Histels.............................................. 4-151

 Descriptions of Great Groups of Histels ................................ 4-152

Turbels Great Groups................................................................. 4-153

 Key to Great Groups of Turbels............................................. 4-155

 Descriptions of Great Groups of Turbels ............................... 4-156

Orthels Great Groups................................................................. 4-157

 Key to Great Groups of Orthels............................................. 4-158

 Descriptions of Great Groups of Orthels ............................... 4-159

Histosols Order................................................................................. 4-161

Histosols Suborders ........................................................................... 4-164

Classification of Histosols................................................................... 4-164

Key to Suborders of Histosols ............................................................ 4-165

Folists Great Groups.................................................................. 4-166

 Key to Great Groups of Folists.............................................. 4-167

 Descriptions of Great Groups of Folists ................................ 4-168 

Wassists Great Groups .............................................................. 4-169

 Key to Great Groups of Wassists .......................................... 4-170

 Descriptions of Great Groups of Wassists............................. 4-171

Fibrists Great Groups................................................................. 4-172

 Key to Great Groups of Fibrists............................................. 4-173

 Descriptions of Great Groups of Fibrists ............................... 4-174

Saprists Great Groups ............................................................... 4-175

 Key to Great Groups of Saprists ........................................... 4-177

 Descriptions of Great Groups of Saprists.............................. 4-178

Hemists Great Groups ............................................................... 4-179

 Key to Great Groups of Hemists ........................................... 4-180

 Descriptions of Great Groups of Hemists.............................. 4-181

ix
Inceptisols Order.............................................................................. 4-183 

Inceptisols Suborders ......................................................................... 4-186 

Classification of Inceptisols ................................................................ 4-186 

Key to Suborders of Inceptisols.......................................................... 4-187 

Aquepts Great Groups ............................................................... 4-188 

 Key to Great Groups of Aquepts ........................................... 4-191 

 Descriptions of Great Groups of Aquepts.............................. 4-192 

Gelepts Great Groups ................................................................ 4-195 

 Key to Great Groups of Gelepts............................................ 4-196 

 Descriptions of Great Groups of Gelepts .............................. 4-197 

Cryepts Great Groups ................................................................ 4-198 

 Key to Great Groups of Cryepts............................................ 4-199 

 Descriptions of Great Groups of Cryepts .............................. 4-200 

Ustepts Great Groups ................................................................ 4-202 

 Key to Great Groups of Ustepts ............................................ 4-204 

 Descriptions of Great Groups of Usteps................................ 4-205 

Xerepts Great Groups ................................................................ 4-207 

 Key to Great Groups of Xerepts............................................ 4-209 

 Descriptions of Great Groups of Xerepts .............................. 4-210 

Udepts Great Groups................................................................. 4-212 

 Key to Great Groups of Udepts............................................. 4-214 

 Descriptions of Great Groups of Udepts ............................... 4-215 

Mollisols Order ................................................................................. 4-217 

Mollisols Suborders ............................................................................ 4-220 

Classification of Mollisols.................................................................... 4-220 

Keys to Suborders of Mollisols ........................................................... 4-222 

Albolls Great Groups.................................................................. 4-223 

 Key to Great Groups of Albolls.............................................. 4-225 

 Descriptions of Great Groups of Albolls ................................ 4-226 

Aquolls Great Groups................................................................. 4-227 

 Key to Great Groups of Aquolls............................................. 4-229 

 Descriptions of Great Groups of Aquolls ............................... 4-230 

Rendolls Great Groups .............................................................. 4-232 

 Key to Great Groups of Rendolls .......................................... 4-234 

 Descriptions of Great Groups of Rendolls............................. 4-235 

Gellols Great Groups ................................................................. 4-236 

 Key to Great Groups of Gelolls ............................................. 4-237 

 Description of the Great Group of Gelolls ............................. 4-238 

Cryolls Great Groups ................................................................. 4-239 

 Key to Great Groups of Cryolls ............................................. 4-241 

 Descriptions of Great Groups of Cryolls................................ 4-242 

Xerolls Great Groups ................................................................. 4-244 

 Key to Great Groups of Xerolls ............................................. 4-247 

 Descriptions of Great Groups of Xerolls................................ 4-248

x 
Ustolls Great Groups.................................................................. 4-250

 Key to Great Groups of Ustolls.............................................. 4-252

 Descriptions of Great Groups of Ustolls ................................ 4-254

Udolls Great Groups .................................................................. 4-256

 Key to Great Groups of Udolls .............................................. 4-258

 Descriptions of Great Groups of Udolls................................. 4-259

Oxisols Order.................................................................................... 4-261

Oxisols Suborders .............................................................................. 4-264

Classification of Oxisols...................................................................... 4-264

Key to Suborders of Oxisols ............................................................... 4-265

Aquox Great Groups .................................................................. 4-266

 Key to Great Groups of Aquox .............................................. 4-268

 Descriptions of Great Groups of Aquox................................. 4-269

Torrox Great Groups .................................................................. 4-270

 Key to Great Groups of Torrox .............................................. 4-272

 Descriptions of Great Groups of Torrox................................. 4-273

Ustox Great Groups ................................................................... 4-274

 Key to Great Groups of Ustox ............................................... 4-276

 Descriptions of Great Groups of Ustox.................................. 4-277

Perox Great Groups................................................................... 4-279

 Key to Great Groups of Perox............................................... 4-281

 Descriptions of Great Groups of Perox ................................. 4-282

Udox Great Groups.................................................................... 4-283

 Key to Great Groups of Udox................................................ 4-285

 Descriptions of Great Groups of Udox .................................. 4-286

Spodosols Order .............................................................................. 4-287

Spodosols Suborders ......................................................................... 4-290

Classification of Spodosols................................................................. 4-290

Key to Suborders of Spodosols .......................................................... 4-291

Aquods Great Groups ................................................................ 4-292

 Key to Great Groups of Aquods ............................................ 4-294

 Descriptions of Great Groups of Aquods............................... 4-295

Gelods Great Groups................................................................. 4-297

 Key to Great Groups of Gelods............................................. 4-298

 Descriptions of Great Groups of Gelods ............................... 4-299

Cryods Great Groups ................................................................. 4-300

 Key to Great Groups of Cryods............................................. 4-302

 Descriptions of Great Groups of Cryods ............................... 4-303

Humods Great Groups ............................................................... 4-304

 Key to Great Groups of Humods........................................... 4-306

 Descriptions of Great Groups of Humods ............................. 4-307

Orthods Great Groups................................................................ 4-308

 Key to Great Groups of Orthods............................................ 4-310

 Descriptions of Great Groups of Orthods ...............................4-311

Ultisols Order.................................................................................... 4-313

Ultisols Suborders .............................................................................. 4-316

xi
Classification of Ultisols...................................................................... 4-316

Key to Suborders of Ultisols ............................................................... 4-317

Aquults Great Groups ................................................................ 4-318

 Key to Great Groups of Aquults............................................. 4-320

 Descriptions of Great Groups of Aquults ............................... 4-322

Humults Great Groups ............................................................... 4-324

 Key to Great Groups of Humults ........................................... 4-326

 Descriptions of Great Groups of Humults.............................. 4-327

Udults Great Groups .................................................................. 4-329

 Key to Great Groups of Udults .............................................. 4-331

 Descriptions of Great Groups of Udults................................. 4-332

Ustults Great Groups ................................................................. 4-334

 Key to Great Groups of Ustults ............................................. 4-336

 Descriptions of Great Groups of Ustults................................ 4-337

Xerults Great Groups ................................................................. 4-339

 Key to Great Groups of Xerults ............................................. 4-340

 Descriptions of Great Groups of Xerults................................ 4-341

Vertisols Order.................................................................................. 4-342

Vertisols Suborders ............................................................................ 4-345

Classification of Vertisols.................................................................... 4-345

Key to Suborders of Vertisols ............................................................. 4-347

Aquerts Great Groups ................................................................ 4-348

 Key to Great Groups of Aquerts ............................................ 4-350

 Descriptions of Great Groups of Aquerts............................... 4-351

Cryerts Great Groups................................................................. 4-353

 Key to Great Groups of Cryerts............................................. 4-354

 Descriptions of Great Groups of Cryerts ............................... 4-355 

Xererts Great Groups................................................................. 4-356

 Key to Great Groups of Xererts............................................. 4-358

 Descriptions of Great Groups of Xererts ............................... 4-359

Torrerts Great Groups ................................................................ 4-360

 Key to Great Groups of Torrerts ............................................ 4-362

 Descriptions of Great Groups of Torrerts............................... 4-363

Usterts Great Groups ................................................................. 4-364

 Key to Great Groups of Usterts............................................. 4-366

 Descriptions of Great Groups of Usterts ............................... 4-367

Uderts Great Groups.................................................................. 4-368

 Key to Great Groups of Uderts.............................................. 4-370

 Descriptions of Great Groups of Uderts ................................ 4-371

xii
Part 1—How to Use This Version 

of the Keys

Part 1—How to Use This Version of the Keys 1-1
Illustrated Guide to Soil Taxonomy

Introduction

The “Illustrated Guide to Soil Taxonomy” is presented in four parts, each 

covering a specific aspect of the USDA’s soil classification system (Soil 

Taxonomy). It is a PDF document designed for use on a PC, tablet, or smart 

phone. Each part contains many internal links, and bookmarks are provided 

for the major items in each part.

Part 1—How to Use This Version of the Keys 

This part has five main sections: 

• An overall summary of each part of the guide.

• A brief discussion of the general steps to follow when classifying a soil.

• A list of a few things to know about classifying soils. This information 

needs to be understood and followed when using these keys.

• A discussion of soil moisture and temperature regimes. This section 

includes generalized maps of the continental United States that can be 

used to estimate the correct class based on location in the country.

• A brief list of important sources of information along with a list of 

abbreviations and acronyms. The sources can be used to find more indepth definitions or explanations of the terms and concepts presented

in any of the parts.

Part 2—Horizon Nomenclature Used for Describing Soil 

Profiles

This section provides a brief description of the master horizon designations, 

letter suffixes, number prefixes and suffixes, special symbols, and conventions 

for using standard horizon nomenclature to describe soil profiles. In addition to 

background information and basic conventions for using the nomenclature,

tables are provided listing the master horizon symbols (table 1) and the 

subordinate suffix symbols (tables 2A and 2B) and a general description of 

each.

Part 3—Diagnostic Horizons and Characteristics

This part has two major sections. In the first section, the eight diagnostic 

surface horizons (“epipedons”) are presented. In the second section, 49

diagnostic horizons and characteristics are presented. A diagnostic horizon 

constitutes a continuous horizon in the soil profile. Diagnostic characteristics

are features found within some horizons (e.g., plinthite or slickensides) but 

that do not form a continuous layer in the soil profile. This part discusses only 

those needed to classify a soil to the great group level (for more, see the 

current edition of the “Keys to Soil Taxonomy”). It also includes information on

Part 1—How to Use This Version of the Keys 1-2
Illustrated Guide to Soil Taxonomy

each of the soil moisture and soil temperature regimes. For each diagnostic 

horizon or feature presented, the following is provided:

• Heading. A brief descriptive phrase is given for the item.

• Concept and background information. A brief narrative describes the 

concept and basic information of the item.

• Generalized characteristics. The criteria given in the “Keys to Soil 

Taxonomy,” 12th edition, are presented in a somewhat simplified and 

illustrated version. Information has been reworded, and, in a few cases, 

the critera have been reorganized. Some of the more complex details 

have been omitted. Generally, a note (italicized text that follows) alerts 

the reader if there are exceptions to the presented information or if 

additional information is available in the full version of the keys.

• Common horizon nomenclature. The master horizon and/or 

subordinate symbols that are commonly used in soil descriptions to 

indicate a particular diagnostic horizon or characteristic are identified. 

• Photos (for most of the diagnostic horizons and characteristics)

Part 4—Keys to the Orders, Suborders, and Great Groups

This part is an illustrated and somewhat simplified version of the keys to the 

orders, suborders, and great groups. Narrative descriptions are provided for 

each category.

Keys to the twelve soil orders and their suborders and great groups

The full version of the keys is somewhat simplified. Information has been

reworded, and, in a few cases, the criteria have been reorganized. Some of 

the more complex details have been omitted. Generally, a note (italicized text 

that follows) alerts the reader if there are exceptions to the presented

information or if additional information is available in the full version of the 

keys.

The last choice in each key has been modified to include one or more 

diagnostic horizons or properties present in the soil profile. This is a potentially 

significant change from the “Keys to Soil Taxonomy,” in which the last choice 

is simply listed as “all others.” Because of this change, it is possible that a soil 

profile will be impossible to classify using this guide.

Narratives for each order

The one-page narrative includes the following:

• Heading. A brief descriptive phrase is given for the basic concept of the 

soil order.

• General characteristics. These are the major horizons and properties 

associated with the order.

Part 1—How to Use This Version of the Keys 1-3
Illustrated Guide to Soil Taxonomy

• Environment and processes. The kind of environments where the 

order occurs and some important processes and characteristics of 

these environments are discussed.

• Location. The places where the soils occur in the world and in the 

United States are identified.

• Photo. An image of the soil profile representing the order is provided.

• Maps. These depict the distribution of the soils globally and within the 

United States.

• Brief narrative. The main soil properties and features considered at 

the suborder and great group level of classification are discussed.

Charts for each order and suborder

A graphical depiction, in the form of a hierarchical chart, shows (in keying 

order) the suborders within the order and the great groups within the suborder.

Descriptions for each suborder and great group

A narrative description describes the important features of the suborder or 

great group (arranged in alphabetical order). For some classes, a 

representative photo is also included.

General Steps to Follow When Classifying a Soil

1. Consider the environment where the soil profile is located. By 

considering the soil-forming processes most likely occurring at the site, you 

can begin to narrow your focus to the soil features most likely to be present.

This includes considering both the broad physiographic region as well as the 

position on the local landscape. Based on these two factors, consider the 

kinds of soil processes likely to be important in the area and the types of 

diagnostic horizons and features that may be present. For example, if you are 

on the prairies of the Northern Plains, it is likely that organic matter 

accumulation has been an important soil-forming process and that a mollic 

epipedon is present. If you are in the Mohave Desert, the accumulation of 

soluble compounds due to little or no leaching is likely to be a more prominent 

process than organic matter accumulation. In this area, diagnostic horizons 

(such as calcic, gypsic, natric, or salic subsoil horizons) are more likely to 

occur than a mollic epipedon or a spodic horizon.

Consider the local landscape. For example, soils on broad upland divides may 

have well developed, thick subsoil horizons reflecting a fairly stable landform, 

while soils on adjacent steep side slopes, assuming that the overall processes 

of erosion are more or less keeping pace with the processes of soil 

development, probably are less well developed. Soils in concave positions 

tend to receive additional runoff water from adjacent slopes while those on 

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convex slopes tend to shed water. This may lead to wetness in the concave 

areas and the formation of redoximorphic features. 

A useful source of information about the soils in many areas is the most 

recently published USDA-NRCS soil survey report (if one exists for the 

area). The chapter on formation of the soils can be particularly helpful in

understanding the overall soil-forming processes in the area and in learning 

which moisture and temperature regimes have been identified for the area.

2. Describe the soil profile. A complete and accurate soil profile 

description is essential for classifying the soil. A previously prepared standard 

form is useful because it will help you to record all of the important 

information. An example of a standard pedon description form can be found in 

the “Field Book for Describing and Sampling Soils” (pages 2-93 and 2-94).

Also, part 2 of this guide provides information on the standard nomenclature 

for describing soil horizons. Click here to see a video about describing soil 

horizons in the field.

Begin by observing the exposed profile as a whole. Is any part composed of 

organic soil material? If so, those parts will be named as O horizons. With 

respect to the lower part, is there evidence of soil development all the way to 

the bottom? Consider features such as structure, color, and presence of clay

films. If soil development does not extend to the bottom of the profile, mark 

where the solum ends and the underlying parent material (i.e., C horizon) 

begins. Next, consider the top part of the profile. Mark the lower boundary of 

the epipedon. Most likely it is at least slightly darkened by organic matter 

accumulation. Now consider the zone under the epipedon and above the C

horizon or the lower boundary of the soil (i.e., a Cr or R layer, if present). Is 

this a B horizon only, or do you see an E horizon (light-colored, leached 

layer)? Mark these major divisions, as appropriate.

Finally, look more closely at each of the major divisions you have identified. 

Consider the features that are present and relate them to the horizon 

nomenclature used for describing soils. For example, if one or more layers of 

the B horizon have clay films, the suffix “t” is needed. If a horizon is engulfed 

by secondary calcium carbonate to such an extent that it will not slake in 

water (continuously cemented) and is root restrictive, the suffix “kkm” is 

appropriate. In this way, you document the kinds of soil processes based on 

the morphology that you observe in the profile.

3. Determine which diagnostic horizons and characteristics are 

present. Using the information provided in part 3 of this guide, determine 

which diagnostic horizons and characteristics are present and record their 

location in the profile (e.g., mollic epipedon, 0-40 cm, redoximorphic features 

60-200 cm, etc.). Remember that the diagnostic horizons and characteristics 

are not a part of a sequential key and, because most profiles have more than 

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one, you need make certain to identify all horizons and characteristics that 

apply. To do this accurately, laboratory data probably will be needed for the 

horizons described. If data cannot be provided, sampling and analysis may be 

needed. In the absence of data, educated assumptions will be required based 

on knowledge and past experience with similar soils in the area. Although Soil 

Taxonomy recognizes many diagnostic horizons and characteristics, only a 

few are likely to occur in a specific profile. 

4. Identify the soil moisture and temperature regimes. It is likely that 

the classification at the suborder or great group level will require determination 

of the soil moisture regime and possibly the soil temperature regime. If no 

data exist, see the information on these topics provided later in this part of the 

guide.

5. Determine the classification. Using part 4 of this guide, determine the 

correct order, suborder, and great group for the soil. Keep in mind that the 

keys are designed to be used in sequential order. Begin with the key to the 

orders. Once a set of criteria is met, stop and go to the section for that order. 

Following the key to suborders, start at the top and work down sequentially to 

the first suborder where all criteria are met. Do the same to determine the 

proper great group.

Soil scientists describing a soil profile in the United 

Arab Emirates. (Photo courtesy of Dr. Craig Ditzler)

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A Few Things to Know About Classifying Soils

Sequential use of the keys. The keys are designed to be used sequentially, 

beginning with the key to the orders, then the suborders, and finally the great 

groups. Start with the first item listed in the key to orders and evaluate the soil

to see if it meets the criteria. If the initial criteria listed are not met, continue 

through the key until the criteria are met. Once the criteria are determined to 

be met for the order, move on to suborder and finally great group. The design 

of the keys is such that the soil is to be placed in the first taxa for which the 

criteria are met.

Operational definitions. Many of the criteria that define diagnostic horizons 

or that are used as class limits in the keys require evaluation of a measured 

physical or chemical property. Examples include bulk density, base saturation, 

clay content, effective cation-exchange capacity, electrical conductivity, 

organic carbon, pH, content of weatherable minerals, and many others. The 

values obtained in the laboratory for these properties are dependent on the 

method used to measure the property. Consider clay content. It can be 

measured in the laboratory by the hydrometer method or by the pipette

method. Values obtained by each method on a given sample will be similar, 

but not identical. 

Soil Taxonomy uses “operational definitions” tied to specific measurement

methods to define the properties used as class limits. In the case of clay 

content, criteria are based data gained by the pipette method. The appendix 

included at the end of the current version of the “Keys to Soil Taxonomy”

provides a discussion of the properties and methods. Specific information 

regarding the analytical procedures is contained in the “Soil Survey 

Laboratory Methods Manual.” Additional information is available in the “Soil 

Survey Laboratory Information Manual.”

Rounding. When evaluating data to see if a criterion has been met, normal 

rules for rounding should be applied. For example, item 2 for Vertisols in the 

keys to the soil orders uses a class limit of > 30% clay. Note that a whole 

number is used. Therefore, if laboratory results show a clay content of 29.6%, 

you first round to 30% and then determine that the requirement is in fact met. 

Similarly, item 1 for Ultisols in the key to the soil orders lists a requirement that

base saturation (by sum of bases) be < 35%. For example, a measured base 

saturation of 34.7% is rounded to 35%, and the criterion is therefore not met

(exactly 35% fails the criteria as given). So when measured data is applied to 

classifying the soil, one must first note the level of precision used as a class 

limit and then round the measured data to the same level of precision.

The conventional rules for rounding numbers are as follows:

• If the digit immediately to the right of the last significant figure is more 

than 5, round up to the next higher digit. For example, 34.8 is rounded

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to 35 (round up because the digit to be dropped is more than halfway 

between 34 and 35).

• If the digit immediately to the right of the last significant figure is less 

than 5, round down to the next lower digit. For example, 34.4 is 

rounded to 34 (round down because the digit to be dropped is less than 

halfway between 34 and 35).

• If the digit immediately to the right of the last significant figure is equal 

to 5, round to the adjacent even number, either up or down. For 

example, 17.5 is rounded to 18 (round up because the result is an even 

number) and 34.5 is rounded to 34 (round down because the result is 

an even number).

Soil color. An important criterion in many of the keys, as well as in the 

definitions of the diagnostic horizons and features, is soil color. The 

operational definition for soil color uses the Munsell Soil-Color Charts ®. Hue, 

value, and chroma are identified for a soil sample. Colors are recorded to the 

closest matching color chip in the charts. The quality of light has an effect on 

the visual perception of color, so it is best to evaluate color outdoors in natural 

sunlight, neither too early nor too late in the day, and without wearing 

sunglasses. Soil moisture state can also influence the hue, value, or chroma 

observed for the soil (value is particularly susceptible to change in soil 

moisture state). Because of this, the keys often specify the moisture state (i.e., 

moist or dry) for the evaluation of color. However, in cases where moisture 

state is not specified, the criterion is met if the required color is observed 

either moist or dry. Further discussion of soil color can be found in chapter 3 of 

the “Soil Survey Manual.”

Horizon designation and diagnostic horizons. It is a common 

misconception that there is equivalence between horizon designations and the 

presence of diagnostic horizons. For example, if a soil contains a subsoil 

horizon labeled Bt, it should not be assumed that this horizon is an argillic 

horizon. Similarly, a Bk horizon should not be assumed to be a calcic horizon. 

While many Bt horizons are in fact argillic horizons and many Bk horizons are 

calcic horizons, this is not always true. The guidelines for applying horizon 

designations are more qualitative than quantitative and thus allow the field 

observer to apply the symbolic designation that best fits his or her 

interpretation of the soil-forming processes. The definitions for diagnostic 

horizons are based on observable and measurable properties defining a class 

limit that must be met. For example, while an observer may note clay films in a 

layer, thus justifying the use of the symbol “Bt” to indicate illuvial clay, the layer 

may not be sufficiently thick, or the increase in clay content may not be great 

enough, to meet the minimum criteria for an argillic horizon.

Depths in the soil used for classification. It is important to know what part 

of the soil to consider when classifying a soil. Key points to remember when 

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using these keys to classify a soil to the great group level are listed below.

Additional considerations are required to classify to the family and series level.

1) The lower depth of consideration is arbitrarily limited to a maximum of 200 

cm. This is simply a practical consideration to facilitate field operations in 

soil survey. This does not imply that soil descriptions below 200 cm are not 

valuable, but the properties observed below this depth are not considered 

in the classification. 

2) Soil properties below any densic, lithic, paralithic, or petroferric contact are 

not considered when classifying a soil. This is so that the depth of 

consideration can be limited to less than 200 cm.

3) Application of these keys often requires one to determine the depth to a 

diagnostic horizon or feature, such as a petrocalcic horizon, permafrost, a 

lithic contact, redoximorphic features, and many others. In most soils, this 

simply means measuring from the soil surface (beginning below any fresh

leaf or needle litter that is undecomposed) to the top of the diagnostic 

horizon, or to the depth where the feature in question begins. (Only the top 

of the horizon needs to be within the listed depth.) In cases where the 

depth to the upper boundary varies laterally (i.e., a wavy or irregular 

boundary), judgment must be used to select a value that best represents 

the observation of the soil. In cases where depth measurements start 

below any surface O horizon, the datum is noted in the keys as the 

“mineral soil surface.”

4) An exception to item 3 is for cases in which a “surface mantle of new

material” covers an older “buried soil.” To be considered a surface mantle 

of new material covering a buried soil, the material must:

a) Be at least 50 cm thick,

b) Have a zone at least 7.5 cm thick at its base that fails the criteria for 

any diagnostic horizon, and

c) Be underlain by an older soil with a sequence of one or more 

genetically developed horizons.

Examples include: recent deposits, such as eolian sands, over a paleosol 

formed in till; a volcanic ash deposit blanketing a soil on a mountain slope;

a landslide which buries an older soil on a stream terrace; and recently 

deposited fill material (human-transported material) covering the surface of 

a naturally occurring soil. In cases like these, two soils are potentially 

present: a new soil (such as an Entisol) covering an older one (such as an 

Alfisol). Which should be classified? To decide, one must determine if the 

older soil meets the definition of a “buried soil” (see chapters 1 and 4 of the 

“Keys to Soil Taxonomy”).

Case 1—The new material fails item a above (is less than 50 cm thick). 

Classify the older soil. Begin measuring from the older soil’s surface 

(except in cases where soil temperature, soil moisture, and/or andic or 

vitrandic properties dictate otherwise).

Case 2—The new material meets items a, b, and c. Classify the new soil. 

Begin measuring from the new soil’s surface. Any diagnostic horizons 

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present in the buried soil are not considered in the classification for 

order, suborder, or great group. Use suffix “b” with any genetic horizons 

in the older soil.

Case 3—The new material fails item b. The materials are considered to 

be just one soil. Begin measuring from the new soil surface and 

consider all diagnostic horizons that are present within a depth of 2 m. 

The older soil is no longer considered “buried.” Recognize a lithologic 

discontinuity for horizons below the contact between the new and old 

parent materials.

Case 4—The underlying (older) material fails item c (has no genetic 

horizons). As in case 3, the materials are considered just one soil. 

Begin measuring from the new soil surface.

For a more in-depth discussion, including example photos explaining how 

to identify a surface mantle and a buried soil, see Soil Survey Technical 

Note No.10 (Buried Soils and Their Effect on Taxonomic Classification).

Soil Moisture and Temperature Regimes

Soil Moisture Regimes

Many of the suborders and some great groups are defined by the moisture 

regime of the soil. The soil moisture regimes are defined by conditions within 

the moisture control section (MCS).The upper boundary of the MCS is the 

depth to which a dry soil will be wetted by 2.5 cm of precipitation in a 24-hour

period. The lower boundary is the depth to which the same soil will be wetted

by 7.5 cm of precipitation in a 24-hour period. Note that the MCS will vary 

depending on the porosity of the soil. As a general rule, the MCS will begin 

and end deeper in the profile as texture becomes progressively coarser, as 

follows: 

Depth is 10-30 cm if the soil is loamy or clayey.

Depth is 20-60 cm if the soil is loamy with < ~18% clay and > ~15% sand.

Depth is 30-90 cm if the soil is sandy. 

(Depth may be even greater for gravelly soils.)

The aridic (torric), udic (and perudic), ustic, and xeric soil moisture regimes 

are defined by the number of days and seasonal patterns that the soil is either 

moist or dry in the MCS. The soil is considered dry if moisture is held at a 

tension > 1500 kPa and moist if held at a tension < 1500 kPa (1500 kPa 

tension is commonly considered the wilting point). The aquic soil moisture 

regime requires the MCS to be periodically saturated and reduced, sometimes 

for as little as a few days in a year when the soil temperature is above 5 °C.

The peraquic soil moisture regime is also a reducing regime that exists in soils 

which are continuously saturated to the soil surface (e.g., subaqueous soils). 

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Specific criteria for each moisture regime are given in chapter 3 of the current

“Keys to Soil Taxonomy.”

The criteria (modified from the “Field Book for Describing and Sampling Soils,”

version 3.0) for each moisture regime are summarized here:

Aquic—A reducing regime for soils that are free of dissolved oxygen and 

saturated when the soil temperature is above 5 °C (seasonal ground-water 

fluctuations are typical). Unlike other regimes, the aquic regime may occur 

temporarily for only a few days. Since this regime is not defined on an annual 

basis, some soils with an aquic moisture regime also have a xeric, ustic, or 

aridic (torric) soil moisture regime. (Note: The aquic moisture regime is not 

used as criteria in the keys. Instead, the presence of aquic conditions is used.)

Aridic (Torric)—The predominantly dry regime for soils of arid and semiarid 

climates that are unsuitable for cultivation without irrigation. In most years, the 

soil is dry (in all parts of the soil moisture control section) > 50% of all days 

annually when the soil is > 5 °C at a depth of 50 cm and moist in some part for 

< 90 consecutive days when the soil is > 8 °C at a depth of 50 cm.

Udic—The predominantly moist regime for soils of humid climates with well 

distributed rainfall. In most years, the soil is dry (in any part of soil moisture 

control section) for < 90 cumulative days.

Perudic—An extremely wet regime for soils of climates where precipitation 

exceeds evapotranspiration in all months in most years. The soil is almost 

always moist; soil tension is rarely > 100 kPa (≈ >1 bar).

Ustic—The temporarily dry regime for soils of climates that are intermediate 

between dry (aridic) and moist (udic). The soil is intermittently moist and dry; 

moisture is limited but usually available during portions of the growing season. 

In most years, the soil is moist > 180 cumulative days or > 90 consecutive 

days.

Xeric—The seasonally dry regime for soils of Mediterranean-type climates, 

which have cool, moist winters and warm, dry summers. In most years, the 

soil is moist in all parts for ≥ 45 consecutive days in the 4 months following the 

winter solstice (i.e., winter and early spring) and dry in all parts for ≥ 45 

consecutive days in the 4 months following the summer solstice (i.e., summer 

and early fall). The soil is also moist in some part > 50% of all days when the 

soil is > 5 °C at a depth of 50 cm or moist in some part for ≥ 90 consecutive 

days when the soil is > 8 °C at a depth of 50 cm.

Determining the correct moisture regime class for a given soil can be difficult. 

There are three approaches:

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1) The moisture state of the MCS is measured in the field using soil 

moisture sensors. This approach is done at selected sites and provides 

very valuable information. However, because it is expensive and timeconsuming to collect the required multiple years of data, this approach

generally is not practical for routine soil classification needs.

2) The expected moist/dry seasonal patterns for the moisture control 

section is determined with a computer simulation model. Inputs 

required include past temperature and precipitation data for the site 

and information about the water storage capacity of the soil profile to 

be modeled. The Java Newhall Simulation Model can be used for this. 

3) Where previous soil survey data collection and field experience are well 

documented, soil moisture regimes are inferred from geographic 

location (including general location in the country, elevation, aspect, 

and topographic relationships) as well as from knowledge of 

relationships between natural vegetation and moisture regimes. 

Individual soil survey projects may have locally developed guides and 

protocols based on local knowledge for assigning moisture regime 

classes to soils. Information about soil moisture regimes for an area 

can be found in previously published soil survey reports.

The map below shows the general pattern of soil moisture regimes across the 

United States. The classes shown include ad hoc moisture intergrade terms,

such as “Typic Ustic” and “Ustic Aridic,” which are used in the subgroup 

category of Soil Taxonomy. The intergrade classes are not needed for this 

guide. For use of these keys, only the second term is needed. For example,

“Ustic Aridic” will be considered as simply “Aridic.” This map can be used as a 

general guide for assigning a moisture regime class, but it is not definitive for 

any specific location. 

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Back to discussion of Soil Moisture Regimes 

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Soil Temperature Regimes

Some of the suborders and great groups are defined with either the gelic or 

cryic soil temperature regime. Although Soil Taxonomy recognizes a total of 10

temperature regimes, only these two coldest regimes are used at the suborder 

and great group levels. The other regimes, which are used in defining the 

lower classes and some diagnostic horizons and are mentioned in some of 

the descriptions of taxa, are included here for reference. The soil temperature

regimes are defined by mean annual soil temperature (MAST) at a depth of 50 

cm from the soil surface (or at a densic, lithic, or paralithic contact if it is 

shallower than 50 cm). Mean summer soil temperature (MSST) at the same 

depth is also used in the cryic regime definition. “Iso” temperature regimes 

(e.g., isothermic, isomesic, etc.) have a difference of < 6 o C between mean

summer and mean winter temperature. The “iso” regimes are not presented 

here since they are not used in this guide. Specific criteria for each 

temperature regime are given in chapter 3 of the current “Keys to Soil 

Taxonomy.” Soil temperature is discussed in detail in “Soil Taxonomy,” 2nd

edition (chapter 3). A good discussion of soil temperature measurement and 

interpretation of data is provided in Soil Survey Investigations Report Number 

48 (TheTemperature Regime for Selected Soils in the United States, USDANRCS, 2002).

The criteria (modified from the “Field Book for Describing and Sampling Soils,”

version 3.0) for each temperature regime are summarized below in text and 

flow charts:

Gelic—Soil is ≤0 °C.

Cryic—Soil is > 0 to < 8 °C but has no permafrost and summer temperatures 

are also cold (see diagram below regarding summer temperatures in the cryic 

regime).

Frigid—Soil is > 0 to < 8 °C (but warmer than cryic soils in summer).

Mesic—Soil is 8 to < 15 °C.

Thermic—Soil is 15 to < 22 °C.

Hyperthermic—Soil is ≥ 22 °C.

Soil temperature is affected by inherent soil properties that determine the 

capacity of the soil to absorb and transmit heat, such as moisture content, 

porosity, and mineralogy. It is also greatly affected by site characteristics, such 

as drainage, vegetative cover, seasonal snow cover, slope steepness and 

aspect, elevation, and latitude.

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Flow chart for determining soil temperature regime.

Back to discussion of Soil Temperature Regimes

Methods for determining soil temperature regime include the following:

1) Soil temperature is measured in the field with manual thermometers, 

thermocouples, or digital sensors. It is relatively easy to install 

automated sensors for recording temperature over time, and this is 

frequently done to obtain data used in classifying soils. This is not 

practical, however, for obtaining immediate results. As few as four 

temperature measurements equally spaced throughout the year (e.g., 

January 1, April 1, July 1, and October 1) give very close 

approximations of mean annual soil temperature.

2) The mean annual soil temperature is estimated from available records 

of air temperature. For the continental United States, adding about 1 or 

2 o

C to the mean annual air temperature will approximate MAST at a 

Gelic < 0 oC MAST

Cryic > 0 oC to <8 oC 

MAST

Mineral Soils

Not Saturated in 

Summer, then 

MSST:

No "O", >0 oC 

to 15 oC

With "O" , >0 oC

to 8 oC

Saturated in 

Summer, then 

MSST:

No "O", >0 oC 

to 13 oC

With "O" or Histic, 

>0 oC to 6 oC

Histosols MSST >0 oC to 6 oC Frigid

MAST >0 to <8 

°C (but warmer 

than cryic in 

summer)

Mesic MAST 8 to <15 °C

Thermic MAST 15 to <22 °C

Hyperthermic MAST ≥22 °C

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depth of 50 cm, although the actual relationship varies from location to 

location and the difference can be greater. The Java Newhall 

Simulation Model can estimate soil temperature regime based on air 

temperature records.

3) Where previous soil survey data collection and field experience are well 

documented, soil temperature regimes are inferred from geographic 

location (including general geographic location, elevation, aspect, and 

topographic relationships) as well as from knowledge of relationships 

between natural vegetation and temperature regimes. Individual soil 

survey projects may have locally developed guides and protocols 

based on local knowledge for assigning temperature regime classes to 

soils. Information about soil temperature regimes for an area can be 

found in previously published soil survey reports.

The map below shows the general pattern of soil temperature regimes across 

the continental United States. The gelic soil temperature regime has not been 

documented in soils of the continental U.S. but occurs in the soils of Alaska. 

This map can be used as a general guide for assigning a soil temperature

regime, but it is not definitive for any specific location. 

Back to discussion of Soil Temperature Regimes

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Important Sources of Information Supporting the Use 

of the Illustrated Guide to Soil Taxonomy

This guide includes many terms that have specific technical definitions. While 

many items are described and defined in Part 2—Diagnostic Horizons and 

Features, other terms are not. It is assumed that the user is either familiar with 

the definitions or knows which source documents can provide the definitions. 

Specifically, it is assumed that the user is familiar with the commonly used 

sources of soil survey standards, including:

Keys to Soil Taxonomy (Soil Survey Staff. 2014. Keys to soil taxonomy, 

12th edition. U.S. Department of Agriculture, Natural Resources 

Conservation Service.)

Soil Survey Manual (Soil Survey Division Staff. 1993. Soil survey manual. 

Soil Conservation Service. U.S. Department of Agriculture Handbook 18.)

Soil Taxonomy (Soil Survey Staff. 1999. Soil taxonomy: A basic system of 

soil classification for making and interpreting soil surveys. 2nd edition. 

Natural Resources Conservation Service. U.S. Department of Agriculture 

Handbook 436.)

Field Book for Describing and Sampling Soils (P.J. Schoeneberger, D.A. 

Wysocki, E.C. Benham, and Soil Survey Staff. 2012. Field book for 

describing and sampling soils, version 3.0. Natural Resources 

Conservation Service, National Soil Survey Center, Lincoln, Nebraska.)

Definitions of many of the terms used can also be found in the Soil Science 

Society of America’s online, searchable Glossary of Soil Science Terms.

Abbreviations and Acronyms

CaCl2 calcium chloride

CaCO3 calcium carbonate

CEC cation-exchange capacity

cmol+ centimoles of charge

COLE coefficient of linear extensibility

dS/m decisemens per meter

ECe electrical conductivity of the saturation extract

ECEC effective cation-exchange capacity

ESP exchangeable sodium percentage

HCl hydrochloric acid

Ksat saturated hydraulic conductivity

KOH potassium hydroxide

MAST mean annual soil temperature

MCS moisture control section

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MSST mean summer soil temperature

NaOH sodium hydroxide

NH4OAc ammonium acetate

NRCS Natural Resources Conservation Service

O.C. organic carbon

ODOE optical density of oxalate extract

P2O5 phosphorus pentoxide (phosphorous oxide)

pH power of hydrogen (hydrogen ion activity)

SAR sodium absorption ratio

USDA United States Department of Agriculture

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Part 2—Horizon Nomenclature Used for 

Describing Soil Profiles

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Background Information

The purpose of horizon nomenclature is to provide a standard short-hand 

method to label soil horizons in profile descriptions. Nomenclature allows soil 

scientists to convey information to the reader about the presence of key

properties and genetic processes that have produced the morphology 

observed in the soil profile. The nomenclature tells a story expressing the 

genetic development of the soil. Click here to see a video about describing 

soil horizons in the field.

The nomenclature for designating kinds of horizons consists of five kinds of 

characters. For any given horizon, however, only those characters needed to 

effectively label it (typically just two to four) are used. 

1. Master horizon symbol. Capital letters, such as A, E, B, and C, are used 

to designate the kind of master horizon. Table 1 lists the nine master 

horizon designations and provides a general description of each.

2. Suffix symbols. Lowercase letters following the master horizon symbol 

are used to further describe one or more important characteristics of the 

horizon. Examples include Ap, Bt, and Bk. Tables 2A and 2B list 34 suffix 

symbols that can be used and provides a general description of each one. 

Table 2A lists those used to indicate physical characteristics, and Table 2B

lists those used to indicate chemical or mineralogical characteristics.

3. Vertical subdivisions. Numbers can be added at the end of the horizon 

designation to subdivide a horizon having the same combination of master 

and suffix letters. Vertical subdivisions are generally used to split master 

horizons based upon minor variations, such as slight changes in color and 

structure or differences in the amount or size of features. 

4. Lithologic discontinuities. A number is placed before the master horizon 

symbol to designate a significant change, such as particle-size distribution 

or mineralogy, in the lithology of mineral soils. 

5. Special symbols

a. Horizons that formed in human-transported materials are indicated by a

caret symbol (^) preceding the master horizon symbol.

b. When a sequence of horizons is repeated in different parts of a soil 

profile (sometimes called a bisequal soil) the prime symbol ( ́) is used 

after the master horizon letter in the second sequence. 

Conventions for the Use of Horizon Nomenclature

Some basic rules are listed below. See the “Keys to Soil Taxonomy” for more 

detailed information.

1. Vertical subdivisions of horizons with the same master and suffix letter 

symbols are indicated by a number at the end of the horizon 

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designation. An example sequence for a soil profile with subdivisions of 

the A and Bt horizons is: A1-A2-E-Bt1-Bt2–C.

2. Lithologic discontinuities in mineral soils are indicated by a number 

preceding the master horizon letter. An example sequence for a profile 

of a soil that formed in loess over glacial till underlain by bedrock is: 

A-E-Bt1-Bt2-2BC-2C-3R. In this sequence, the A, E, and Bt horizons 

developed in loess while the BC and C horizons are glacial till (and 

therefore preceded by the number 2). Since the bedrock (R) is not the 

source of parent material for the soil above, it is preceded with the 

number 3, which indicates another discontinuity. Note that the number 

1 is understood and therefore not used.

3. Horizons that are composed of human-transported material are 

indicated by using a caret symbol (^) as a prefix to the master horizon 

letter. An example sequence for a profile of a soil that formed in 

human-transported organic soil material over a naturally occurring 

mineral soil is: ^Oau1-^Oau2-A-B-C.

4. In cases where two or more horizons having identical number prefixes 

and master and suffix letters are separated by a different kind of 

horizon, the prime symbol ( ́) is used after the master horizon letter of 

the lower sequence. For example, the profile of a soil having two E 

horizons separated by a Bt horizon is designated as: A-E-Bt-E ́-Btx-C. 

Only horizons with identical number prefixes and master and suffix 

letters are designated with the prime symbol (thus no prime symbol is 

used for the sequence: A-E-Bt-2E-2Btx-2C). Number suffixes used at 

the end of the horizon designation to indicate vertical subdivisions of 

horizons are not considered when using the prime symbol. For 

example, the sequence A-E-Bt-E ́-B ́t1-B ́t2-C shows the appropriate 

use the prime symbol.

5. Master horizon symbols may be combined in the following situations:

a. Transitional horizons. In cases where a horizon is in a position of 

transition and is dominated by the properties of one master 

horizon type, but has subordinate properties of another master 

horizon type, two master horizon letters may be used. For 

example, an AB horizon has characteristics of both an overlying 

A horizon and an underlying B horizon but is more like an A than 

a B horizon (thus A comes first). Other examples include BA, 

EB, BE, BC, and CB. 

b. Horizons with two distinct parts. In cases where a horizon has 

distinct parts, each having recognizable characteristics of two 

different forms of master horizons, both master horizon symbols 

Part 2—Horizon Nomenclature Used for Describing Soil Profiles 2-3
Illustrated Guide to Soil Taxonomy

are used and separated by a virgule (/). The dominant part is 

listed first. Examples include A/B, B/A, E/B, and B/C.

c. Horizons containing lamellae. In cases where a horizon has 

lamellae, the two master horizon letters are combined by the

word “and.” Examples include “E and Bt” and “Bt and E.”

Part 2—Horizon Nomenclature Used for Describing Soil Profiles 2-4
Illustrated Guide to Soil Taxonomy

Table 1.—Master Soil Horizons and the Nomenclature (Capital Letters) Used to Designate

Them in Soil Profile Descriptions

Horizon 

Symbol

General Description

A Surface mineral soil horizon, generally darkened by accumulating humus. May 

be buried by newer deposits or underlie an organic horizon.

B Subsoil mineral horizon, generally characterized by accumulation, removal, or 

redistribution of constituents such as iron, aluminum, silica, clay, humus, 

calcium carbonate, calcium sulfate, or sesquioxides. May be at the surface in a 

truncated soil.

C Relatively unaltered mineral soil layer underlying the pedogenically developed 

soil profile. Generally unconsolidated, but includes soft bedrock.

E Subsurface mineral soil horizon characterized by a loss of iron, aluminum, clay, 

or organic matter. Generally has lighter color and coarser texture compared to 

horizon above.

L Soil horizon composed of organically derived limnic material that was 

deposited in water by chemical precipitation or aquatic organisms or came 

from plants (as modified by organisms). Examples include coprogenous earth, 

diatomaceous earth, and marl. 

M Root-limiting, human-manufactured layer. Examples include a buried layer of 

continuous asphalt, landfill liner, and geotextile fabric.

O Soil horizon composed dominantly of organic soil materials (but not limnic 

materials). Has relatively low bulk density compared to mineral soil. May be at 

the surface or buried.

R Consolidated hard, continuous bedrock.

W Layer of water within or under the soil (not at the surface). Examples include 

floating bogs and segregated ice layers.

Part 2—Horizon Nomenclature Used for Describing Soil Profiles 2-5
Illustrated Guide to Soil Taxonomy

Table 2A.—Subordinate Lowercase Suffixes Describing Physical Characteristics of the Soil 

Suffix General Description

a Highly decomposed organic matter. Has < 17%, by volume, rubbed fiber. 

Used only with master horizon O.

b Buried genetic horizon. Exhibits past soil-forming development.

c Presence of cemented concretions or nodules.

d Physically root-restrictive layer due to high bulk density. May have natural 

or human-induced compaction. Examples include dense basal till and plow 

pans.

e Moderately decomposed organic matter. Has 17 to < 40%, by volume, 

rubbed fiber. Used only with master horizon O.

f Permanently frozen layer that contains ice (permafrost).

ff Dry permafrost. Permanently frozen layer, not cemented by ice.

i Slightly decomposed organic matter. Has > 40%, by volume, rubbed fiber.

Used only with master horizon O.

jj Evidence of cryoturbation in the active layer above permafrost. Examples 

include broken or irregular horizons boundaries, organic bodies within 

mineral horizons, and sorted rock fragments.

m Root-restrictive, pedogenically cemented horizon. More than 90% of layer

is cemented by agents such as calcium carbonate, iron, silica, gypsum, or 

other salts.

p Disturbance of the surface layer, commonly by plowing, cattle trampling, 

vehicle traffic, or other mechanical means. Used with master horizon A.

r Weathered or soft bedrock. Although cemented, it can be dug with hand 

tools, such as a spade or pick. Used with master horizon C.

ss Presence of slickensides formed by shear movement in clayey soils as a 

result of shrinking and swelling.

t Illuvial accumulation of clay as evidenced by the presence of clay films, 

clay bridges, or lamellae.

u Presence of human-manufactured material (artifacts) such as bricks, 

metal, coal ash, fabric, rubber, plastic, glass, and garbage.

w Weakly expressed color or structural development or minimal 

accumulation of pedogenic constituents. Used with master horizon B, but 

not with transitional horizons.

x Genetically developed horizon that is firm, brittle, and physically root 

restrictive, at least in part.

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Illustrated Guide to Soil Taxonomy

Table 2B.—Subordinate Lowercase Suffixes Describing Chemical or Mineralogical 

Characteristics of the Soil

Suffix General Description

co Limnic layer composed primarily of coprogenous earth (fecal material from 

aquatic animals). Used only with master horizon L.

di Limnic layer composed primarily of diatomaceous earth (sedimentary 

siliceous diatom remains). Used only with master horizon L.

g Strong gleying (iron reduction and loss due to saturation and anaerobic 

conditions). Chroma is 2 or less.

h Illuvial accumulation of amorphous, dispersible, organic matter and 

aluminum-dominated sesquioxides coating sand and silt particles and 

sometimes filling pores. Used with master horizon B.*

j Accumulation of jarosite (iron hydroxy sulfate mineral with yellow hue 

produced in acid-sulfate soils).

k Accumulation of visible pedogenic calcium carbonate (< 50%, by volume).

Forms include filaments, soft masses, nodules, pendants, and finely 

disseminated carbonates. 

kk Engulfment of the horizon by pedogenic calcium carbonate (> 50%, by 

volume). Carbonates coat particles and fill pores, effectively plugging the 

soil fabric.

ma Limnic layer composed primarily of marl (soft, muddy deposit of

sedimentary calcium carbonate and clay). Reacts with dilute HCl. Used 

only with master horizon L.

n Accumulation of exchangeable sodium.

o Residual accumulation of sesquioxides (iron- and aluminum-oxides).

q Accumulation of secondary silica as concretions, durinodes, opal, etc.

s Illuvial accumulation of amorphous, dispersible sesquioxides (iron- and 

aluminum-oxides) and organic matter. Used with master horizon B.*

se Presence of sulfides in mineral or organic layers. Often associated with 

sulfurous odor (rotten-egg smell). Typically dark in color (value < 4 and

chroma < 2).

v Presence of plinthite (firm, iron-rich, humus-poor, red concentration in 

association with clay, quartz, and other minerals). Irreversibly hardens with 

repeated wet/dry cycles (as in a road cut).

y Accumulation of gypsum (or rarely, anhydrite). Because amounts are 

sufficiently low (< 50%, by volume) the gypsum does not disrupt or 

obscure other features of the horizon.

yy Dominance of the horizon by accumulated gypsum (or rarely, anhydrite). 

Amounts are sufficiently high (> 50%, by volume) for the growth of gypsum 

crystals to disrupt or obscure other features of the horizon. Color is 

typically white (value > 7 and chroma < 2).

z Accumulation of salts more soluble than gypsum, such as sodium chloride.

 *

Suffixes h and s are combined, as in Bhs, where the sesquioxide component is significant 

but the color has both moist value and chroma of 3 or less.

Part 2—Horizon Nomenclature Used for Describing Soil Profiles 2-7

Part 3—Diagnostic Horizons and 

Characteristics

Part 3—Diagnostic Horizons and Characteristics 3-1 
Illustrated Guide to Soil Taxonomy

Part 3—Diagnostic Horizons and Characteristics 3-2

Introduction

The taxonomic classes defined in Soil Taxonomy group soils that have similar 

properties and that formed as a result of similar pedogenic processes. Rather 

than define classes based directly on theories of soil genesis, however, the 

classes are based largely on the presence of diagnostic horizons and 

characteristics. These diagnostic horizons and characteristics reflect the 

important pedogenic processes (i.e., additions, removals, transfers, and 

transformations) that are either occurring now, or have occurred in the past, to 

produce the kinds of soil profiles we see today. Some of the criteria used to 

define the diagnostic horizons and characteristics can be observed directly in 

the field (color, texture, thickness, etc.). Other properties can only be 

determined in the laboratory (cation-exchange capacity, percent gypsum, 

organic carbon content, etc.).

The following descriptions are divided into two sections. In the first section, 

the eight diagnostic surface horizons (epipedons) are presented. In the 

second section, 49 diagnostic subsurface horizons and characteristics needed 

to classify a soil to the great group level are discussed. There are more 

diagnostic characteristics, not discussed in this guide, that are used to define 

categories below the great group level. For information on these, see the 12th

edition of the “Keys to Soil Taxonomy.” In addition, six soil moisture regimes 

and six soil temperature regimes are briefly described because they are used 

either in some of the criteria for taxonomic classes or in criteria for some of the 

diagnostic horizons.

For each diagnostic horizon or characteristic presented, the following is 

provided:

 Heading. A brief descriptive phrase is given for the item.

 Concept and background information. A brief narrative describes the 

concept of the item and basic information about it.

 Generalized characteristics. The criteria given in the “Keys to Soil 

Taxonomy,” 12th edition, are presented in a somewhat simplified and 

illustrated version. Information has been reworded, and, in a few cases, 

the critera have been reorganized. Some of the more complex details 

have been omitted. Generally, a note (italicized text that follows) alerts 

the reader if there are exceptions to the presented information or if 

additional information is available in the full version of the keys.

 Common horizon nomenclature. The master horizon and/or 

subordinate symbols that are commonly used in soil descriptions to 

indicate a particular diagnostic horizon or characteristic are identified. 

 Photos (for most of the diagnostic horizons and characteristics)

It is important to note that, unlike the keys to the orders, suborders, and great 

groups (given in part 4 of this guide), the information presented in this section 
Illustrated Guide to Soil Taxonomy

Part 3—Diagnostic Horizons and Characteristics 3-3

is NOT a key (i.e., the user does not start at the beginning and stop when the 

criteria are met). To become fully competent, a soil scientist needs to be 

familiar with the basic concepts and criteria of all the diagnostic horizons and 

characteristics (or at least all of those found in the local geographic area). 

Some of the diagnostic horizons are mutually exclusive, but not all. Two 

examples follow:

 The upper part of an argillic horizon that has significant accumulations 

of organic carbon could also qualify as part of a mollic or umbric 

epipedon, depending on base saturation.

 A layer with significant accumulations of both gypsum and calcium 

carbonate may qualify as both a calcic and gypsic horizon.

Epipedons

Diagnostic Subsurface Horizons and Characteristics (including temperature 

and moisture regimes)
Illustrated Guide to Soil Taxonomy

Part 3—Diagnostic Horizons and Characteristics 3-4

Epipedons

The epipedon (Gr. epi, over, upon, and pedon, soil) is a horizon that forms at 

or near the surface and in which most of the rock structure has been 

destroyed. It is darkened by organic matter or shows evidence of eluviation, or 

both. Rock structure includes fine stratification (less than 5 mm) in 

unconsolidated sediments (eolian, alluvial, lacustrine, or marine) as well as 

saprolite derived from consolidated rocks in which the unweathered minerals 

and pseudomorphs of weathered minerals retain their relative positions to 

each other. An epipedon is not the same as an A horizon. It may include part 

or all of an illuvial B horizon if the darkening by organic matter extends from 

the soil surface into or through the B horizon.

Soil Taxonomy recognizes eight epipedons. The most common in the United 

States are the ochric, mollic, and umbric epipedons. In general, they are 

distinguished based on organic matter accumulation, color, thickness, and 

degree of base saturation. Two epipedons form in organic soil materials—

histic and folistic epipedons. The more common histic forms on wet soils, 

while the less common folistic epipedon forms on more or less well drained 

soils. Provision is made to allow mineral versions of the folistic and histic 

epipedons if they are mixed by plowing. The melanic epipedon is unique to 

some soils with andic soil properties. Two epipedons are the result of human 

activity – the anthropic and plaggen epipedons. These are rare in the United 

States but are found in some local areas.

Brief Descriptions of the Epipedons

(Click on the links for more detailed information)

Anthropic—A thick horizon that formed in human-altered or humantransported material

Folistic—A more or less freely drained horizon that formed in organic 

materials

Histic—A saturated horizon that formed in organic materials

Melanic—A thick, dark-colored, humus-rich horizon in which organic carbon is 

associated with poorly crystalline, short-range-order minerals or aluminumhumus complexes

Mollic—A thick, dark-colored, humus-rich horizon with high base status

Ochric—A more or less minimally developed surface horizon, typically thin or 

light colored, that does not meet the criteria for any other epipedon
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Part 3—Diagnostic Horizons and Characteristics 3-5

Plaggen—A human-developed surface layer resulting from long-term 

manuring and spading

Umbric—A thick, dark-colored, humus-rich horizon with low base status
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Part 3—Diagnostic Horizons and Characteristics 3-6

Soil profile (of a Torriorthent in the United Arab 

Emirates) showing rock structure as fine to 

coarse stratifications throughout. The upper 

60 cm is recent eolian sand. The lower part

consists of old alluvial deposits interspersed 

with eolian strata. Scale is in cm. (Photo 

courtesy of Dr. Craig Ditzler)

Back to introduction of Epipedons
Illustrated Guide to Soil Taxonomy

Part 3—Diagnostic Horizons and Characteristics 3-7

Anthropic Epipedon

A thick horizon that formed in human-altered or human-transported 

material

Concept and Background Information

The anthropic epipedon is a thick horizon that formed in human-altered or 

human-transported material. The key feature is that it formed as the result of 

intentional human alteration (but not simply by the common agricultural 

practices of plowing and amending the soil with fertilizers). Its landscape 

setting (such as a raised surface due to filling) and/or presence of human 

artifacts are key characteristics.

Generalized Characteristics 

1) When dry, horizon has structural units < 30 cm in size.

2) The original rock structure, including fine stratification, has been mostly 

obliterated.

3) Horizon consists of human-altered or human-transported material.

4) Soil is on an anthropogenic landform, contains artifacts or kitchenmiddens, or has been compacted to impede drainage (as in paddy 

cultivation).

5) Minimum thickness is 25 cm.

6) When moist, horizon is not fluid.

Common Horizon Nomenclature

Commonly used horizon nomenclature includes master horizon A, which may 

be in combination with a preceding caret symbol (^) and with suffix p or u.

Examples include: ^Ap, ^Au, and Ap.
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Part 3—Diagnostic Horizons and Characteristics 3-8

This soil in New York City formed in humantransported material with human artifacts in the 

profile, such as brick, glass, and metal. The 

surface layer is an anthropic epipedon.

Back to Anthropic Epipedon
Illustrated Guide to Soil Taxonomy

Part 3—Diagnostic Horizons and Characteristics 3-9

Folistic Epipedon

An organic surface horizon that is freely drained

Concept and Background Information

The folistic epipedon is a layer that is more or less freely drained and that has 

sufficiently high amounts of organic carbon (O.C. > 20%, by weight) to be 

considered organic soil material. Typically, it is at (or near) the surface and is 

at least 15 cm thick.

Included within the concept of the folistic epipedon are some layers which 

have been plowed. As a consequence of mixing with mineral soil material, 

and/or loss of organic carbon due to oxidation, these layers now have slightly 

lower amounts of organic carbon (8 to 16%, by weight, depending on clay 

content) and no longer qualify as organic soil material. This form of the folistic 

epipedon must be > 25 cm thick.

The folistic epipedon is used only with mineral soils (i.e., it is not recognized in 

Histosols).

Generalized Characteristics

Horizon is:

1) Freely drained, and

2) High in organic material (generally > ~ 15% O.C., by weight, depending on 

clay content), and

3) At least 15 cm thick.

Note: The layer is saturated < 30 days cumulative annually. Minimum 

thickness and organic carbon requirements vary depending on the kind of 

organic matter (affecting bulk density) and whether or not the site is plowed. 

For the plowed condition, the minimum required O.C. weight % is 8 + 

(clay% / 7.5). If the layer is > 60% clay, O.C must be > 16%, by weight.

Common Horizon Nomenclature

Commonly used horizon nomenclature includes master horizon O (and less 

commonly A), which may be in combination with suffix p, e, or i. Examples 

include: Oi, Oe, and Ap.
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Part 3—Diagnostic Horizons and Characteristics 3-10

Histic Epipedon

An organic surface horizon that is periodically saturated

Concept and Background Information

The histic epipedon is a horizon that is periodically saturated with water and 

that has sufficiently high amounts of organic carbon (O.C. 12 to 18%, by 

weight, depending on clay content) to be considered organic soil material. 

Typically, it is at (or near) the surface and is peat or muck at least 20 cm thick. 

Included within the concept of the histic epipedon are some layers which have 

been plowed. As a consequence of mixing with mineral soil material, and/or 

loss of organic carbon due to oxidation, these layers now have slightly lower 

amounts of organic carbon (8 to 16%, by weight, depending on clay content) 

and no longer qualify as organic soil material. This form of the histic epipedon 

must be > 25 cm thick.

The histic epipedon is used only with mineral soils (i.e., it is not recognized in 

Histosols).

Generalized Characteristics 

Horizon is:

1) Poorly drained (saturated periodically), and

2) High in organic material (generally > ~ 15% O.C., by weight, depending on 

clay content), and

3) At least 20 cm thick.

Note: The layer is saturated > 30 days cumulative annually. Minimum 

thickness and organic carbon requirements vary depending on the kind of 

organic matter (affecting bulk density) and whether or not the site is plowed. 

For the undisturbed condition, the minimum required O.C. weight % is 12 + 

(clay% X 0.1). If the layer is > 60% clay, O.C must be > 18%, by weight. For 

the plowed condition, the minimum required O.C. weight % is 8 + 

(clay% / 7.5). If the layer is > 60% clay, O.C must be > 16%, by weight.

Common Horizon Nomenclature

Commonly used horizon nomenclature includes master horizon O (and less 

commonly A), which may be in combination with suffix p, a, e, or i. Examples 

include: Oa, Oe, and Ap.
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Part 3—Diagnostic Horizons and Characteristics 3-11

Soil profile (in Alaska) that has a histic epipedon extending to a 

depth of about 38 cm. The upper 20 cm is fibric soil material that

has a high content of sphagnum. Scale is in cm. (Photo courtesy

of Dr. David Weindorf)

Back to Histic Epipedon
Illustrated Guide to Soil Taxonomy

Part 3—Diagnostic Horizons and Characteristics 3-12

Melanic Epipedon

A thick, very dark-colored, humus-rich horizon with andic properties

Concept and Background Information

The melanic epipedon is a thick (> 30 cm), very dark-colored (commonly 

black), humus-rich horizon with unique chemical and physical properties. It is 

typically at the soil surface, but may begin as much as 30 cm below the soil 

surface in places where it is covered by a younger layer, such as an ash fall or 

alluvial deposit. It is generally associated with soils that formed in materials of 

volcanic origin and has high concentrations of organic carbon (> 6%, by 

weight) derived mostly from the root residue of grasses and other similar 

vegetation. The humus is chemically associated with aluminum and poorly

crystalline minerals, such as allophane, imogolite, and ferrihydrite (andic soil

properties). As a result, the soil exhibits unique chemical and physical

properties, including a high water-holding capacity, a relatively low bulk 

density, and an ability to bind with the important plant nutrient phosphorus so 

tightly that the nutrient is not available to plants.

Generalized Characteristics 

Horizon:

1) Begins at (or within 30 cm of) the surface and has a cumulative thickness 

of > 30 cm.

2) Has color value (moist) < 2.5 and chroma < 2.

3) Has melanic index value of < 1.7.

4) Has an overall average of organic carbon > 6%, by weight.

5) Meets the criteria for andic soil properties criteria throughout.

Note: The melanic index value of < 1.70 indicates that fulvic acids make up 

more than about 40% of the combined total of humic and fulvic acids in the 

organic fraction of the soil, thus verifying the significant contribution of grass 

or grass-like plants. See 12th edition of the “Keys to Soil Taxonomy” for the 

criteria for andic soil properties.

Common Horizon Nomenclature

Commonly used horizon nomenclature includes master horizon A, which may 

be in combination with suffix p, for example, A or Ap.
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Part 3—Diagnostic Horizons and Characteristics 3-13

An Andisol in Japan that has a thick, dark 

melanic epipedon. A paleosol is below

a depth of 90 cm. Scale is in cm.

Back to Melanic Epipedon
Illustrated Guide to Soil Taxonomy

Part 3—Diagnostic Horizons and Characteristics 3-14

Mollic Epipedon

A thick, dark-colored, humus-rich horizon with high base status

Concept and Background Information

The mollic epipedon is a thick, friable, dark-colored (mostly very dark brown to 

black), humus-rich surface horizon with high base status. Because of the 

favorable base status, it has high native fertility and calcium, magnesium, and 

other important positively charged elements dominate the cation exchange 

sites. The properties of the mollic epipedon are, to a significant degree, the 

result of inputs of organic matter and nutrients from deep-rooted vegetation 

that is highly productive of below-ground biomass, such as that associated 

with grassland or savannah ecosystems. The organic matter and nutrients are 

further cycled by earthworms and other soil organisms, resulting in a thick, 

fertile, humus-rich surface horizon.

Generalized Characteristics

1) When dry, horizon is not both massive and hard.

2) The original rock structure, including fine stratification, is mostly 

obliterated.

3) Moist value and chroma are < 3 and dry value is < 5. (Lighter colors are 

allowed if calcium carbonate [CaCO3] is > 15%).

4) Minimum thickness is 25 cm for most soils. Exceptions include:

a. 10 cm if the epipedon rests directly on bedrock (and is not sandy) or 

rests on a densic contact, a petrocalcic horizon, or a duripan.

b. 18-25 cm if the epipedon is not sandy and it is at least:

i. 1/3 the depth to carbonates, or to a calcic horizon, petrocalcic

horizon, duripan, or fragipan, or

ii. 1/3 the solum thickness (as indicated by the base of an argillic, 

cambic, natric, oxic, or spodic horizon).

5) Base saturation is > 50% throughout the layer (by NH4OAc sum of bases).

6) Organic carbon is > 0.6% (by weight), about 1% organic matter content. 

(Exceptions include a higher required O.C. for soils with high calcium 

carbonate equivalent or for soils that formed in naturally dark-colored 

parent materials).

7) Horizon is moist in some part for > 90 total days during the growing season 

(not from irrigation).

8) When moist, horizon is not fluid.

Common Horizon Nomenclature

Commonly used horizon nomenclature includes master horizon A, which may 

be in combination with suffix p. In addition, the mollic epipedon may extend 
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Part 3—Diagnostic Horizons and Characteristics 3-15

into the subsoil and include horizons with master symbol AB, BA, or B and 

various suffixes, including k, n, t, ss, and w. Examples include: A, Ap, AB, and 

Bt.

A mollic epipedon with granular structure (soil profile in Romania). (Photo

courtesy of Dr. David Weindorf)

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Illustrated Guide to Soil Taxonomy

Part 3—Diagnostic Horizons and Characteristics 3-16

Ochric Epipedon

A minimally developed surface horizon that is typically thin and/or lightcolored

Concept and Background Information

The ochric epipedon typically includes surface layers that are only slightly or 

moderately darkened by organic matter (mostly light yellowish brown to 

brown) as well as any lighter-colored eluvial horizons below (A and E horizon

sequence), extending to the first underlying diagnostic subsoil horizon. 

However, rather than providing a definition based on the properties of the 

horizon, Soil Taxonomy simply defines as ochric epipedons those surface 

horizons that fail to meet the definitions for any of the other seven epipedons. 

For example, a horizon may meet all the criteria for a mollic epipedon except 

that it is too thin or too light in color. In other cases, a horizon may be 

composed of organic soil material but be too thin to qualify as either a histic or 

folistic epipedon. As a consequence, the ochric epipedon can closely 

resemble in form any of the other epipedons but fails to have one or more of 

their characteristics, possibly by just a small amount.

Generalized Characteristics

Soil Taxonomy does not provide a list of required characteristics for the ochric 

epipedon. This is due to the nature of the ochric as a “catch-all” epipedon for 

those horizons that fail to meet the requirements given for any of the other 

seven epipedons. It must be remembered, however, that to qualify as an 

ochric epipedon, the horizon in question must:

1) Meet the definition for an epipedon: 

a. Horizon formed at or near the surface.

b. Most rock structure, including fine stratification, has been destroyed.

c. Horizon has been darkened by organic matter and/or shows evidence 

of eluviation.

2) Fail to have one or more of the required characteristics of all other 

epipedons.

Common Horizon Nomenclature

Commonly used horizon nomenclature includes master horizons A, E, and O, 

which may be in combination with suffix a, e, i, or p. Examples include: A, Ap, 

E, and Oi.
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Part 3—Diagnostic Horizons and Characteristics 3-17

Profile of an Andisol with a thin ochric epipedon 

that has been darkened by organic matter.

Scale is in cm.

Back to Ochric Epipedon
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Part 3—Diagnostic Horizons and Characteristics 3-18

Plaggen Epipedon

A thick, dark-colored, human-made surface layer produced by long-term 

manuring and spading

Concept and Background Information

The plaggen epipedon is a thick, dark-colored (typically black to dark grayish 

brown) human-made mineral surface layer produced by long-term manuring 

and spading. It typically contains a few artifacts, such as bits of brick and 

pottery, throughout. It was originally recognized in Europe and has generally 

not been recognized in the United States. In medieval times, sod or other 

materials commonly were used for bedding livestock and the manure was 

spread on nearby cultivated fields. The mineral materials brought in by this 

kind of manuring eventually produced an appreciably thickened Ap horizon 

(as much as 1 m or more thick). In northwestern Europe, this custom was 

associated with the poorly fertile, sandy Spodosols. The practice more or less 

ceased at the turn of the 19th century, when synthetic fertilizers became 

readily available. An area of soil with a plaggen epipedon tends to have 

straight-sided rectangular borders that are higher than the adjacent soils by as 

much as or more than the thickness of the plaggen epipedon.

Generalized Characteristics

1) Horizon occurs in soils on elevated landforms which contain artifacts (not 

including agricultural amendments) or spade marks.

2) Color value is < 4 moist (< 5 dry), and chroma is < 2.

3) Organic carbon content is > 0.6% (by weight), about 1% or more organic 

matter.

4) Horizon has > 50 cm of human-transported material.

5) Horizon is moist in some part for > 90 total days during the growing season 

(not from irrigation).

Common Horizon Nomenclature

Commonly used horizon nomenclature includes master horizon A, which may 

be preceded by the caret symbol (^) and be in combination with suffixes p 

and/or u. Examples include: Ap and ^Apu.
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Part 3—Diagnostic Horizons and Characteristics 3-19

Profile of a soil from Holland with a plaggen 

epipedon that formed from long-term 

manuring. Scale colors are in decimeters.

Back to Plaggen Epipedon
Illustrated Guide to Soil Taxonomy

Part 3—Diagnostic Horizons and Characteristics 3-20

Umbric Epipedon

A thick, dark-colored, humus-rich horizon with low base status

Concept and Background Information

The umbric epipedon is a thick, dark-colored (mostly very dark brown to 

black), humus-rich surface horizon with low base status. The base status is 

relatively low (< 50% in some part or the entire horizon) because relatively 

high proportions of aluminum and possibly hydrogen (rather than calcium, 

magnesium, sodium, and potassium) occupy a significant number of the 

exchange sites. This is generally characteristic of acid soils with low to 

moderate native fertility levels. The umbric epipedon is thought to be formed 

mainly by the decomposition of organic residues. The decomposed organic 

residues are partly roots and partly surface organic residues that have been 

taken underground by soil fauna. Accumulation and turnover of the organic 

matter probably are slower in the umbric epipedon than in the mollic 

epipedon. The aluminum ions may be somewhat toxic to some kinds of soil 

micro-organisms.

Generalized Characteristics

1) When dry, horizon is not both massive and hard.

2) The original rock structure, including fine stratification, is mostly 

obliterated.

3) Moist value and chroma are < 3, and dry value is < 5.

4) Minimum thickness is 25 cm (for most soils). Exceptions include:

a. 10 cm if the epipedon rests directly on bedrock (and is not sandy) or

rests on a densic contact, a petrocalcic horizon, or a duripan.

b. 18-25 cm if the epipedon is not sandy and it is at least:

i. 1/3 the depth to carbonates or to a calcic horizon, petrocalcic

horizon, duripan, or fragipan, or

1. 1/3 the depth of solum thickness (as indicated by the base of an 

argillic, cambic, natric, oxic, or spodic horizon).

5) Base saturation is < 50% in at least part of the horizon (by NH4OAc sum of 

bases).

6) Organic carbon is > 0.6% (by weight), about 1% organic matter content.

(Exceptions exist for soils that formed in naturally dark-colored parent 

materials).

7) Horizon is moist in some part for > 90 total days during the growing season 

(not from irrigation).

8) When moist, horizon is not fluid.

9) Horizon has no human artifacts (as required for the plaggen epipedon).
Illustrated Guide to Soil Taxonomy

Part 3—Diagnostic Horizons and Characteristics 3-21

Common Horizon Nomenclature

Commonly used horizon nomenclature includes master horizon A, which may 

be in combination with suffix p. In addition, the umbric epipedon may extend 

into the subsoil and include horizons with master symbol AB, BA, or B and 

various suffixes, including h, s, t, and w. Examples include: A, Ap, AB, and Bt.
Illustrated Guide to Soil Taxonomy

Part 3—Diagnostic Horizons and Characteristics 3-22

Profile of a soil in Japan with a thick umbric 

epipedon that was darkened by organic 

matter. Scale is in cm.

Back to Umbric Epipedon
Illustrated Guide to Soil Taxonomy

Part 3—Diagnostic Horizons and Characteristics 3-23

Diagnostic Subsurface Horizons and Characteristics

(including moisture and temperature regimes)

(Click the links for more detailed information.)

Abrupt Textural Change—A considerable increase in clay content over a short 

vertical distance

Agric Horizon—Subsoil horizon with accumulation of silt, clay, and humus

Albic Horizon—Light-colored, leached subsoil horizon

Albic Materials—Soil materials whose color is determined by uncoated 

primary sand and silt particles

Andic Soil Properties—Unique soil properties associated with materials that 

are rich in volcanic glass or poorly crystalline minerals

Anhydritic Horizon—Horizon with an accumulation of anhydrite

Anhydrous Conditions—Very cold and very dry soil conditions

Aquic Conditions—Saturation in the soil to the extent that it results in the 

depletion of oxygen

Aquic Moisture Regime—Regime characteristic of poorly and very poorly 

drained soils

Argillic Horizon—Subsoil horizon with an illuvial accumulation of clay

Aridic (Torric) Moisture Regime—Regime characteristic of arid and semiarid 

climates that are unsuitable for cultivation without irrigation

Calcic Horizon—Subsoil horizon with an illuvial accumulation of calcium 

carbonate (CaCO3)

Cambic Horizon—Subsoil horizon with minimal development

Coefficient of Linear Extensibility (COLE)—A measure describing the 

proportional change in linear dimension of a soil clod due to shrinking and 

swelling

Cryic Soil Temperature Class

Cryoturbation—Intense frost churning
Illustrated Guide to Soil Taxonomy

Part 3—Diagnostic Horizons and Characteristics 3-24

Densic Contact—The contact between unconsolidated soil material and the 

underlying root-restrictive, dense, compact materials

Densic Materials—Root-restrictive, noncemented, dense, compact material

Duripan—Subsoil layer that is cemented by silica

Fibric Soil Materials—Organic soil materials that are only slightly decomposed

Fragipan—A root-restrictive subsoil layer that is firm and brittle, but not 

cemented

Frigid Soil Temperature Class

Gelic Soil Temperature Class

Gelic Materials—Soil materials (mineral or organic) above permafrost that 

show evidence of frost churning

Glacic Layer—Layer of ice in the soil

Glossic Horizon—A degrading argillic, kandic, or natric horizon in which loss 

of clay and iron oxide is occurring

Gypsic Horizon—Surface or subsoil horizon with an accumulation of gypsum

Hemic Soil Materials—Organic soil materials that are moderately decomposed

Human-Altered Materials—Soil material significantly altered by intentional 

human activity

Human-Transported Materials—Soil material transported by intentional human 

activity

Hyperthermic Soil Temperature Class

Identifiable Secondary Carbonates—Visible calcium carbonate (CaCO3) that 

has been precipitated in the soil

Interfingering of Albic Materials—Narrow penetrations of light-colored, leached 

material into a subsoil horizon

Kandic Horizon—Subsoil horizon with a low nutrient-holding capacity and 

significantly more clay than the overlying surface layer
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Part 3—Diagnostic Horizons and Characteristics 3-25

Lamellae—Two or more thin layers with accumulation of illuvial clay

Lithic Contact—Contact between unconsolidated soil material and the 

underlying root-restrictive hard bedrock

Mesic Soil Temperature Class

n Value—A value that describes the ability of a saturated soil to support a load

Natric Horizon—Subsoil horizon with an illuvial accumulation of clay and high 

levels of sodium

Oxic Horizon—Subsoil horizon that is extremely weathered and has a very low 

nutrient-holding capacity

Paralithic Contact—Contact between unconsolidated soil material and the 

underlying root-restrictive soft bedrock

Permafrost—Frozen layer within the soil

Perudic Moisture Regime—Regime in which precipitation exceeds

evapotranspiration every month

Petrocalcic Horizon—Root-restrictive subsoil horizon that is cemented by 

calcium carbonate (CaCO3)

Petroferric Contac—Boundary between unconsolidated soil and a layer 

cemented with iron

Petrogypsic Horizon—Root-restrictive subsoil horizon that is cemented by 

gypsum

Placic Horizon—Root-restrictive subsoil horizon that is cemented by iron and 

organic matter

Plinthite—Subsoil feature consisting of a firm iron-oxide-rich mass that 

irreversibly hardens after exposure to repeated wet-dry cycles

Redoximorphic Features—Morphological features caused by wetness that are

characterized by distinctive reddish and grayish color patterns (also see Aquic 

conditions)

Salic Horizon—Saline horizon with an accumulation of salts

Sapric Soil Materials—Organic soil materials that are highly decomposed
Illustrated Guide to Soil Taxonomy

Part 3—Diagnostic Horizons and Characteristics 3-26

Slickensides—Subsoil feature consisting of polished and grooved surfaces 

caused by shrinking and swelling

Soil Moisture Regimes (generalized map)

Aquic

Aridic

Perudic

Udic

Ustic

Xeric

Soil Temperature Regimes (generalized map and classes)

Sombric Horizon—Subsoil horizon with an illuvial accumulation of humus

Spodic Horizon—Subsoil horizon with an illuvial accumulation of organic 

matter in complex with aluminum and also commonly iron

Spodic Materials—Illuvial, active amorphous materials composed of organic 

matter in complex with aluminum and also commonly iron

Sulfidic Materials—Soil materials containing acid-producing, oxidizable sulfur 

compounds

Sulfuric Horizon—Subsoil horizon that has become highly acid due to 

oxidation of sulfide minerals

Thermic Soil Temperature Class

Udic Moisture Regime—Regime characteristic of humid regions with 

seasonally well distributed precipitation

Ustic Moisture Regime—Regime characteristic of semiarid climates where 

moisture is limited but available for portions of the growing season

Xeric Moisture Regime—Regime characteristic of a Mediterranean-type 

climate, which has cool, moist winters and warm, dry summers
Illustrated Guide to Soil Taxonomy

Part 3—Diagnostic Horizons and Characteristics 3-27

Abrupt Textural Change

A considerable increase in clay content over a short vertical distance

Concept and Background Information

An abrupt textural change is a specific kind of clay increase into an underlying 

diagnostic subsoil horizon. Only silicate clay (i.e., noncarbonated clay) is 

considered. The clay increase must occur over a vertical distance of < 7.5 cm 

and meet a minimum, as follows:

 If the layer above the subsoil horizon has < 20% clay, then the clay 

content doubles within 7.5 cm. For example, if an E horizon has 15% 

clay, then clay content is at least 30%. 

 If the layer above the subsoil horizon has > 20% clay, then the clay 

content increases by at least 20% (absolute) within 7.5 cm. For 

example, if an E horizon has 25% clay, then clay content is at least 

45%.

Generalized Characteristics 

1) Clay content increases considerably over a vertical distance of < 7.5 cm.

2) The noncarbonated clay content of the subsoil layer is > 8% (by weight).

3) If there is < 20% clay in the eluvial layer, then clay content doubles.

4) If there is > 20% clay in the eluvial layer, then clay content increases by 

> 20% (absolute).

Common Horizon Nomenclature

An abrupt textural change occurs in the narrow zone of transition from an A or 

E horizon to the underlying B horizon. There is no horizon nomenclature 

specifically indicating the presence of an abrupt textural change. Examples of 

an abrupt textural change include (if the criteria listed above are met) sandy 

loam over clay, fine sandy loam over silty clay, and loamy sand over sandy 

clay.
Illustrated Guide to Soil Taxonomy

Part 3—Diagnostic Horizons and Characteristics 3-28

This soil from California has an abrupt textural change from the ochric 

epipedon to the argillic horizon (at a depth of about 6 inches). The clay 

increase is not readily apparent visually due to the rather uniform red 

color. However, it is easily observed tactilely. The soil is an Abruptic 

Durixeralf (duripan is at a depth of about 20 inches). Scale is in inches.

Back to Abrupt Textural Change
Illustrated Guide to Soil Taxonomy

Part 3—Diagnostic Horizons and Characteristics 3-29

Agric Horizon

Subsoil horizon with an accumulation of silt, clay, and humus

Concept and Background Information 

The agric horizon forms in response to cultivation and the subsequent illuvial 

accumulation of silt, clay, and humus. It is always directly below a plowed 

surface layer (Ap horizon). After precipitation events, turbulent muddy water 

moves through large pores in the plow layer. Large connecting pores (such as 

wormholes), root channels, and voids between peds directly below the plow 

layer fill with this turbid water. As it moves into the peds, the suspended 

material (silt, clay, and humus) is left behind to coat the pore surfaces. The 

illuvial material is dark-colored, generally brown or dark grayish brown to 

black. Less commonly, the accumulated silt, clay, and humus form thin, 

continuous lamellae, probably as a result of movement through stratified 

sandy sediments with contrasting pore sizes between strata.

The agric horizon has only been identified rarely in the United States.

Generalized Characteristics 

1) Horizon is directly below a plow layer.

2) Thickness is > 10 cm.

3) Horizon has illuvial silt/clay/humus (with color value of < 4 moist and

chroma of < 2) as wormhole linings or thin lenses comprising > 5%, by 

volume.

Common Horizon Nomenclature

The agric horizon is directly below an Ap horizon. Commonly used horizon 

nomenclature includes master horizon B and various suffixes, including h, t, 

and w. Examples include: Bh, Bw, and Bt.
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Part 3—Diagnostic Horizons and Characteristics 3-30

Albic Horizon

Light-colored, leached subsoil horizon

Concept and Background Information

The albic horizon is a light-colored (mostly grayish brown to white), eluvial 

subsoil horizon. It has been leached of clay and free iron-oxides to such a 

degree that the color of the horizon is mostly due to uncoated sand and silt 

particles. The albic horizon is generally in a position between a darkened 

surface layer and an underlying diagnostic horizon (e.g., a cambic, argillic, or 

spodic horizon). It can also be between two subsoil horizons. 

Generalized Characteristics 

1) Thickness is > 1 cm.

2) > 85% of the horizon is composed of albic materials. Color is related to

uncoated sand and silt grains. Generally, chroma is < 3 and value is > 4

moist and > 5 dry.

Note: Albic materials are eluvial soil materials that have their color determined 

by the primary silt and sand particles because clay and/or free iron-oxides

have been lost. See Albic Materials for specific criteria and for more detailed 

color criteria.

Common Horizon Nomenclature

Commonly used horizon nomenclature includes master horizon E, which may 

be in combination with one or more suffixes, such as c, g, or x. Examples 

include: E, Ex, and Ecg.
Illustrated Guide to Soil Taxonomy

Part 3—Diagnostic Horizons and Characteristics 3-31

An albic horizon (white) above a spodic horizon (dark brown) in a

soil in New Zealand. Scale is in cm.

Back to Albic Horizon
Illustrated Guide to Soil Taxonomy

Part 3—Diagnostic Horizons and Characteristics 3-32

Albic Materials

Soil materials whose color is determined by uncoated primary sand and 

silt particles

Concept and Background Information

Albic materials are soil materials that have been subject to eluvial processes 

to the extent that their color is determined by primary sand and silt particles. 

They are essentially void of coatings (such as clay and iron oxides). Some 

parent materials (such as volcanic ash or eolian sands) have colors meeting 

the definition of albic materials but, unless they are eluviated, they are not 

considered albic materials. Albic materials are typically found in association 

with illuvial features in the soil profile, such as an argillic, natric, or spodic 

horizon.

Generalized Characteristics 

Albic materials have one of the following colors:

1) Chroma < 2, and either

a. Value 3 moist and > 6 dry, or

b. Value > 4 moist and > 5 dry, or

2) Chroma 3, and either

a. Value > 6 moist, or

b. Value > 7 dry, or

3) Hue 5YR or redder, and either

a. Value 3 moist and > 6 dry, or

b. Value > 4 moist and > 5 dry.

Common Horizon Nomenclature

Albic materials may, or may not, be suggested by the horizon nomenclature. 

Where they make up a significant portion of the horizon, they may be denoted 

with master horizon E in combination with B. Where they make up the entire 

horizon (albic horizon), they are noted as E. Examples include: E, B/E, and

E/B.
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Part 3—Diagnostic Horizons and Characteristics 3-33

Profile of a soil in Michigan that has a horizon composed of 

albic materials (albic horizon) in the upper part (white). The

white color is indicative of the uncoated sand and silt 

particles that make up the albic horizon. (Scale is in feet).

Back to Albic Materials
Illustrated Guide to Soil Taxonomy

Part 3—Diagnostic Horizons and Characteristics 3-34

Andic Soil Properties

Unique soil properties associated with materials that are rich in volcanic 

glass or poorly crystalline minerals

Concept and Background Information

Andic soil properties are characterized by either:

1) Weakly weathered soils that have relatively high contents of volcanic glass 

and only low to moderate amounts of the minerals that result from 

weathering of the glass, or

2) Moderately weathered soils that are rich in short-range-order (poorly 

crystalline) weathering by-products and have lesser amounts of glass 

remaining.

The common short-range-order (poorly crystalline) minerals include allophane, 

imogolite, and ferrihydrite. These short-range-order minerals are responsible 

for imparting andic soil properties to the soil. These properties include a high 

water-holding capacity, good friability, and low bulk density. In addition, there 

is a significant propensity to adsorb phosphorus strongly, making it 

unavailable to plants. Laboratory tests of phosphorus retention and 

ammonium oxalate extractions of aluminum, iron, and silica are used to infer 

the relative amounts of the short-range-order minerals.

It is important to note that andic soil properties, while most commonly found in 

soils of volcanic origin, can also form in other parent materials with no volcanic 

influence. Andic soil properties in areas with cool, moist climates and

abundant carbon may generate organo-metal complexes.

Generalized Characteristics

Soil with andic soil properties:

1) Has < 25% organic carbon (by weight), and 

2) Meets all of either item A or B:

A. Has any amount volcanic glass:

1. Bulk density < 0.90 g/cm3, and 

2. P-retention > 85%, and

3. Al + 1⁄2 iron > 2.0%,

OR

B. Has > 5% volcanic glass:

1. > 30% material that is 0.02 – 2.0 mm in size (coarse silt and sand), 

and

2. P-retention > 25%, and
Illustrated Guide to Soil Taxonomy

Part 3—Diagnostic Horizons and Characteristics 3-35

3. Al + 1⁄2 iron > 0.4%, and

4. [(Al + 1⁄2 iron %) X 15.625] + (glass %) > 36.25

Common Horizon Nomenclature

There is no horizon nomenclature indicating the presence of andic soil 

properties. They are most often associated with O, A, or B horizons in soils of

volcanic origin but can only be confirmed with laboratory data.
Illustrated Guide to Soil Taxonomy

Part 3—Diagnostic Horizons and Characteristics 3-36

Anhydritic Horizon

Horizon with an accumulation of anhydrite

Concept and Background Information 

The anhydritic horizon is a horizon in which anhydrite (CaSO4) has 

accumulated either through direct formation or as a transformation 

(dewatering) of gypsum (CaSO4 2H2O). It typically occurs as a subsurface 

horizon and is commonly associated with a salic horizon.

Generalized Characteristics 

1) Thickness is > 15 cm.

2) Anhydrite is the dominant calcium sulfate mineral present and its content is 

> 5% (by weight), and

a. (Anhydrite % wt.) X (thickness in cm) > 150. (For example, if 

gypsum is 6% and thickness is 20 cm: 6% gypsum X 20 cm = 120; 

example fails anhydritic.)

3) Horizon has hue of 5Y with chroma of 1 or 2 (moist or dry) and value of 7 

or 8.

Common Horizon Nomenclature

Commonly used horizon nomenclature includes master horizon A or B and 

suffix y or yy. In some cases, suffix k or z may also be used. Examples 

include: Ayy, Byk, and Ayz.
Illustrated Guide to Soil Taxonomy

Part 3—Diagnostic Horizons and Characteristics 3-37

Profile of a soil in the United Arab Emirates that has an anhydritic 

horizon (white layer at a depth of 0 to about 60 cm). Depth is

in cm. (Photo courtesy of Dr. Shabbir A. Shahid)

Back to Anhydritic Horizon
Illustrated Guide to Soil Taxonomy

Part 3—Diagnostic Horizons and Characteristics 3-38

Anhydrous Conditions

Very cold and very dry soil conditions

Concept and Background Information

Anhydrous conditions occur in very cold deserts. Precipitation tends to be very 

low, generally < 50 mm per year. Consequently, the soil is very dry (generally 

< 3% moisture, by weight).

Generalized Characteristics 

1) Mean annual soil temperature is < 0

oC.

2) At a depth of 10 to 70 cm:

a. Temperature is < 5 oC all year, and

b. Soil has no ice-impregnated permafrost, and

c. When temperature is > 0

oC (> 1500 kPa tension), soil is very dry,

or

d. When temperature is < 0

oC, soil is loose to just slightly hard (unless 

there is pedogenic cementation).

Common Horizon Nomenclature

There is no horizon nomenclature indicating the presence of anhydrous

conditions. Under such extreme conditions, most master horizons are C in 

combination with suffix ff, for example, Cff. Note: Suffix ff is also used in cases 

that are not extreme enough to qualify for anhydrous conditions.
Illustrated Guide to Soil Taxonomy

Part 3—Diagnostic Horizons and Characteristics 3-39

A soil climate monitoring station in the cold, dry desert of the Wright Valley 

of Antarctica at the base of Bull Pass (elevation of 150 m). The soil is 

an Anhyorthel.

Return to Anhydrous Conditions
Illustrated Guide to Soil Taxonomy

Part 3—Diagnostic Horizons and Characteristics 3-40

Aquic Conditions

Saturation in the soil to the extent that it results in the depletion of 

oxygen

Concept and Background Information

Aquic conditions are characterized by saturation that is of long enough 

duration to result in the depletion of oxygen by microbes, thus becoming 

anaerobic. The time required to deplete oxygen is unique to each site. It 

generally occurs faster in warmer environments than in colder ones but is 

generally a few days or weeks. In the anaerobic environment, microbes must 

rely on elements other than oxygen to carry out their metabolism. In so doing, 

these elements are “reduced” (i.e., they gain an electron). Two important 

elements involved in this process are manganese and iron. When these 

elements are converted from their oxidized state to their reduced state, they 

become mobile and move in solution within the soil. As a result, manganese 

and iron ions tend to move along a gradient from areas within the soil that are 

void of oxygen to areas (such as faces of peds and pores) where oxygen is 

present. In the oxygenated areas, the ions are returned to their oxidized state 

(i.e., they lose an electron) and are immobilized. This process is marked by 

the formation of redoximorphic features in the soil. Areas depleted of iron and 

manganese tend to be gray in color, forming redox depletions, while areas 

where the iron and manganese have accumulated are redder (or black for 

manganese), forming redox concentrations. To be recognized as having aquic 

conditions, there must be periodic saturation by water, reducing conditions 

strong enough to reduce iron, and the presence of redoximorphic features. 

Aquic conditions may occur throughout the soil profile or in one or more layers 

anywhere within the soil profile. 

Link to Aquic Moisture Regime

Generalized Characteristics 

Soils with aquic conditions have:

1) Saturation by water, and

2) Reducing (anaerobic) conditions, and

3) Presence of redoximorphic features.

Common Horizon Nomenclature

There is no horizon nomenclature specifically indicating the presence of aquic 

conditions. Master horizons can be O, A, E, B, C, or L. Suffix a is commonly 

used with O, and suffix g (alone or in combination with other suffixes) is used 
Illustrated Guide to Soil Taxonomy

Part 3—Diagnostic Horizons and Characteristics 3-41

with the other master horizons if aquic conditions exist. Examples include: Oa, 

Btg, and Cg.
Illustrated Guide to Soil Taxonomy

Part 3—Diagnostic Horizons and Characteristics 3-42

Redoximorphic features in a periodically saturated 

and reduced soil near College Station, Texas. 

Redox depletions are gray; redox concentrations 

are red or yellow. (Photo courtesy of Dr. David 

Weindorf)

Return to Aquic Conditions

Return to Redoximorphic Features
Illustrated Guide to Soil Taxonomy

Part 3—Diagnostic Horizons and Characteristics 3-43

Aquic Moisture Regime

Regime characteristic of poorly and very poorly drained soils

Concept and Background Information

The aquic soil moisture regime occurs in poorly drained and very poorly 

drained soils that are saturated close to the surface when soil temperatures 

are warm enough to allow microbial activity to occur (generally > 5 degrees C) 

and for periods that are long enough to result in the near depletion of 

dissolved oxygen.

Soil Taxonomy does not use the aquic moisture regime as a criterion in any 

taxa. Rather it uses the identification of aquic soil conditions at specific depths 

to define some taxa. However, it is appropriate to identify an aquic soil 

moisture regime in soil descriptions.

See the generalized map of the soil moisture regime regions in the continental 

United States.

Generalized Characteristics 

1) Saturation and reduction of the upper part of the soil occur to the extent 

that the soil is virtually free of dissolved oxygen.

Note: For information regarding the estimation of this criterion, see the 

discussion of soil moisture regimes in Part 1—How to Use This Version of the 

Keys.
Illustrated Guide to Soil Taxonomy

Part 3—Diagnostic Horizons and Characteristics 3-44

The soils adjacent to this open water area are saturated near the surface 

and have an aquic soil moisture regime.

Return to Aquic Moisture Regime
Illustrated Guide to Soil Taxonomy

Part 3—Diagnostic Horizons and Characteristics 3-45

Argillic Horizon

Subsoil horizon with an illuvial accumulation of clay

Concept and Background Information

The argillic horizon forms as a result of the translocation of clay from an 

eluvial layer (generally near the surface) into the subsoil, where it 

accumulates in the argillic horizon. As a result, the argillic horizon has a 

significantly higher content of clay than the layer(s) above. Not all of the 

increase in clay content from the surface layers to the argillic horizon may be

due to the physical movement (illuviation) of clay into the argillic horizon. 

Other processes, such as the chemical dissolution of clay in the surface 

layers, selective erosion of the finer (clay) particles from the surface layers, 

and the chemical formation of clay within the argillic horizon, can also 

contribute to the increase in clay content. Illuviation of clay, however, is 

responsible for at least some of the increase.

Generalized Characteristics 

1) Thickness is > 7.5 cm (> 15 cm if horizon is sandy or composed of 

lamellae).

2) There is evidence of clay illuviation, either:

a. Field evidence (clay films in pores or on peds or clay bridging of 

sand grains), or

b. Laboratory evidence (oriented clay in thin section or significantly 

more fine clay [as a fraction of total clay] in the argillic horizon as 

compared to the eluvial layer above.

3) Clay percent increases significantly within a vertical distance of < 30 cm. 

(Minimum required increase ranges from 3 to 8 percent, depending on the 

clay content of the eluvial layer.)

Common Horizon Nomenclature

Commonly used horizon nomenclature includes master horizon B and suffix t,

alone or in combination with other suffixes, such as g, k, n, ss, v, or x.

Examples include: Bt, Btg, Btk, and Btv.
Illustrated Guide to Soil Taxonomy

Part 3—Diagnostic Horizons and Characteristics 3-46

Clay films (slightly darker red) on surfaces of peds throughout 

an argillic horizon.

Back to Argillic Horizon

Clay bridging of sand grains.
Illustrated Guide to Soil Taxonomy

Part 3—Diagnostic Horizons and Characteristics 3-47

Aridic (Torric) Moisture Regime

Regime characteristic of arid and semiarid climates that are unsuitable 

for cultivation without irrigation

Concept and Background Information

The aridic soil moisture regime is a very dry regime in which not enough water 

moves through the soil to leach minerals from the profile. Soluble salts, if 

present in the parent materials or if introduced by dust or other sources, tend 

to accumulate in some of these soils. Soils that occur in the slightly wetter 

semiarid regions but have an aridic moisture regime have some restriction to 

infiltration of precipitation, such as steep slopes or a surface crust. Not 

included in the aridic soil moisture regime are soils in very cold, very dry polar 

regions or at high elevations that have anhydrous conditions. The terms 

“aridic” and “torric” refer to the same soil moisture regime but are used in 

different categories in Soil Taxonomy. 

See the generalized map of soil moisture regimes in the continental United 

States.

Generalized Characteristics 

1) During the growing season:

a. The soil is dry more than half of the total days. 

b. The soil is moist < ~ 90 consecutive days.

See the discussion of soil moisture regimes in Part 1—How to Use This 

Version of the Keys.
Illustrated Guide to Soil Taxonomy

Part 3—Diagnostic Horizons and Characteristics 3-48

Small-scale farm plots in Saudi Arabia in an area with an aridic soil moisture

regime. Growing crops in such areas requires access to water for 

irrigation.

Return to Aridic Moisture Regime
Illustrated Guide to Soil Taxonomy

Part 3—Diagnostic Horizons and Characteristics 3-49

Calcic Horizon

Subsoil horizon with an illuvial accumulation of calcium carbonate

Concept and Background Information

The calcic horizon is a subsoil horizon with a significant illuvial accumulation 

of calcium carbonate. Evidence for the pedogenic accumulation of calcium 

carbonate (CaCO3) is either the presence of secondary forms, such as 

masses, threads coatings, pendants, or nodules, or a calcium carbonate 

equivalent that is higher than that of the underlying horizon. The carbonates 

typically have moved downward in solution with the percolating waters and 

subsequently reprecipitated and accumulated at the wetting front. In some 

soils with a water table, the calcium carbonate may move upward due to 

capillary rise and accumulate as the water evaporates. The calcic horizon 

generally has no cementation, but the definition includes layers with 

cementation that is not sufficiently developed to meet the criteria for a 

petrocalcic horizon. Calcic horizons are commonly found in arid or semiarid 

environments where limited precipitation only moves soluble salts, including 

calcium carbonate, though the soil as a function of depth of moisture 

penetration.

Generalized Characteristics 

1) Thickness is > 15 cm.

2) Calcium carbonate equivalent is > 15% (or > 5% if the horizon has < 18% 

clay and > 15% sand) and horizon either:

a. Is 5% higher by weight (absolute) than an underlying horizon, or

b. Has > 5%, by volume, visible secondary CaCO3 forms (such as 

masses, threads, coatings, and nodules).

3) Horizon has no (or minimal) cementation (air-dry fragments disintegrate 

when submerged in water).

Common Horizon Nomenclature

Commonly used horizon nomenclature includes master horizon A or B and 

suffix k or kk, alone or in combination with other suffixes, such as n, q, y, or z.

Examples include: Ak, Bk, and Bkknz.
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Part 3—Diagnostic Horizons and Characteristics 3-50

Profile of a soil in New Mexico. A calcic horizon is 

below a depth of about 75 cm. The white color 

is due to calcium carbonate concentrations.

Scale is in decimeters (left) and feet (right).

Back to Calcic Horizon
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Part 3—Diagnostic Horizons and Characteristics 3-51

Cambic Horizon

Subsoil horizon with minimal development

Concept and Background Information

The cambic horizon is a subsoil horizon with minimal development.

Development is evidenced either by physical alteration, chemical 

transformation, removal of soil constituents, or a combination of these. 

Physical alteration is commonly the result of processes like freezing and 

thawing, shrinking and swelling, and biological activity to the extent that most 

rock structure is obliterated. In addition, the alteration commonly includes the 

formation of soil structural units (peds). Chemical transformation includes 

processes such as the reduction of iron, manganese, and other elements 

under anaerobic conditions in wet soils, the weathering of primary minerals to 

form clays and/or sesquioxides, and/or the dissolution and redistribution of 

salts (such as calcium carbonate and gypsum) into secondary forms in the 

horizon. Removal of constituents originally present in the parent material 

includes the leaching of materials such as calcium carbonate and gypsum

from the layer.

The common feature of all cambic horizons is the loss of at least half of their 

original rock structure. Most cambic horizons have some degree of soil 

structure but, if they formed in contrasting environments, can look quite 

different from each other. For example, in well drained, humid climates, the 

cambic horizon commonly occurs as a bright-colored and/or slightly reddened 

layer (due to the accumulation of sesquioxide coatings). Under saturated 

conditions, the cambic horizon has red and gray iron accumulations and 

depletions (redoximorphic features) as a common characteristic. In semiarid 

and arid environments, many cambic horizons have redistribution and/or loss 

of soluble salts.

Generalized Characteristics 

1) Thickness is > 15 cm.

2) Texture is loamy or clayey (i.e., very fine sand, loamy very fine sand, or 

finer).

3) Horizon has evidence of alteration (pedogenesis) as follows:

a. Soil structure (or at least loss of rock structure in > 50% of the 

volume), and

b. In wet soils (those periodically saturated within 50 cm of the soil 

surface):

i. Gray colors and no color change on exposure to air, and

1. Value < 3 and chroma = 0, or

2. Value > 4 and chroma < 1, or
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3. Any value and chroma < 2, plus redox concentrations 

(reddish mottles).

c. In “non-wet” soils (those not saturated within 50 cm of the soil 

surface):

i. When compared to the horizon above or below:

1. Higher value or chroma, or

2. Redder hue, or

3. Higher clay content, or

ii. Carbonates or gypsum have been removed.

4) Horizon is not significantly brittle (i.e., it deforms gradually when 

compressed). 

Note: The cambic horizon cannot also meet the requirements for an anthropic, 

histic, folistic, melanic, mollic, plaggen, or umbric epipedon; for a duripan or

fragipan; or for an argillic, calcic, gypsic, natric, oxic, petrocalcic, petrogypsic, 

placic, salic, spodic, or sulfuric subsoil horizon.

Common Horizon Nomenclature

Commonly used horizon nomenclature includes master horizon B and suffix g, 

ss, or w. However, many other suffixes can be applied (such as h, k, n, o, q, s, 

ss, t, or x) so long as the properties noted do not meet the criteria for any 

other diagnostic horizon. Examples include: Bw, Bg, and Bk.
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Part 3—Diagnostic Horizons and Characteristics 3-53

In this soil, the cambic horizon has subangular blocky structure and

redder hue and higher chroma than the overlying ochric epipedon. It 

also has a lower carbonate content than the horizons below (not 

visible in photo). The soil is a Haplocambid in the United Arab 

Emirates. (Photo courtesy of Dr. Craig Ditzler)

Back to Cambic Horizon
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Part 3—Diagnostic Horizons and Characteristics 3-54

Coefficient of Linear Extensibility (COLE)

A measure describing the proportional change in linear dimension of a 

soil clod due to shrinking and swelling

Concept and Background Information

The coefficient of linear extensibility (COLE) is a measure describing the 

proportional change in linear dimension of a soil clod due to shrinking and 

swelling. Higher values indicate greater potential for soil movement due to

shrinking and swelling. COLE is used in the criteria for argillic and natric 

horizons to identify conditions with high shrink-swell potential (COLE > 0.4) 

where the visual evidence of clay illuviation (such as oriented clay as clay 

films on faces of peds and in pores) is likely to be destroyed due to soil 

movement. 

COLE is calculated as:

COLE = (length(moist) – length(dry)) / length(dry)

In the laboratory, COLE is derived by simply taking the difference between the 

bulk density measurements for moist and oven-dry states. It can be estimated 

in the field by placing two pins in a clod at field capacity and observing the 

change in distance between the pins at the air-dry state.

Note: A similar property (linear extensibility, or LE) is used at the subgroup 

level of Soil Taxonomy.
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Part 3—Diagnostic Horizons and Characteristics 3-55

Coefficient of linear extensibility is a quantitative measure describing a soil’s 

potential for shrinking and swelling. In extreme cases, large cracks can 

form when the soil dries, as in this photo.

Return to COLE
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Part 3—Diagnostic Horizons and Characteristics 3-56

Cryoturbation

Intense frost churning

Background Information

Cryoturbation is the mixing of the soil as a result of intense frost churning due 

to freeze-thaw cycles. (See Gelic Materials)

Generalized Characteristics 

1) Evidence of cryoturbation includes:

a. Irregular or broken horizons.

b. Involutions.

c. Organic matter accumulation on the permafrost table.

d. Orientated rock fragments.

e. Silt caps on rock fragments.

Common Horizon Nomenclature

Cryoturbation is specifically indicated by use of the suffix jj. It can be 

associated with any of the master horizons O, A, E, B, C, or L, which may be 

combined to reflect the mixing (such as O/A and B/C). Examples include: 

O/Ajj, Bjj, and Bjjg.
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Part 3—Diagnostic Horizons and Characteristics 3-57

Cryoturbated organic materials in the subsoil of a low-centered polygon 

rim in northern Alaska. (Photo courtesy of Dr. David Weindorf)

Back to Cryoturbation
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Densic Contact

The contact between unconsolidated soil material and the underlying 

root-restrictive, dense, compact materials

Concept and Background Information

The densic contact is the boundary between unconsolidated soil material and 

the underlying root-restrictive, noncemented, dense and compact layer. Since 

it is not cemented, an air-dry fragment will slake in water. See Densic 

Materials.

Generalized Characteristics

1) The contact is to a dense, compact, noncemented layer.

2) Spaces where roots can penetrate are more than 10 cm apart

Common Horizon Nomenclature

A densic contact is commonly identified at the upper boundary of some

horizons (A, E, B, or C) by the suffix d. Examples include: Bd, Cd, and ^Cdu. 
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Densic Materials

Root-restrictive, noncemented, dense, compact material

Concept and Background Information

Densic materials are root-restrictive, noncemented, dense, and compact. They 

do not meet the definition of any diagnostic horizon (e.g., a fragipan). Densic 

materials are not cemented, so an air-dry fragment will disintegrate in water. 

Spaces where roots can penetrate are more than 10 cm apart. Densic 

materials occur naturally in materials such as glacial till and volcanic 

mudflows. However, they can also result from human activities that cause 

significant compaction of unconsolidated soil material (natural or humantransported).

Generalized Characteristics 

1) Materials do not meet criteria for any diagnostic horizon.

2) Spaces where roots can penetrate are more than 10 cm apart.

3) Materials are noncemented.

Common Horizon Nomenclature

Densic materials are commonly identified in some horizons (A, E, B, and C) by 

the suffix d. Examples include: Bd, Cd, and ^Cdu. 
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Part 3—Diagnostic Horizons and Characteristics 3-60

Profile of a soil in Rhode Island that has a densic contact at a depth of 

about 70 cm. The densic materials (gray) below this depth consist of 

root-restrictive, dense, compacted lodgment glacial till. Scale is in cm.

Return to Densic Contact

Return to Densic Materials
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Part 3—Diagnostic Horizons and Characteristics 3-61

Duripan

Subsoil layer that is cemented by silica

Concept and Background Information

The duripan is a root-restrictive subsoil layer that is primarily cemented by 

illuvial silica. Other cementing agents, such as calcium carbonate, may also 

be present. Duripans occur in areas where silica may be illuviated into the 

subsoil but is not removed entirely from the profile. These conditions may 

occur in arid or semi-arid regions with seasonal wetness. Duripans are 

commonly associated with volcanic materials, such as ash deposits, which 

supply ample silica as they weather. They are also associated with other 

silica-supplying, non-volcanic parent materials, especially those with 

ferromagnesian minerals and feldspars.

Generalized Characteristics 

1) More than 50% of the layer is cemented with illuvial silica.

2) The slake test (air dry in 1N HCl) results in < 50% dissolution of cemented 

fragments.

Note: There is no minimum thickness requirement for the duripan. The slake 

test is used to confirm that silica is the main cementing agent. Calcium 

carbonate cement will be dissolved by 1N HCl. To dissolve silica cement,

concentrated KOH or NaOH is required.

Common Horizon Nomenclature

Commonly used horizon nomenclature includes master horizon B and suffixes

q and m used together, with or without other suffixes. Examples include: Bqm 

and Bkqm.
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Part 3—Diagnostic Horizons and Characteristics 3-62

A duripan in a Haplodurid near Culberson County, Texas. Rock fragments in 

photo are medium to coarse gravel in a silica-cemented matrix. (Photo 

courtesy of Dr. David Weindorf)

Back to Duripan
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Part 3—Diagnostic Horizons and Characteristics 3-63

Fragipan

A root-restrictive subsoil layer that is firm and brittle, but not cemented

Concept and Background Information

The fragipan is a root- and water-restrictive subsoil layer that is firm and brittle 

but not cemented. Air-dry fragments mostly slake in water, thus confirming the 

absence of a cementing agent. Fragments are firm or harder when dry. When 

moist, they have a brittle manner of failure when increasing pressure is 

applied (they rupture suddenly rather than deform gradually). The fragipan is 

commonly located below an argillic, cambic, or spodic horizon. It commonly 

has sufficient illuvial clay to also be an argillic horizon. Many fragipans have 

vertical ped surfaces coated with light-colored eluvial material that form a 

polygonal pattern when viewed in cross-section on a horizontal plane. The 

streaks commonly surround brittle, browner material that has redoximorphic 

features in the form of iron-manganese accumulations. Most fragipans restrict 

water movement, and water perches above them. Fragipans commonly form 

in transported parent materials. They are generally loamy in texture and

commonly have a lithologic discontinuity.

Generalized Characteristics

1) Thickness is > 15 cm.

2) Layer has evidence of pedogenesis (e.g., it is not simply mechanically 

compacted).

3) Layer has structure that does not allow roots to penetrate at spaces less 

than 10 cm apart, or it is massive.

4) Layer is not cemented (air-dry fragments mostly disintegrate when 

submerged in water).

5) In > 60% of volume, peds are firm or hard and brittle when moist.

6) Layer is noneffervescent in dilute HCl.

Note: Evidence of pedogenesis includes features such as oriented clay, albic 

materials, structure, and redoximorphic features.

Common Horizon Nomenclature

Commonly used horizon nomenclature includes master horizon B, and less 

commonly E, combined with suffix x. Additional suffixes, such as g and t, may

also be used. A number prefix denoting a lithologic discontinuity is often used 

for fragipans. Examples include: Ex, 2Btx, and Bxg.
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Profile a Fragiudalf (in Tennessee) that has a fragipan below a depth of 

about 60 cm. The gray soil material consists of eluvial coatings 

surrounding the browner soil material of the prism interiors. Scale is 

in feet (left) and centimeters (right). 

Back to Fragipan
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Part 3—Diagnostic Horizons and Characteristics 3-65

Horizontal cross-section through a fragipan with prismatic structure. View is

from above. Soil is a Fragiudalf in Tennessee. Scale is in feet.

Back to Fragipan
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Part 3—Diagnostic Horizons and Characteristics 3-66

Fibric Soil Materials

Organic soil materials that are only slightly decomposed

Concept and Background Information

Fibric soil materials are organic soil materials that are only slightly 

decomposed. They have high amounts of fibers that can be identified as to 

their biological origin. Three quarters or more of intact fibers remain after 

rubbing. Fibric materials tend to be found in cool, moist environments with 

rates of decomposition relatively slow compared to the rate of organic matter 

accumulation. 

A simple field test involves rubbing the material between the fingers (about ten 

times) to observe the amount of intact fiber that persists after rubbing. Fibric 

soil materials generally have low bulk density (<~0.1 g/cm3) and a high water 

content when saturated (commonly ~850 - > 3,000%, by weight, compared to 

oven dry). Fibric soil materials are commonly light yellowish brown, dark 

brown, or reddish brown.

Note: For comparison, see the more decomposed hemic and sapric soil 

materials.

Generalized Characteristics 

Materials contain:

1) > 3/4 (by volume) fibers after rubbing or

2) > 2/5 fibers and yield a light gray to very pale brown extract of sodium 

pyrophosphate solution. 

Note: Fibers are > 0.15 mm in cross-section, show evidence of cellular 

structure, and are < 20 mm in the smallest dimension (or can easily be 

crushed to this size).

Common Horizon Nomenclature

Commonly used horizon nomenclature includes master horizon O with suffix I 

(i.e., Oi).
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Part 3—Diagnostic Horizons and Characteristics 3-67

Organic soil materials, including fibric (top), hemic 

(middle), and sapric (lower) materials. (Photo 

courtesy of John Kelley)

Return to Fibric Soil Materials
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Part 3—Diagnostic Horizons and Characteristics 3-68

Gelic Materials

Soil materials (mineral or organic) above permafrost that show evidence 

of frost churning

Concept and Background Information 

Gelic materials are soil materials (mineral or organic) that show evidence of 

frost churning (see Cryoturbation). Gelic materials formed as a result of 

cryopedogenic processes occurring in the periodically frozen soil layers above 

the permafrost. These processes include volume changes associated with 

water/ice transformations, movement of moisture across thermal gradients in 

the frozen soil, and thermal contraction during continued rapid cooling

episodes.

Generalized Characteristics 

Gelic materials have:

1) Macro structural evidence that includes:

a. Platy, blocky, or granular peds.

b. Particle sorting, as in patterned ground.

2) Micro structural evidence that includes:

a. Orbiculic, conglomeric, banded, or vesicular soil fabric types.

3) Ice segregations in the form of:

a. Ice lenses, vein ice, segregated ice crystals, or ice wedges.

Common Horizon Nomenclature

Gelic materials are commonly indicated by use of suffix jj in horizons above a 

permafrost layer. They can be associated with any of the master horizons O, 

A, E, B, C or L, and these may be combined to reflect the mixing (O/A, B/C, 

etc.). Examples include: O/Ajj, Bjj, and Bjjg.
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Part 3—Diagnostic Horizons and Characteristics 3-69

A layer with gelic materials from a soil in Alaska. The presence of ice crystal 

segregations and ice lenses is evidence of gelic materials. (Photo courtesy 

of Dr. David Weindorf)

Return to Gelic Materials
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Part 3—Diagnostic Horizons and Characteristics 3-70

Glacic Layer

Layer of ice in the soil

Concept and Background Information

A glacic layer is a layer composed primarily of ice in the soil.

Generalized Characteristics 

1) Thickness is > 30 cm.

2) Layer contains > 75% visible ice.

Common Horizon Nomenclature

Commonly used horizon nomenclature includes master horizon W and suffix f

(i.e, Wf).

An ice wedge forms a glacic layer in a frozen soil in Alaska. (Photo 

courtesy of Dr. David Weindorf)
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Glossic Horizon

A degrading argillic, kandic, or natric horizon in which loss of clay and 

iron oxide is occurring

Concept and Background Information

The glossic horizon is a degrading argillic, kandic, or natric horizon. In this 

horizon, eluviation of clay and iron oxides is occurring. This results in the 

formation of albic materials to the extent that they make up 15 to 85 percent of 

the volume of the horizon. The albic materials appear as light-colored tongues 

extending into the horizon between ped surfaces. As the degradation process 

proceeds, the tongues occupy progressively more of the horizon, until only 

remnants of the former argillic, kandic, or natric horizon remain.

Generalized Characteristics 

1. Horizon has thickness of > 5 cm.

2. Horizon has two distinct parts (eluvial and illuvial), each making up 15-85% 

of the horizon:

a. Illuvial part consists of remnants of argillic, kandic, or natric horizon 

(Bt).

b. Eluvial part consists of tongues of albic materials (E).

Common Horizon Nomenclature

Commonly used horizon nomenclature includes master horizons B and E 

used in combination. Additional suffixes, such as g, n, t, or x, may also be 

used. Examples include: Bt/E, E/B, and Btn/E.
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Part 3—Diagnostic Horizons and Characteristics 3-72

This glossic horizon is a degrading argillic horizon. Albic material 

(white) surrounds remnant argillic horizon peds (brown).

Scale is in inches.

Back to Glossic Horizon
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Part 3—Diagnostic Horizons and Characteristics 3-73

Gypsic Horizon

Surface or subsoil horizon with an accumulation of gypsum

Concept and Background Information

The gypsic horizon is a horizon with an accumulation of gypsum. It typically is 

in the subsoil but may be at the surface in some soils. Gypsic horizons form in 

arid or semiarid environments and are typically associated with gypsiferous 

parent materials. The secondary gypsum in the horizon commonly results 

from a combination of illuviation of gypsum into the horizon as well as 

dissolution and precipitation locally within the horizon. The gypsum typically 

has moved downward in solution with rainwater and then accumulated at the 

wetting front. In some soils with a water table, the gypsum may move upward 

due to capillary rise and accumulate at the surface as the water evaporates. 

The gypsic horizon generally has no cementation, but the definition includes 

layers with cementation that is not sufficiently developed to meet the criteria 

for a petrogypsic horizon. Gypsic horizons are commonly found in arid or 

semiarid environments where deep leaching and removal of soluble salts (like 

gypsum) does not occur. The limited precipitation translocates soluble salts to 

only the depth of moisture penetration, which is typically still within the solum.

Generalized Characteristics 

1. Thickness is > 15 cm.

2. Horizon has > 1% visible secondary gypsum and > 5% gypsum (by 

weight), and

a. (Gypsum % wt.) X (thickness in cm) > 150. (For example, if gypsum 

is 6% and thickness is 20 cm: 6% gypsum X 20 cm = 120; example 

fails gypsic).

3. Horizon has as no, or minimal, cementation (air-dry fragments disintegrate 

when submerged in water).

Note: Secondary gypsum may be in forms such as masses or clusters of 

crystals, pendants beneath rock fragments, or uniformly distributed fine 

crystals throughout the soil matrix.

Common Horizon Nomenclature

Commonly used horizon nomenclature includes master horizon B, and 

sometimes A, combined with suffix y or yy. Additional suffixes, such as k and

z, may also be used. Examples include: Ayy, Bky, and By.
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Part 3—Diagnostic Horizons and Characteristics 3-74

A gypsic horizon with nests of gypsum crystals. (Photo courtesy of Dr. David 

Weindorf)

Back to Gypsic Horizon
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Part 3—Diagnostic Horizons and Characteristics 3-75

Hemic Soil Materials

Organic soil materials that are moderately decomposed

Concept and Background Information

Hemic soil materials are organic soil materials that are moderately 

decomposed. They are intermediate between the highly decomposed sapric 

soil materials and the slightly decomposed fibric soil materials. Before rubbing, 

hemic soil materials have moderate amounts of fibers that can be identified as 

to their biological origin. The organic material has undergone some physical 

and chemical alteration, but about 1/3 to 2/3 of the volume is fibers before 

rubbing. After rubbing however, few intact fibers remain. 

A simple field test involves rubbing the material between the fingers (about ten 

times) to observe the amount of intact fiber that persists after rubbing. Hemic 

soil materials generally have bulk density of <~0.07-0.18 g/cm3and an 

intermediate water content when saturated (commonly ~450-850%, by weight, 

compared to oven dry). They are commonly dark grayish brown to dark 

reddish brown.

Note: For comparison, see the more decomposed sapric and less 

decomposed fibric soil materials.

Generalized Characteristics 

1) Hemic soil materials are intermediate in decomposition between fibric and 

sapric soil materials. 

Common Horizon Nomenclature

Commonly used horizon nomenclature includes master horizon O with suffix e

(i.e., Oe).
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Organic soil materials, including fibric (top), hemic 

(middle), and sapric (lower) materials. (Photo 

courtesy of John Kelley)

Return to Hemic Soil Materials
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Human-Altered Material

Soil material significantly altered by intentional human activity

Concept and Background Information

Human-altered soil materials have undergone some form of intentional 

human-induced alteration, including deep plowing to break up root-restrictive 

subsoil layers, excavation (such as for a gravel pit), and surface compaction in 

order to puddle water (such as for rice paddies). Although highly altered from 

their natural state, they have not been transported from another location.

Generalized Characteristics 

1) Materials are either:

a. Tilled to > 50 cm to break up a root-restrictive subsoil layer, or

b. Occur on a destructional anthropogenic landform (such as a gravel 

pit), or

c. Are purposely compacted to puddle water for agricultural purposes,

OR

2) Materials have not been transported and contain evidence of purposeful 

human alteration.

Note: See 12th edition of the “Keys to Soil Taxonomy” for soil properties to 

consider as evidence of human alteration.

Common Horizon Nomenclature

There is no specific horizon nomenclature to indicate human-altered material. 

Some human-altered materials may be indicated with the suffix p or d. 

Examples include: Ap and Bdg. (Note that the caret symbol prefix [^] is used 

for human-transported material).
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These human-constructed terraces in Nepal are used for growing rice and

vegetables. They make up a large-scale anthropogenic landform with both 

human-altered and human-transported materials. (Photo courtesy of Dr. 

John Galbraith)

Back to Human-Altered Material
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Part 3—Diagnostic Horizons and Characteristics 3-79

Human-Transported Material

Soil material transported by intentional human activity

Concept and Background Information

Human-transported soil materials are essentially a unique kind of parent 

material. These materials have been intentionally moved by human activity 

from a location away from the site where they are now located. Humantransported material is commonly underlain at some depth by a lithologic 

discontinuity and/or a buried soil horizon from the original soil that formed at 

the site before deposition of the new material.

Generalized Characteristics 

1) Material occurs on a constructional landform (such as a filled-in area).

2) Material contains evidence of purposeful transport by humans from an 

area outside of the current pedon.

Note: See 12th edition of the “Keys to Soil Taxonomy” for soil properties to 

consider as evidence of human transport.

Common Horizon Nomenclature

Commonly used horizon nomenclature includes the use of the caret symbol 

(^) for a prefix in combination with master horizons, such as O, A, B, or C.

These may be combined with suffix d, j, u, or w. Examples include: ^Ap, ^Cj, 

and ^Cdu. Soil material above a layer designated with master symbol M is 

human-transported.
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Profile of a Virginia soil that formed in humantransported material. The upper 52 cm was 

deposited by truck from nearby sources of fill 

material and contains scattered broken peds 

of diagnostic horizons that formed at their 

original locations. The dark layer at a depth of 

60 cm has a high content of wood and coal 

ash. The lower part of the profile consists of

deposits of dredged marine sediments. Scale 

is in cm. (Photo courtesy of Dr. John Galbraith)
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Part 3—Diagnostic Horizons and Characteristics 3-81

Identifiable Secondary Carbonates

Visible calcium carbonate that has been precipitated in the soil

Concept and Background Information

Identifiable secondary carbonates are forms of calcium carbonate in the soil 

that are the result of pedogenic processes. The source of the carbonates may 

have been in the original parent material, or it may have been introduced from 

outside the soil profile through dust-fall or the lateral movement of water in the 

subsoil. Regardless of the source, however, the carbonates must have been 

translocated and precipitated within the soil to qualify as identifiable 

secondary carbonates. In addition, the feature must be thick enough to be 

visible even when moist, generally about 1 mm or more in size. Forms include 

masses, nodules, concretions, coatings on peds or in pores, filaments in the 

soil matrix, and pendants on the undersides of rock fragments. Some horizons 

are completely engulfed in calcium carbonates, and their light color is 

determined largely by the calcium carbonates. These are within the concept of 

identifiable secondary carbonates.

Generalized Characteristics

1) Carbonates are pedogenic in nature.

2) Carbonates are thick enough to be visible when moist.

Common Horizon Nomenclature

Commonly used horizon nomenclature includes master horizon B, or 

sometimes A or C, and the use of suffix k or kk alone or in combination with 

other suffixes, such as n, y, or z. Examples include: Ak, Bkny, and Bkk. In 

addition, the description of the horizon includes information about the kind and 

amount of carbonates present.
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Part 3—Diagnostic Horizons and Characteristics 3-82

A pendant of secondary calcium carbonate coating

the underside of a rock fragment.

Back to Identifiable Secondary Carbonates
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Part 3—Diagnostic Horizons and Characteristics 3-83

Profile of a Calciustoll with identifiable secondary carbonates 

(white bodies) below a depth of about 28 inches. Scale is 

in feet.

Back to Identifiable Secondary Carbonates
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Part 3—Diagnostic Horizons and Characteristics 3-84

Interfingering of Albic Materials

Narrow penetrations of light-colored, leached material into a subsoil 

horizon

Concept and Background Information

Interfingering of albic materials is a soil feature consisting of thin penetrations 

of albic materials (light-colored, leached soil material) extending 5 cm or more 

into a clay-enriched horizon. The penetrations are along vertical, and to a 

lesser degree horizontal, ped surfaces. There may or may not be a continuous 

albic horizon above. The albic materials make up less than 15% of the 

horizon. They are mostly light gray when moist and nearly white when dry. 

The color is controlled by the primary sand and silt particles making up the 

feature.

Generalized Characteristics

1) Feature consists of light-colored, leached material (albic materials).

2) Albic materials extend > 5 cm into a clay-enriched subsoil layer.

3) Penetrations are > 2 mm thick.

4) Albic materials constitute < 15% of the volume of the horizon.

Common Horizon Nomenclature

Commonly used horizon nomenclature includes master horizons B and E 

used in combination. In some cases, where the amount of albic materials is 

quite low, the horizon may only be designated as a B and the nature of the 

albic material penetrations described in the horizon description. Suffixes such 

as g, n, t, or x may also be used. Examples include: B/E, Btn, and Btx.
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Part 3—Diagnostic Horizons and Characteristics 3-85

Interfingering of albic materials (white) into an argillic horizon

(brown).

Back to Interfingering of Albic Materials
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Part 3—Diagnostic Horizons and Characteristics 3-86

Kandic Horizon

Subsoil horizon with a low nutrient-holding capacity and significantly 

more clay than the overlying surface layer

Concept and Background Information

The kandic horizon is a subsoil horizon that has a significantly higher content 

of clay than the horizons above and has a low nutrient-holding capacity. The 

difference in clay content may be the result of eluviation/illuviation processes, 

but this is not a requirement for the kandic horizon (as it is for the argillic and

natric horizons). Other processes leading to the increase in clay content from 

the surface layer to the kandic horizon include the chemical dissolution of clay 

in the surface layers, the selective erosion of the finer (clay) particles from the 

surface layers, and the chemical formation of clay within the kandic horizon. 

The clay minerals making up the kandic horizon are generally referred to as 

low-activity clays due to their low nutrient-holding capacity (or low cationexchange capacity). Soils with kandic horizons tend to have low native fertility. 

The presence of a kandic horizon generally suggests that the soil has 

undergone a high degree of weathering in a warm, humid climate.

Generalized Characteristics 

1) Horizon underlies a coarser textured surface layer > 18 cm thick.

2) Horizon is > 30 cm thick.

3) Clay percent increases significantly within a vertical distance of < 15 cm.

(Minimum required increase ranges from 4 to 8 percent, depending on the 

clay content of the overlying layer.)

4) Horizon begins generally at a depth of < 125 cm. (See 12th edition of the

“Keys to Soil Taxonomy” for exceptions.)

5) Texture is loamy or clayey.

6) CEC (cation-exchange capacity) is < 16 and ECEC (effective cationexchange capacity) is < 12 (cmol+

per kg clay).

7) Horizon is not alluvial in nature (there is no fine stratification and organic

carbon content decreases regularly with increasing depth).

Note: CEC or ECEC sometimes is reported for the fine-earth fraction rather 

than just the clay fraction. To convert to clay basis: 

CEC (fine earth) X 100 / % clay. 

For example, 12 (cec fine earth) X 100 / 30% clay = CEC 40 cmol+kg clay.
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Part 3—Diagnostic Horizons and Characteristics 3-87

Common Horizon Nomenclature

There is no horizon nomenclature to designate the presence of the low-activity 

clays that characterize the kandic horizon. Commonly used horizon 

nomenclature includes master horizon B, often in combination with suffixes

such as g, t, or v. Examples include: Bt, Btv, and Btg.
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Part 3—Diagnostic Horizons and Characteristics 3-88

Profile of a soil in Zambia that has a kandic horizon below 

an ochric epipedon. Scale colors are in decimeters. 

Back to Kandic Horizon
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Part 3—Diagnostic Horizons and Characteristics 3-89

Lamellae

Two or more thin layers with accumulation of illuvial clay

Concept and Background Information

Lamellae consist of two or more thin layers with accumulation of illuvial clay. 

Each individual lamella is associated with an overlying eluvial layer containing 

less clay than the lamella below it. Each lamella is < 7.5 cm thick and 

commonly just a few millimeters thick. Lamellae form in unconsolidated 

materials at least 50 cm thick. Lamellae that collectively total > 15 cm qualify 

for either a cambic horizon (if they are very fine sand, loamy very fine sand, or 

finer) or, if they are at least 0.5 cm thick and have a sufficiently large increase 

in clay content, an argillic horizon.

Generalized Characteristics

1) Lamellae contain illuvial clay.

2) Thickness of individual lamella is < 7.5 cm.

3) Lamellae occur in vertical series of two or more.

4) Each lamella has an eluvial layer above it.

Common Horizon Nomenclature

Commonly used horizon nomenclature includes the combination of master 

horizons E and B and the word “and.” Examples include: “E and Bt” and “E 

and B.”
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Part 3—Diagnostic Horizons and Characteristics 3-90

Profile of a soil that has an argillic horizon composed of lamellae. (Photo 

courtesy of John Kelley)

Back to Lamellae
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Part 3—Diagnostic Horizons and Characteristics 3-91

Lithic Contact

Contact between unconsolidated soil material and the underlying rootrestrictive hard bedrock

Concept and Background Information

The lithic contact is the boundary between unconsolidated soil material and 

the underlying root-restrictive hard bedrock. Any cracks that allow penetration 

by roots are spaced more than 10 cm apart. The underlying bedrock is hard 

enough to make hand digging impractical, if not impossible (the cementation 

class is strongly cemented to indurated).

Generalized Characteristics

1) The contact is to underlying hard bedrock.

2) Cementation class is strongly cemented to indurated.

3) Spaces where roots can penetrate are more than 10 cm apart.

4) Underlying material is not a diagnostic horizon (such as a duripan or a 

petrocalcic horizon).

Common Horizon Nomenclature

A lithic contact is identified at the boundary between soil and continuous hard 

bedrock, such as the upper boundary of an R horizon. Note: A lithic contact is 

NOT recognized at the upper boundary of a pedogenically cemented 

diagnostic horizon, such as a petrocalcic horizon or a duripan.
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Soil profile with a lithic contact over hard limestone. Note that the upper 

part of the profile consists of soil and loose rock fragments. The lithic 

contact is at the top of the continuous bedrock layer, which comprises

about the lower third of the photo.

Return to Lithic Contact
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Part 3—Diagnostic Horizons and Characteristics 3-93

n Value

A value that describes the ability of a saturated soil to support a load

Concept and Background Information

The n value describes the ability of a saturated soil to support a load, such as 

grazing cattle or small machinery. It considers the percentage of water held 

under field conditions as well as percentages of silt, sand, clay, and organic 

matter in the soil (see the 12th edition of the “Keys to Soil Taxonomy” for the 

formula used). For soil classification purposes, a critical n value of 0.7 is used. 

A simple field test involves squeezing a fist-full of soil. If the soil flows between 

the fingers but with difficulty (i.e., slightly fluid), the n value is likely between 

0.7 and 1.0. If the soil flows easily (i.e., moderately fluid or very fluid), it is 

greater than 1.0. If no soil flows between the fingers (non-fluid), it is less than 

0.7.

Common Horizon Nomenclature

There is no horizon nomenclature to infer the n value. Where appropriate, the 

fluidity class is mentioned in the description of the horizon and commonly 

used horizon nomenclature includes master horizon C, often in combination 

with suffix g or se, such as Cg and Cseg.
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Part 3—Diagnostic Horizons and Characteristics 3-94

This moderately fluid saturated soil flows fairly easily between the fingers

when squeezed, indicating it has an n value greater than 1.0.

Return to n Value
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Part 3—Diagnostic Horizons and Characteristics 3-95

Natric Horizon

Subsoil horizon with an illuvial accumulation of clay along and high 

levels of sodium

Concept and Background Information

The natric horizon is a subsoil horizon that, in addition to having all the 

characteristics of an argillic horizon (i.e., significant clay accumulation), has 

high levels of sodium. In effect, it is a special kind of argillic horizon. The high 

amount of sodium causes clay to disperse, thus enhancing the movement of 

clay into the subsoil. Natric horizons commonly have columnar or prismatic 

structure, and they may also have light-colored uncoated sand or silt grains on 

ped surfaces, especially on the upper surface of the columns. Soils with a 

high amount of sodium (and no other salts) tend to have rather high pH (> 9.0

in some cases). They may also have poor physical structure, which impedes 

the movement of air and water through the soil.

Generalized Characteristics 

(Note: Items 1, 2, and 3 are shared with the argillic horizon.)

1) Thickness is > 7.5 cm (> 15 cm if horizon is sandy or composed of 

lamellae).

2) There is evidence of clay illuviation, either

a. Field evidence (clay films in pores or on peds or clay bridging of 

sand grains), or

b. Laboratory evidence (oriented clay in thin section or significantly 

more fine clay [as fraction of total clay] in the Btn horizon as 

compared to the eluvial layer above).

3) Clay percent increases significantly within a vertical distance of < 30 cm. 

(Minimum required increase ranges from 3 to 8 percent, depending on the 

clay content of the eluvial layer.)

4) Structure is columnar or prismatic, or it is blocky and there are also lightcolored uncoated sand or silt grains extending > 2.5 cm into the horizon.

5) Within 40 cm of the top of the natric horizon, horizon has either:

a. And exchangeable sodium percentage (ESP) > 15% or a sodium 

adsorption ratio (SAR) > 13, or

b. If ESP ≥ 15 or SAR is ≥ 13, anywhere within 200 cm of the soil 

surface, more exchangeable Mg + Na than Ca + extractable acidity.
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Part 3—Diagnostic Horizons and Characteristics 3-96

Common Horizon Nomenclature

Commonly used horizon nomenclature includes master horizon B, along with 

combined suffixes t and n. (These may be in combination with other suffixes

such as g, ss, y, or z). Examples include: Btn, Btny, and Btnz.
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Part 3—Diagnostic Horizons and Characteristics 3-97

This natric horizon (lower half of profile) has characteristic 

columnar structure and light-colored silt coatings on the 

top surface of the peds.

Back to Natric Horizon
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Part 3—Diagnostic Horizons and Characteristics 3-98

Oxic Horizon

Subsoil horizon that is extremely weathered and has a very low nutrientholding capacity

Concept and Background Information

The oxic horizon has undergone extreme weathering. As a result, it has few 

primary weatherable minerals remaining. It is dominated by quartz and other 

highly resistant minerals in the sand and silt fraction. The clay fraction is 

dominated by clay minerals such as kaolinite and sesquioxides (iron- and

aluminum-oxides) that have a very low nutrient-holding ability (i.e., low activity 

clays). The combination of a low ability to hold existing nutrients and little 

capacity to supply new nutrients through weathering of primary minerals 

results in very low native fertility. Biocycling of nutrients between the soil, 

microbes, and plant tissues is critical to sustaining adequate fertility and 

growth of native vegetation. Cultivated areas require substantial and 

continued application of fertilizers, especially phosphorous because it tends to 

become “fixed” in the soil and unavailable to plant roots. Oxic horizons 

commonly have granular or fine subangular blocky structure, which causes

them to be friable and very porous.

Generalized Characteristics

1) Thickness is > 30 cm.

2) Texture is loamy or clayey.

3) Horizon has < 10% weatherable minerals in the fine and very fine sand 

fraction (i.e., 0.05 - 0.2 mm).

4) Horizon has < 5% rock structure.

5) Clay percent increases little between the surface layer and the oxic

horizon. (Maximum permitted increase within 15 cm ranges from 3 to 7 

percent, depending on the clay content of the overlying layer).

6) CEC (cation-exchange capacity) is < 16 and ECEC (effective cationexchange capacity) is < 12 (cmol+

per kg clay).

Note: CEC/ECEC may be reported for the fine-earth fraction rather than just 

clay. To convert to clay basis: CEC (fine earth) X 100 / % clay. For example,

12 (cec fine earth) X 100 / 30% clay = CEC 40 cmol+kg clay.

Common Horizon Nomenclature

Commonly used horizon nomenclature includes master horizon B and suffix o, 

which are combined with suffix g, t, or, v in some cases. Examples include: 

Bo, Bto, and Bov.
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Part 3—Diagnostic Horizons and Characteristics 3-99

Profile of a soil in Brazil that has a highly weathered clayey oxic horizon 

with a fine grade of structure below a darkened ochric epipedon.

Back to Oxic Horizon
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Paralithic Contact

Contact between unconsolidated soil material and the underlying rootrestrictive soft bedrock

Concept and Background Information

The paralithic contact is the boundary between unconsolidated soil material 

and the underlying root-restrictive soft bedrock (paralithic materials). Cracks 

that allow penetration by roots are spaced more than 10 cm apart. The 

underlying bedrock is soft enough to make hand digging possible, although it 

may be difficult (the cementation class is extremely weak to moderate).

Because the material is cemented, air-dry fragments do not slake in water. 

The material underlying the paralithic contact commonly is weathered bedrock 

or bedrock that may never have been strongly consolidated.

Generalized Characteristics

1) The contact is to underlying soft bedrock.

2) The cementation class is extremely weak to moderate.

3) Spaces where roots can penetrate are more than 10 cm apart.

4) Underlying material is not a diagnostic horizon (such as a duripan or a 

petrocalcic horizon).

Common Horizon Nomenclature

A paralithic contact is identified at the boundary between soil and continuous 

soft bedrock (indicated by master horizon C and suffix r), i.e., the upper 

boundary of a Cr horizon.
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This soil has an abrupt way boundary at the paralithic contact. The material 

below the contact is relatively soft, weathered bedrock that can be dug 

with a spade. (Photo courtesy of John Kelley)

Return to Paralithic Contact
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Permafrost

Frozen layer within the soil

Concept and Background Information

A frozen layer with a temperature that has remained below 0 oC for 2 or more 

consecutive years is considered permafrost. It may be impregnated with ice,

or it may be dry. Layers above the permafrost freeze and thaw annually.

Generalized Characteristics

1) Layer has temperature of < 0 oC for 2 or more consecutive years.

Common Horizon Nomenclature

Commonly, soil horizons with permafrost are identified through the use of 

either suffix f or ff (for moist or dry permafrost, respectively). Additional 

suffixes commonly used with permafrost layers are a, e, and or g. Master 

horizons are mostly indicated as O or C, but A and B are also used. Examples 

include: Cff, Oaf, and Bfg.
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Part 3—Diagnostic Horizons and Characteristics 3-103

A soil in Alaska that is permanently frozen below a depth of about 60 cm.

Scale is in cm. (Photo courtesy of Dr. David Weindorff)

Return to Permafrost
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Perudic Moisture Regime

Regime characteristic of areas where precipitation exceeds

evapotranspiration every month

Concept and Background Information

The perudic soil moisture regime occurs in areas with very high rainfall. 

Precipitation exceeds evapotranspiration by plants in every month. The 

perudic moisture regime is a special type of udic regime. Areas of soils with a 

perudic regime are generally rather small. Most commonly, they occur at high 

elevations in mountains with very high precipitation.

See the generalized map of soil moisture regime regions in the continental 

United States. (Note: The perudic areas are too small to indicate at this map

scale.)

Generalized Characteristics 

1) Precipitation exceeds evapotranspiration by plants in every month.

Common Horizon Nomenclature

There is no horizon nomenclature indicating that a soil has a perudic moisture 

regime.
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Petrocalcic Horizon

Root-restrictive subsoil horizon that is cemented by calcium carbonate

Concept and Background Information

The petrocalcic horizon is a root-restrictive subsoil horizon that is cemented by 

calcium carbonate. Lateral continuity is such that spaces where roots can 

penetrate are more than 10 cm apart. This horizon is essentially an advancedstage calcic horizon where so much secondary calcium carbonate has 

accumulated in the layer that the pores have become plugged and cemented. 

Air-dry fragments do not slake in water but will slake in HCL, thus confirming 

calcium carbonate as the cementing agent. The carbonates may have been 

deposited (e.g., as dust) from outside the profile, or, in naturally calcareous 

parent materials, the carbonates may be dissolved and reprecipitated locally 

within the horizon. Petrocalcic horizons are commonly found in arid or 

semiarid environments where deep leaching and removal of soluble salts like 

calcium carbonate do not take place.

Generalized Characteristics

1) Horizon is cemented by carbonates to such an extent that spaces where 

roots can penetrate are more than 10 cm apart.

2) Horizon is > 10 cm thick (or > 1 cm thick, if it is a laminar cap resting on 

bedrock).

Common Horizon Nomenclature

Commonly used horizon nomenclature includes master horizon B combined 

with suffixes kk and m. In addition, suffixes n, q, y, and z are sometimes used.

Examples include: Bkkm, Bkkzm, and Bkkym.
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Part 3—Diagnostic Horizons and Characteristics 3-106

Old alluvial gravel cemented in a matrix of illuvial calcium carbonate (white) 

form a very strongly cemented petrocalcic horizon (in a soil in the United 

Arab Emirates). (Photo courtesy of Dr. Craig Ditzler)

Back to Petrocalcic Horizon
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Part 3—Diagnostic Horizons and Characteristics 3-107

This New Mexico soil has a petrocalcic horizon below a depth of 

about 80 cm. Scale is in cm.

Back to Petrocalcic Horizon
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Part 3—Diagnostic Horizons and Characteristics 3-108

Cross-section of the upper part of a petrocalcic horizon with a laminar cap. 

The penciled numbers are the ages of various parts of the layer. Scale is

in inches.

Back to Petrocalcic Horizon
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Part 3—Diagnostic Horizons and Characteristics 3-109

Petroferric Contact

Boundary between unconsolidated soil and a layer cemented with iron

Concept and Background Information

The petroferric contact is the boundary between unconsolidated soil material 

and a root-restrictive subsoil feature that is cemented primarily by iron 

(ironstone). Organic matter as part of the cementing agent is either absent or 

in small amounts (this distinguishes the cemented layer below the petroferric 

contact from the placic horizon). The petroferric contact is generally 

recognized in some tropical and subtropical areas where layers of ironstone 

have formed in the soil as sesquioxides accumulated. The ironstone is 

generally more or less horizontal but may have a wavy or irregular boundary. 

It is commonly only a few centimeters thick but can be a few meters thick.

Generalized Characteristics

1) The contact is to an iron-cemented layer (ironstone).

2) Spaces that roots can penetrate are more than 10 cm apart.

3) Thickness of the ironstone below the contact generally ranges from a few 

centimeters to a few meters.

Common Horizon Nomenclature

A petroferric contact is identified at the boundary between soil and a layer or 

thin sheet of continuous ironstone. In some cases, especially if the ironstone 

sheet is less than 1 cm thick, the contact may only be described within the text 

of the horizon description and not as a distinct layer.
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This soil has a petroferric contact at the upper boundary between the 

unconsolidated soil above and the indurated ironstone sheet below.

The soil material below the ironstone is unconsolidated. (Photo 

courtesy of John Kelley)

Back to Petroferric Contact
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Petrogypsic Horizon

Root-restrictive subsoil horizon that is cemented by gypsum

Concept and Background Information

The petrogypsic horizon is a root-restrictive subsoil horizon that is cemented 

by gypsum. Lateral continuity is such that spaces where roots can penetrate 

are more than 10 cm apart. The horizon is essentially an advanced-stage 

gypsic horizon where so much secondary gypsum has accumulated in the 

layer that the pores have become plugged and cemented. Air-dry fragments 

do not readily slake in water. Commonly, the gypsum was naturally occurring 

in the parent materials, where it was dissolved and reprecipitated locally within 

the horizon. Petrogypsic horizons are commonly found in arid or semiarid 

environments where deep leaching and removal of soluble salts like gypsum 

does not take place.

Generalized Characteristics

1) Horizon is cemented by gypsum to such an extent that spaces where roots 

can penetrate are more than 10 cm apart.

2) Thickness is > 5 mm.

3) Gypsum content (by weight) is > 40%.

Common Horizon Nomenclature

Commonly used horizon nomenclature includes master horizon B combined 

with suffixes yy and m. In addition, suffixes k and z may be used. Examples 

include: Byym, Byykm, and Byyzm.
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Soil landscape and close-up of a petrogypsic horizon (inset) in the United

Arab Emirates. (Photo courtesy of John Kelley)

Back to Petrogypsic Horizon
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Placic Horizon

Root-restrictive, dark-colored subsoil horizon that is cemented by iron 

and organic matter

Concept and Background Information

The placic horizon is a root-restrictive subsoil horizon that is cemented by iron 

and organic matter. Lateral continuity is such that spaces where roots can

penetrate are more than 10 cm apart. The placic horizon is dark in color, 

ranging from black to dark red. It is generally thin, commonly 2 to 10 mm in 

thickness, and is mostly found within 50 cm of the soil surface. It generally 

occurs in cool, moist climates with low evapotranspiration rates but is also 

known to occur in warm, humid areas. It is frequently associated with stratified 

deposits, such as ash layers.

Generalized Characteristics

1) Horizon is cemented by iron and organic matter to such an extent that 

spaces where roots can penetrate are more than 10 cm apart.

2) Horizon is > 1 mm thick (where it is associated with spodic materials, is it 

no more than 25 mm thick).

Common Horizon Nomenclature

Commonly used horizon nomenclature includes master horizon B combined 

with suffixes s and m and sometimes h. Examples include: Bsm and Bhsm.
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Part 3—Diagnostic Horizons and Characteristics 3-114

Profile of a soil in Alaska that has a thick, cemented placic 

horizon (black) about 20 cm thick over a reddish spodic 

horizon. Scale is in feet.

Return to Placic Horizon
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Plinthite

Subsoil feature consisting of a firm iron-oxide-rich mass that irreversibly 

hardens after exposure to repeated wet-dry cycles

Concept and Background Information

Plinthite is a reddish firm mass consisting of an accumulation of iron-oxide in 

association with clay, quartz, and other minerals. It has little or no organic 

matter. It can occur as individual masses or as a continuous phase where it 

forms platy, polygonal, or reticulate patterns in the soil. Plinthite is a discrete 

body that can be removed intact from the soil. It will irreversibly harden to form 

ironstone if it is subjected to repeated wet-dry cycles, particularly after 

exposure to heat and sunlight (such as in a road cut or gully sidewall). Once 

hardened, it is ironstone and no longer considered plinthite.

Generalized Characteristics

1) Feature is a firm or very firm mass with an accumulation of iron and little or 

no organic matter.

2) Mass will harden irreversibly after repeating wetting and drying upon 

exposure.

3) Feature generally has a dark red color.

Common Horizon Nomenclature

Commonly used horizon nomenclature includes master horizon B combined 

with suffix v and generally t. Additional suffixes c, g, and o may also be used. 

Examples include: Btv, Bov, and Btgv.
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Plinthite (firm, dark red concentrations) in a soil near College Station, 

Texas. (Photo courtesy Dr. David Weindorf)

Bricks made by hand from dried, plinthite-rich soil material (India).

Back to Plinthite
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Redoximorphic Features

Morphological features caused by wetness that are characterized by 

distinctive reddish and grayish color patterns

Concept and Background

The distinctive color patterns associated with redoximorphic features form as 

a result of change in soil color due to the loss or gain of pigment. This change 

of pigment is due to the saturation of the soil with water to the extent that most 

oxygen is depleted from it. In this anaerobic environment, microbes must rely 

on elements other than oxygen to carry out their metabolism. In so doing, 

these elements are reduced (i.e., they gain an electron). Two important 

elements involved in this process are manganese and iron. When they are 

converted from their oxidized state to their reduced state, they become mobile 

and move in solution within the soil. As a result, manganese and iron ions 

tend to move along a gradient from areas within the soil that are void of 

oxygen to areas where oxygen is present (such as ped faces and pores). In 

the oxygenated areas, these ions return to their oxidized state (i.e., they lose 

an electron) and are immobilized. This process is marked by the formation of 

redoximorphic features in the soil. Areas depleted of iron and manganese 

tend to be gray in color, forming redox depletions, while areas where the iron 

and manganese have accumulated are redder (or black for manganese), 

forming redox concentrations.

For more information see Aquic Conditions.

Common Horizon Nomenclature

Other than the suffix g (for strong gleying), there is no horizon nomenclature 

that specifically indicates the presence of redoximorphic features. 

Redoximophic features are identified as either concentrations or depletions 

within the horizon description.
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Salic Horizon

Saline horizon with an accumulation of salts

Concept and Background Information

The salic horizon is a saline horizon in which salts more soluble than gypsum 

have accumulated. Halite is one of the more common salts in salic horizons, 

but other salts also occur. The salic horizon may be either in the subsoil 

(where precipitation leaches salts downward) or at the surface (where salts 

are wicking upward from a water table within the profile). Where this horizon is 

at the surface, there may be a polygonal pattern of slightly elevated ridges 

caused by surface heaving as the salt crystals accumulate. In the laboratory, 

the salt concentration is evaluated by measuring electrical conductivity of a 

saturated paste extract. The level of salt concentration may vary seasonally 

as salts are partially leached during rainy periods and accumulate through 

evapotranspiration during dry periods.

Generalized Characteristics

1) Thickness is > 15 cm.

2) For > 90 consecutive days in most years:

a. Electrical conductivity in a saturated paste extract (ECe) is > 30 

dS/m, and

b. Product of ECe X horizon thickness (cm) is > 900.

Common Horizon Nomenclature

Commonly used horizon nomenclature includes master horizon A or B 

combined with suffix z. Additional suffixes g, k, or y may also be used. 

Examples include: Az, Bzg, and Bzky.
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A salic horizon has formed in the upper part of this profile 

due to upward movement of salts from the water table

(below the view of the photo). The land surface is 

white due to the accumulating salt crust.

Back to Salic Horizon
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Sapric Soil Materials

Organic soil materials that are highly decomposed

Concept and Background Information

Sapric soil materials are organic soil materials that are highly decomposed. 

They have few fibers (<~ 1/3, by volume) that can be identified, before 

rubbing, as to their biological origin. After rubbing, <~ 1/6 (by volume) intact 

fibers remain. 

A simple field test involves rubbing the material between the fingers (about ten 

times) to observe the amount of intact fiber that persists after rubbing. Sapric 

soil materials generally have relatively high bulk density (>~ 0.2 g/cm3) and a 

relatively low water content when saturated (commonly <~ 450%, by weight, 

compared to oven dry). Sapric soil materials are commonly very dark gray to 

black.

Note: For comparison, see the less decomposed hemic and fibric soil 

materials.

Generalized Characteristics 

1) Material contains < 1/6 (by volume) fibers after rubbing, and

2) Material yields a brownish yellow to black extract of sodium pyrophosphate 

solution. 

Common Horizon Nomenclature

Commonly used horizon nomenclature includes master horizon O combined 

with suffix a, as in Oa.
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Organic soil materials, including fibric (top), hemic 

(middle), and sapric (lower) materials. (Photo 

courtesy of John Kelley)

Return to Sapric Soil Materials
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Slickensides

Subsoil feature consisting of polished and grooved surfaces caused by 

shrinking and swelling

Concept and Background Information

Slickensides are polished and grooved features on subsoil structural surfaces 

that are produced as one soil mass slips past another. The soil masses move 

because of shear failure due to significant shrinking and swelling caused by

wetting and drying cycles. Slickensides are only found in soils with high 

contents of swelling clay minerals (smectite) that are subject to at least 

periodic wetting and drying. They are one of the diagnostic criteria used to 

identify Vertisols. 

Generalized Characteristics

1) Surfaces are polished (commonly shiny) and grooved.

2) Dimensions are generally > 5 cm and commonly much larger.

Common Horizon Nomenclature

Commonly used horizon nomenclature includes master horizon B or C 

combined with suffix ss, sometimes in combination with suffix g, k, n, or t.

Examples include: Bss, Bssg, and Css.

Slickensides in a Vertisol.
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Soil Temperature Regimes

Concept and Background Information

The soil temperature regimes are defined by mean annual soil temperature 

(MAST) at a depth of 50 cm from the soil surface, or shallower depth if a rootlimiting layer is present. Mean summer soil temperature (MSST) at the same 

depth is used in the cryic regime definition.

See the generalized map of soil temperature regime regions in the continental 

United States.

Note: For information regarding the estimation of these criteria, see the 

discussion of soil temperature regimes in Part 1—How to Use This Version of 

the Keys.

General Characteristics

The temperature regimes, arranged from coldest to warmest:

1. Gelic – Too cold for virtually all crops; very cold MAST (< 0

oC)

2. Cryic – Too cold for virtually all crops; cold MAST (< 8 oC) and also 

cold in summer 

3. Frigid – Too cold for all but cold weather crops, such as wheat, oats, or 

barley; cold MAST (< 8 oC) but warmer in summer than the cryic 

regime 

4. Mesic – Suited to crops such as corn, wheat, and soybeans; moderate 

MAST (8 - < 15 oC)

5. Thermic – Suited to warm weather crops such as cotton; warm MAST

(15 - < 22 oC)

6. Hyperthermic – Suited to citrus and other freeze-intolerant crops; hot 

MAST (> 22 oC)

Note: MAST = mean annual soil temperature; MSST = mean summer soil 

temperature.
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Sombric Horizon

Subsoil horizon with an illuvial accumulation of humus

Concept and Background Information

The sombric horizon is a dark-colored subsoil horizon with an illuvial 

accumulation of humus. Compared to the layer above, the sombric horizon 

has lower color value and/or chroma. The humus is not complexed with 

aluminum (as in the spodic horizon), is not dispersed with sodium (as in some 

natric horizons), and is not associated with illuvial silt and clay (as in the agric 

horizon). There is no albic horizon above a sombric horizon. 

Sombric horizons form in cool, moist, high-elevation areas in tropical and 

subtropical regions. As a result of high leaching in this environment, base 

saturation (measured by NH4OAc) is < 50%. The cation-exchange capacity 

(CEC) associated with the clay fraction is much lower than that of the spodic 

horizon; the spodic horizon has a significant amount of amorphous minerals 

(not present in the sombric horizon) that provide high CEC. Because of its 

dark color, the sombric horizon may be mistaken for a buried surface layer.

However, upon close examination, it is evident that the dark-colored humus is 

rather concentrated on ped surfaces and pore linings than occurring 

throughout the matrix. 

Generalized Characteristics

1) Horizon is a dark-colored subsoil layer with illuvial humus (not complexed 

with aluminum) that has:

a. Lower color value and/or chroma than layer above, and

b. The appearance of a buried surface layer (but is not).

2) Base saturation (NH4OAc) is < 50%.

3) Horizon does not underlie an albic horizon.

4) Horizon occurs in cool, moist, high-elevation areas in tropical and 

subtropical regions.

Common Horizon Nomenclature

Commonly used horizon nomenclature includes master horizon B combined 

with suffix h and generally o, as in Bho.
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Profile of a high-elevation, tropical soil. The dark layer in the 

middle is a sombric horizon.

Return to Sombric Horizon
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Spodic Horizon 

Subsoil horizon with an illuvial accumulation of organic matter in 

complex with aluminum and also commonly iron

Concept and Background Information

The spodic horizon is a subsoil horizon with an illuvial accumulation of active 

amorphous materials that consist of organic matter in complex with aluminum 

and also commonly iron. These materials are defined as spodic materials. The 

spodic horizon contains > 85% spodic materials. The spodic materials impart 

unique properties to the soil, including a high cation-exchange capacity, a

large surface area, high water retention, reddish color, low pH, and a high 

content of organic carbon.

The spodic horizon forms in humid environments, both cold and hot. Native 

vegetation favoring the production and illuviation of organo-metal complexes 

is important to the formation of the spodic horizon. In cool climates, this 

vegetation includes heath as well as both coniferous and mixed broadleaf 

forest. In warm climates, it includes savannah, palm, and mixed forest. Spodic 

horizons tend to form on sandy or loamy parent materials with < ~ 20% clay. 

Some are gravelly. They form in both well drained and poorly drained 

conditions. In wet conditions, however, it seems important that the water table

fluctuate and not remain at the surface.

Generalized Characteristics

1) Thickness is > 2.5 cm.

2) Horizon contains > 85% spodic materials.

3) Horizon formed as a subsoil horizon.

Common Horizon Nomenclature

Commonly used horizon nomenclature includes master horizon B combined 

with suffix s and/or h. Examples include: Bs, Bhs, and Bh.
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Reddish spodic horizon below a light gray albic horizon.

Back to Spodic Horizon
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Spodic Materials

Illuvial, active amorphous materials composed of organic matter in 

complex with aluminum and also commonly iron

Concept and Background Information

Spodic materials are active amorphous materials that have been illuviated into 

a subsurface horizon. They are composed of organic matter that is in complex 

with aluminum and also commonly iron and are moderately acid or more 

strongly acid in reaction. They impart unique properties to the soil, including a 

high cation-exchange capacity, a large surface area, high water retention, 

reddish color, low pH, and a high content of organic carbon. Spodic materials 

are used in the definition of the spodic horizon.

In many cases, the presence of spodic materials can be confirmed by low pH, 

a high content of organic carbon, certain black to dark brown or reddish 

colors, and the presence of an overlying albic horizon. In cases where either 

there is no albic horizon or the colors below the albic are too high in chroma, 

additional evidence (as described in the next paragraph) is required.

In some cases, the active amorphous materials form cracked coatings on 

sand grains that can be seen with a 40-60X hand lens (especially when dry). 

In extreme cases, this can lead to the cementation of a substantial part of the 

layer. Evidence for the illuvial nature of the material can be gained using the

optical-density-of-the-oxalate-extract test (ODOE). The ODOE value of illuvial 

spodic materials is higher than that of the overlying eluvial layer. This reflects 

the translocation of organic materials into the illuvial layer. The Al + 1⁄2 Fe (by 

ammonium oxalate) test is used to conform illuviation of aluminum and iron. 

Generalized Characteristics

1) All spodic materials have:

a. pH < 5.9 (1:1 water), and

b. > 0.6% organic carbon.

2) If a mostly continuous albic horizon is present, color below the albic has:

a. Hue 5YR or redder, or

b. Hue 7.5YR, value < 5, and chroma < 4, or

c. Hue 10YR (or neutral hue) and value and chroma < 2, or

d. Hue 10YR 3/1, or

e. Hue 7.5YR, value < 5, and chroma 5 or 6.

For item e only, go to item 4; otherwise stop.

3) If there is no albic horizon above, color is:

a. Any color listed for item 2

Plus
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4) Additional properties are required in cases of items 2e or 3a. In these 

cases, materials must:

a. Be very firm or firmer and cemented (air-dry fragment does not 

slake), or

b. Have > 10% cracked coatings on sand grains, or

c. Have Al + 1⁄2 Fe (by ammonium oxalate) > 0.50 and > 2 times more 

than in the layer above, or

d. Have ODOE > 0.25 and > 2 times more than in the layer above.

Common Horizon Nomenclature

There is no horizon nomenclature specifically used to indicate the presence of 

spodic materials, although they are often associated with a spodic horizon and 

the nomenclature associated with it. In some areas, plowing has mixed spodic 

materials into the surface layer, which is simply indicated with master horizon 

A and suffix p.
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Sulfidic Materials

Soil materials containing acid-producing, oxidizable sulfur compounds

Concept and Background Information

Sulfidic materials contain oxidizable sulfur compounds, such as elemental 

sulfur, pyrite, or iron-monosulfides. They generally accumulate in permanently 

saturated sediments associated with brackish waters, such as coastal 

marshes, estuaries, and mouths of rivers. They can also accumulate in fresh 

water systems if sulfur is present in the water. While commonly associated 

with coastal environments, sulfidic materials also are known to occur in deep 

strata in upland positions where the sulfidic materials were deposited in the 

geologic past under previous environmental conditions. If exposed to the 

atmosphere, the sulfur compounds will oxidize to form sulfuric acid and the pH 

will become very low, thus forming a sulfuric horizon. The conversion of 

sulfidic materials into a sulfuric horizon can occur over a period of a few 

weeks to a few months.

Generalized Characteristics

1) Initial pH is > 3.5 (1:1 water).

2) Materials have one or both of the following:

a. After aerobic incubation for 16 weeks, a pH that drops to > 0.5 unit 

(to final pH of < 4.0), and/or

b. A sulfur dry weight that is:

i. > 0.75% and

ii. More than 1/3 the calcium carbonate equivalent.

Common Horizon Nomenclature

Commonly used horizon nomenclature includes master horizon O, A, B, or C 

with suffix se, often in combination with a, e, or g. Examples include: Oase, 

Bseg, and Cseg.
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Sulfuric Horizon

Horizon that has become highly acid due to oxidation of sulfide minerals

Concept and Background Information

The sulfuric horizon is a horizon that has become highly acid due to oxidation 

of sulfide minerals. It can be in the subsoil or at the surface. Sediments 

deposited in salt- or brackish-water environments, whether in today’s coastal 

areas or in inland ancient seas, may contain iron-sulfide minerals that, upon 

exposure to air, produce sulfuric acid. The sulfuric horizon commonly forms as 

a result of human activity, such as drainage, dredging, surface mining, or 

other earth-moving activities that expose the sulfur-bearing materials to the 

air. Natural processes, such as landslides on mountain slopes, can also 

expose sulfur-bearing soil materials to oxidation. After exposure, sulfuric acid 

forms. In some cases, distinctive minerals such as jarosite (with a 

characteristic straw-yellow color) also form. The highly acidic materials are 

toxic to plants and negatively affect structures built upon them.

Generalized Characteristics

1) Thickness is > 15 cm.

2) pH is < 3.5 (or < 4.0 if sulfide or sulfur-bearing minerals are present).

3) Horizon shows evidence that low pH is due to sulfuric acid. It has:

a. Jarosite concentrations (straw-yellow color, commonly hue of 2.5Y

or yellower and chroma of > 6), or

b. > 0.05 water-soluble sulfate, or

c. Sulfidic materials directly below.

Common Horizon Nomenclature

Commonly used horizon nomenclature includes master horizon O, A, or B 

combined with suffix j, often in combination with a, e, g, or se. Where the 

horizon formed in human-transported material, the master horizon is preceded 

by the caret symbol (^). Examples include: Oaj, Bjseg, and ^Apj.
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Part 3—Diagnostic Horizons and Characteristics 3-132

Jarosite concentrations (yellow) within peds from a sulfuric horizon.

Return to Sulfuric Horizon
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Part 3—Diagnostic Horizons and Characteristics 3-133

Udic Moisture Regime (Humid Areas)

Regime characteristic of humid regions with seasonally well distributed 

precipitation

Concept and Background Information

The udic soil moisture regime is in humid regions with seasonally well 

distributed precipitation. The soils are not dry for > 90 cumulative days in most 

years. In general, the combination of stored moisture and moisture from 

summer equals or exceeds that lost through evapotranspiration.

See the generalized map of soil moisture regime regions in the continental 

United States.

Generalized Characteristics 

1) Soil is dry < 90 cumulative days in most years.

2) Soil is dry < 45 consecutive days in summer and early fall (except in very 

warm areas and locations where there is little difference between summer 

and winter soil temperatures).

Note: For information regarding the estimation of these criteria, see the 

discussion of soil moisture regimes in Part 1—How to Use This Version of the

Keys.
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Udic Moisture Regime (Semiarid Areas)

Regime characteristic of semiarid climates where moisture is limited but 

available for portions of the growing season

Concept and Background Information

The ustic soil moisture regime is intermediate between the aridic (dry) and 

udic (moist) soil moisture regimes. Although the overall annual moisture 

amounts are limited, moisture is generally available during portions of the 

growing season. Unlike soils with a xeric regime, where they receive moisture 

mostly in winter, soils with an ustic soil moisture regime receive precipitation 

mostly in spring and summer or spring and fall. The ustic soil moisture regime 

is not used for soils with permafrost.

See the generalized map of soil moisture regime regions in the continental 

United States.

Generalized Characteristics 

1) In areas that have very warm average annual soil temperature (> 22 oC) or 

that have little difference (< 6 oC) between winter and summer soil 

temperatures:

a. The soil is dry for > 90 cumulative days during the year, but

b. The soil is moist during the year for > 90 consecutive days or > 180 

cumulative days.

2) In other areas with cold to warm soil temperatures (< 22 oC):

a. The soil is dry for > 90 cumulative days, but

b. The soil is moist is for > 50% of the growing season (when soil 

temperature > 5 oC).

3) In other areas where the soil is moist for > 45 consecutive days in winter 

and early spring, the soil is dry for < 45 consecutive days in summer and 

early fall.
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Xeric Moisture Regime

Regime characteristic of a Mediterranean-type climate with cool, moist 

winters and warm, dry summers

Concept and Background Information

The xeric soil moisture regime is characterized by a Mediterranean-type 

climate, in which winters are cool and moist and summers are warm and dry. 

Most precipitation falls during winter, when evapotranspiration is low, which 

generally allows ample storage of moisture for the following growing season. 

The xeric soil moisture regime is not used in areas that have very warm 

average annual soil temperatures (> 22 oC) or that have little difference 

(< 6 oC) between winter and summer soil temperatures. The soils are dry for 

> 45 consecutive days in summer and early fall and moist for > 45 

consecutive days in winter and early spring.

See the generalized map of soil moisture regime regions in the continental 

United States.

Generalized Characteristics 

1) The soil is moist for > 45 consecutive days in winter and early spring and 

dry for > 45 consecutive days in summer and early fall.

2) The soil is moist is for > 50% of the growing season (when soil temp is 

> 5 oC).
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Part 3—Diagnostic Horizons and Characteristics 3-136

Moisture Regime Map

This map shows the general pattern of soil moisture regimes across the 

United States. The classes shown include moisture intergrades, such as typic 

ustic and xeric aridic, that are used at the subgroup level of Soil Taxonomy. 

For classification at the great group level, only consider the second term (e.g.,

consider xeric aridic as just aridic).This map can be used as a general guide 

for assigning a moisture regime class, but it is not definitive for any specific 

location.

Return to:

Aquic Soil Moisture Regime

Aridic Soil Moisture Regime

Perudic Soil Moisture Regime

Udic Soil Moisture Regime

Ustic Soil Moisture Regime

Xeric Soil Moisture Regime
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Temperature Regime Map

This map shows the general pattern of soil temperature regimes across the 

continental United States. Gelic temperature regimes have not been 

documented in the continental United States but are found in Alaska. This 

map can be used as a general guide for assigning a moisture regime class, 

but it is not definitive for any specific location.

Return to Soil Temperature Regimes

Part 4—Keys to the Orders, Suborders, and Great Groups 4-1

Part 4—Keys to the Orders, Suborders, 

and Great Groups
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-2

Key to Soil Orders

Introduction

The keys are designed to be used sequentially, beginning with the key to the 

orders, then that of the suborders, and finally that of the great groups. Start 

with the first item listed in the key to orders and determine whether or not the 

soil meets the criteria. If the initial criteria listed are not met, continue through 

the key until the criteria are met. Once the criteria for the order are determined 

to be met, move on to the suborder and then the great group. The design of 

the keys is such that a soil is placed in the first class for which the criteria are 

met.

The criteria presented in the keys refer to the presence of specific diagnostic 

horizons and characteristics. In addition, some require a determination of the 

proper moisture or temperature regime. See Part 3—Diagnostic Horizons and 

Characteristics for detailed information. Unless otherwise specified, depths 

are measured from the soil surface.

At the highest level of the classification system, twelve soil orders are 

recognized. The orders, along with a brief description capturing the general 

concept of each, are presented (alphabetically) in the list below. After these 

descriptions is a key for determining the order in which a soil belongs. Keys 

for determining the proper suborder and great group follow.

Brief Description of the Soil Orders

1. Alfisols are naturally fertile soils with high base saturation and a clayenriched subsoil horizon.

2. Andisols are relatively young soils, mostly of volcanic origin, that are 

characterized by unique minerals with poorly organized crystalline 

structure.

3. Aridisols are the dry soils of deserts.

4. Entisols are young soils with little or no profile development.

5. Gelisols are very cold soils with permafrost in the subsoil.

6. Histosols are soils that formed in decaying organic material.

7. Inceptisols are youthful soils with a weak, but noticeable, degree of 

profile development. 
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-3

8. Mollisols are very dark-colored, naturally very fertile soils of 

grasslands.

9. Oxisols are highly weathered tropical soils with low natural fertility.

10.Spodosols are acid soils with low fertility and accumulations of organic 

matter and iron and aluminum oxides in the subsoil.

11.Ultisols are soils with low base status and a clay-enriched subsoil. 

12.Vertisols are very clayey soils that shrink and crack when dry and 

expand when wet.

Key to Soil Orders

A. Soils (mineral or organic) with permafrost within a depth of 

200 cm ---------------------------------------------------------------------------- Gelisols

If the permafrost is below a depth of 100 cm, then gelic materials must also 

be present (i.e., evidence of cryoturbation) above 100 cm.

B. Soils with organic material 40 cm or more thick ----------------- Histosols

In most cases, the organic soil materials must be > 40 cm thick. There are 

exceptions, however, that allow thinner amounts, i.e., where the organic 

layers directly overlie very coarse textured materials and where the soils 

are relatively shallow to some root-limiting layers. The organic layers must

be > 60 cm thick if they are from sphagnum moss or other organic material 

with a very low bulk density. Also, Histosols are not allowed to contain 

significant amounts of andic soil materials in the upper part.

C. Soils with a spodic (accumulation of translocated organic matter in 

complex with aluminum and also commonly iron) horizon that is 10 

cm or more thick within a depth of 50 cm (or within 200 cm if the soils 

have sandy texture above the spodic horizon)------------------ Spodosols

Spodosols cannot have a plaggen, argillic, or kandic horizon above the 

spodic horizon. If andic soil materials are present in the upper part of the 

soil, then there must be an albic horizon above the spodic. See 12th 

edition of “Keys to Soil Taxonomy” for specific criteria regarding Spodosols 

that are very cold, loamy, cemented, or plowed or that have root-limiting 

layers.

D. Soils with andic soil properties (rich in volcanic glass or 

poorly crystalline minerals) in 36 cm or more of the upper 

60 cm ---------------------------------------------------------------------------- Andisols

Begin measuring from the top of the mineral soil or the top of an O horizon 

that has andic properties, whichever is shallowest. For soils with certain
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-4

root-limiting layers above a depth of 60 cm, andic soil properties must 

constitute > 60% of the zone above the root-limiting layer.

E. Soils with an oxic (extremely weathered) horizon within a depth of 

150 cm -----------------------------------------------------------------------------Oxisols

A kandic horizon may be present within a depth of 100 cm providing it has 

< 10% weatherable minerals in the fine and very fine sand fractions and 

there is > 40% clay in the upper 18 cm of the soil (after mixing).

F. Soils with all of the following:

a) 30 percent or more clay to a depth of at least 50 cm, AND

b) Evidence of shrinking and swelling (slickensides) in a layer 25 cm 

or more thick above a depth of 100 cm, AND

c) Cracks that open and close periodically ------------------------- Vertisols

The top 18 cm of surface soil (or a plow layer) may have sublayers with 

less than 30% clay, as long as the overall average is > 30%. Also, the 50-

cm depth criterion is waved if the layer with a high clay content is underlain 

directly by certain root-limiting layers.

G. Soils that have either:

a) An aridic soil moisture regime (too dry for mesophytic plants) and

a diagnostic subsoil horizon, OR

b) A salic (high content of salts) horizon underlain by a seasonal high 

water table within a depth of 100 cm --------------------------------- Aridisols

Aridisols must have either an ochric (typically thin and/or light-colored) or 

anthropic (human-modified) epipedon. Other diagnostic horizons, except 

argillic (clay accumulation) or natric (high levels of illuvial clay and sodium)

horizons, must begin above a depth of 100 cm. Aridisols cannot have a 

sulfuric (highly acid due to oxidation and production of sulfuric acid) 

horizon within a depth of 150 cm. The moisture control section of Aridisols 

with a seasonal high water table must be dry at some time during the year.

H. Soils with both: 

a) An argillic (clay accumulation) or kandic (very low cation-exchange

capacity) subsoil horizon AND

b) Base saturation (by sum of cations) of less than 35 percent at the 

check depth (generally 125 cm below the top of the argillic or kandic

horizon) --------------------------------------------------------------------------- Ultisols
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-5

The check depth varies for soils with sandy surface layers, for soils with a 

kandic or argillic horizon that begins deeper than 55 cm, and for soils with 

a fragipan or certain other root-limiting layers.

I. Soils with both: 

a) A mollic (rich in humus and bases) epipedon AND

b) Base saturation of 50 percent or more in all layers, generally to a 

depth of 180 cm -------------------------------------------------------------- Mollisols

Exceptions to the criterion of a depth of 180 cm are 125 cm below the top 

of an argillic, kandic, or natric horizon or to a densic, lithic, or paralithic

contact, providing any of these are shallower than 180 cm. Also, special 

provisions are made for some soils with an intervening albic horizon within 

the mollic epipedon.

J. Soils with an argillic (clay accumulation), kandic (very low cationexchange capacity), or natric (high levels of illuvial clay and sodium)

subsoil horizon or a fragipan (firm and brittle but not cemented

layer)--------------------------------------------------------------------------------Alfisols

To qualify as an Alfisol, the fragipan must have clay films more than 1 mm 

thick.

K. Soils that have either: 

a) Within a depth of 100 cm, a cambic (minimal soil development), 

calcic (calcium carbonate accumulation), petrocalcic (cemented by 

calcium carbonate), gypsic (gypsum accumulation), petrogypsic 

(cemented by gypsum), or placic (cemented by iron and organic 

matter) horizon or a duripan (layer cemented by silica), OR

b) Within a depth of 150 cm, a sulfuric (highly acid due to oxidation 

and production of sulfuric acid) horizon, OR

c) Within a depth of 200 cm, a fragipan (firm and brittle but not 

cemented layer) or an oxic (extremely weathered), sombric (dark 

layer in which organic matter has accumulated), or spodic 

(accumulation of translocated organic matter in complex with 

aluminum and also commonly iron) horizon, OR

d) Within the upper 50 cm, a salic (high content of salts) horizon or a 

histic (wet, organic surface layer), folistic (freely drained organic 

surface layer), mollic (rich in humus and bases), umbric (humus-rich 

with low base saturation), or plaggen (human-modified) epipedon, 

OR
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-6

e) Within the upper 50 cm, an excahangeable sodium percentage > 15 

(sodium adsorption ratio > 13), and a seasonal high water table within 

a depth of 100 cm ----------------------------------------------- Inceptisols

The base of a cambic horizon must be below a depth of 25 cm, unless the 

soil has a cryic or gelic temperature regime. For items d and e above, 

sulfidic materials must not occur within a depth of 50 cm and the soil must 

be non-fluid or have < 8% clay at some depth between 20 and 50 cm.

L. Soils generally lacking in pedological development ------------- Entisols
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-7

Alfisols Order

Alfisols are naturally fertile soils with high base saturation and a clayenriched subsoil horizon.

General Characteristics

Typically, Alfisols have a surface horizon consisting of an ochric (typically thin 

and/or light-colored) epipedon and a subsurface consisting of a clay-enriched 

argillic horizon. Between the surface horizon and subsoil, there is commonly a 

light-colored zone of leaching. In addition, these soils have moderate to high 

base saturation. Alfisols may have other subsoil horizon forms, including a 

fragipan (firm and brittle but not cemented layer), a duripan (layer cemented 

by silica), a kandic (very low cation-exchange capacity) horizon, a natric (high 

levels of illuvial clay and sodium) horizon, a petrocalcic (cemented by calcium 

carbonate) horizon, plinthite (firm, iron oxide-rich concentration), or other 

features, and these are used in defining the great groups within the order. A 

few wet Alfisols have an umbric (humus-rich with low base saturation) 

epipedon.

Environment and Processes

Alfisols formed from a wide range of parent materials and occur under broad 

environmental conditions, ranging from tropical to boreal. Most Alfisols have a 

humid (udic) or subhumid to semiarid (ustic) moisture regime. Those with a 

Mediterranean-type climate, where precipitation occurs mostly in winter and 

summers are dry, have a xeric moisture regime. Some Alfisols are wet and 

have an aquic moisture regime. 

Alfisols have developed primarily (though not exclusively) under forest 

vegetation. The movement of clay and other weathering products from the 

upper layers of the soil and their subsequent accumulation in the subsoil are 

important processes in Alfisols. The soil-forming processes are in relative 

balance. As a result, nutrient bases (such as calcium, magnesium, and 

potassium) are supplied to the soil through weathering and the leaching 

process is not sufficiently intense to remove them from the soil before plants 

can use and recycle them.

Location

Alfisols are extensive worldwide, occurring on every continent (excluding 

Antarctica). Globally, they occupy about 10% of the ice-free land area. The 

Alfisols of the warmer climates tend to form a zone between the Aridisols of 

arid regions and the Inceptisols, Ultisols, and Oxisols of areas with warm, 

humid climates. Where the climate is cooler, the Alfisols in the United States 
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-8

tend to form a belt between the Mollisols of the grasslands and the Spodosols 

and Inceptisols of very humid climates. 

Profile of an Alfisol (specifically a Haplustalf) in Stephenville,

Texas. (Photo courtesy of Dr. David Weindorf)

(Back to Key to Soil Orders)
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-9

Alfisols by suborder in the United States.

Global distribution of Alfisols.
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-10

Alfisols Suborders

Classification of Alfisols

In the definitions of the suborders, emphasis is placed entirely on soil climate 

in the form of soil wetness (Aqualfs), cold soil temperature (Cryalfs), and soil 

moisture regime (Ustalfs, Xeralfs, and Udalfs). 

The great group level reflects a combination of important properties, including 

the presence of diagnostic horizons other than an argillic horizon, cold soil 

temperature, an abrupt texture change to a slowly permeable layer, dark red 

colors, iron-cemented nodules in the subsoil, plinthite (firm, iron oxide-rich 

concentration) in the subsoil, patterns of soil saturation, intense bioturbation 

from earthworms or other organisms, and morphology reflecting strong soil 

development on stable landforms.

The five suborders are:

1. Aqualfs—wet Alfisols (aquic conditions in upper part)

2. Cryalfs—cold Alfisols (cryic or isofrigid temperature regime)

3. Ustalfs—moderately dry Alfisols (limited moisture)

4. Xeralfs—moderately dry Alfisols (limited moisture that is supplied in winter

and a Mediterranean-type climate)

5. Udalfs—Alfisols of humid regions with well-distributed rainfall

Alfisols

Aqualfs

Cryalfs

Ustalfs

Xeralfs

Udalfs
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-11

Key to Suborders of Alfisols

Alfisols that:

1) Have a seasonal high water table within a depth of 50 cm ----- Aqualfs

Redoximorphic features are evidence of a seasonal high water table. 

Artificially drained sites are included in Aqualfs. See 12th edition of “Keys 

to Soil Taxonomy” for specific details regarding color, depths, and location 

for redoximorphic features. 

2) Have cold average soil temperature ------------------------------------- Cryalfs

These soils have a cryic or isofrigid soil temperature regime.

3) Have somewhat limited soil moisture available for plant 

growth ----------------------------------------------------------------------------- Ustalfs

These soils have an ustic soil moisture regime. Moisture is limited, but 

available, during portions of the growing season.

4) Have a Mediterranean-type climate -------------------------------------- Xeralfs

These soils have a xeric soil moisture regime (cool and moist in winter and 

warm and dry in summer).

5) Other Alfisols with seasonally well-distributed precipitation ---- Udalfs

These soils have a udic soil moisture regime.
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-12

Aqualfs Great Groups

Aqualfs are the wet Alfisols. They are saturated close to the surface by 

ground water for a long enough period during the year to become devoid of 

oxygen (aquic conditions). Some are artificially drained. Their appearance is 

commonly characterized by a gray and red mottled color pattern 

(redoximorphic depletions and concentrations). In some of these soils, ground 

water is near the surface during a considerable part of the year but drops in 

another part of the year. In others, the ground water may be deep most of the 

year but horizons that have low hydraulic conductivity restrict the downward 

movement of water and extend the period of saturation.

Aqualfs occur in low landscape positions, such as flood plains, depressions, 

and broad flats. A few may be on side slopes where water seeps laterally over 

restrictive layers to the surface. 

Many Aqualfs are drained and used for cultivated crops. Rice is a common 

crop on the Aqualfs that have a thermic or warmer temperature regime. Before 

they were cultivated, some Aqualfs were under forest vegetation or under 

grass. Nearly all Aqualfs are believed to have supported forest vegetation at 

some time in the past.
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-13

Aqualfs

Cryaqualfs

Plinthaqualfs

Duraqualfs

Natraqualfs

Fragiaqualfs

Kandiaqualfs

Vermaqualfs

Albaqualfs

Glossaqualfs

Epiaqualfs,

Endoaqualfs
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-14

Profile of an Aqualf (specifically an Endoaqualf) in Wyoming. The 

dark surface layer is an umbric epipedon. The subsoil has the 

characteristic gray and red mottled color pattern indicating the

presence of aquic conditions. Scale is in cm.

Return to Aqualfs
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-15

Key to Great Groups of Aqualfs (Back to key to suborders)

Aqualfs that have:

1. Cold average soil temperature (cryic temperature

regime) ----------------------------------------------------------------------- Cryaqualfs

2. Plinthite making up > 50% of a layer between depths of 30 and 

150 cm -------------------------------------------------------------------- Plinthaqualfs

Begin measuring depth below any O horizon. Plinthite is a firm, iron oxiderich mass that irreversibly hardens after exposure to repeated wet-dry 

cycles.

3. A duripan (layer cemented by silica) ------------------------------ Duraqualfs

4. A natric (high levels of illuvial clay and sodium) 

horizon ---------------------------------------------------------------------- Natraqualfs

5. A fragipan (firm and brittle but not cemented layer) within a depth of 

100 cm --------------------------------------------------------------------- Fragiaqualfs

Begin measuring depth below any O horizon.

6. A kandic (very low cation-exchange capacity) horizon--- Kandiaqualfs

7. Significant evidence of bioturbation ---------------------------- Vermaqualfs

A total of > 25 cm (within a depth of 100 cm) with > 50% evidence of 

bioturbation is required. Begin measuring depth below any O horizon.

8. An abrupt increase in clay content to the argillic 

horizon ----------------------------------------------------------------------- Albaqualfs

See “abrupt texture change” definition for specific criteria. Also, the Ksat

class must be moderately low or lower in the argillic horizon (most likely to 

be met if the texture is sandy clay, silty clay, or clay).

9. A glossic (degraded argillic) horizon --------------------------- Glossaqualfs

10.Episaturation (perched water table) -------------------------------- Epiaqualfs

11.Endosaturation (saturated throughout the profile) --------- Endoaqualfs
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-16

Descriptions of Great Groups of Aqualfs

Cryaqualfs—These are the cold Aqualfs. These soils are not known to occur 

in the United States. The group has been proposed for other countries, but 

definitions of subgroups have not been suggested. (Back to Aqualfs key)

Plinthaqualfs—These soils have plinthite (firm, iron oxide-rich concentration

that irreversibly hardens after exposure to repeated wet-dry cycles)

constituting one-half or more of the volume in some subhorizon between 

depths of 30 and 150 cm. They are mainly in depressions in areas of wet-dry 

tropical and subtropical climates. They are not known to occur in the United 

States, but they are reported to be extensive in Africa south of the Sahara. 

They are perhaps the Alfisols the least used for agriculture, at least in part 

because they tend to be too wet in the rainy season and too dry in the dry 

season for most crops. On most of these soils, the vegetation is or was 

savanna or a deciduous broadleaf forest. (Back to Aqualfs key)

Duraqualfs—These soils have a silica-cemented duripan. Duraqualfs are not 

known to occur in the United States. The group has been proposed for other 

countries, but definitions of subgroups have not been suggested. (Back to 

Aqualfs key)

Natraqualfs—These soils have a clay-enriched subsoil with high levels of 

sodium saturation (natric horizon). Typically, ground water perches above the 

natric horizon at some period and saturates the soil at another period. 

Natraqualfs are allowed, but not required, to have a glossic (degraded argillic) 

horizon. In the United States, most of these soils have a mesic, thermic, or 

hyperthermic temperature regime but a few are frigid. If undisturbed, 

Natraqualfs commonly have a thin A horizon overlying a thin albic (lightcolored and leached) horizon that, in turn, overlies the natric subsoil horizon. If 

the soils are plowed, the two upper horizons and part of the natric horizon or 

only part of the two upper horizons may be mixed. In the United States, the 

vegetation on Natraqualfs before cultivation was most commonly grass or 

mixed grass and drought-tolerant trees. In humid regions where annual 

precipitation is 100 cm or more, the presence of sodium generally is attributed 

to very slow permeability in the natric horizon. The permeability is so slow that 

there is thought to be less leaching of sodium than there is release of sodium 

by the weathering of feldspars. Many Natraqualfs in the United States formed 

in loess or alluvium of Wisconsinan age. Some Natraqualfs are in basins or on 

lowlands and are subject to flooding, and the sodium in them may be supplied 

by salty ground water or sea water. Characteristically, areas of Natraqualfs 

are small. (Back to Aqualfs key)

Fragiaqualfs—These soils have a dense, brittle layer (fragipan) within 100 cm 

of the mineral soil surface. Most have ground water that is perched above the 

fragipan at some period and saturates the soil at another period. In the United 
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-17

States, the soils generally have frigid to thermic temperature regimes. In most 

years, the albic horizon generally does not become dry but the ground water 

drops below the base of the fragipan during summer and the soil moisture 

content is below field capacity at some period. Most Fragiaqualfs in the United 

States formed in Wisconsinan deposits of late-Pleistocene age and had 

broadleaf deciduous forest vegetation before they were cultivated. Most of the 

soils are nearly level. Fragiaqualfs as a group have lower base saturation than 

other Aqualfs. (Back to Aqualfs key)

Kandiaqualfs—These soils have a clay-enriched subsoil with a very low 

nutrient-supplying capacity (kandic horizon). They are allowed, but not 

required, to have a glossic (degraded argillic) subsoil horizon. 

Characteristically, they are the Aqualfs with the warmest and most humid 

climates and the most water passing through the profile. They have a 

relatively low base saturation for soils of this order. The vegetation is mostly 

tropical or subtropical hardwood forest. Slopes are nearly level or concave. 

Kandiaqualfs are mostly in tropical and subtropical areas. They are rare in the 

United States. (Back to Aqualfs key)

Vermaqualfs—These soils have significant bioturbation in the form of filled 

animal burrows, wormholes, or casts. Krotovinas (filled animal burrows) 

restrict water movement because they are dense, massive, compact, and 

stratified. Soil horizons are obliterated where krotovinas occur. Significant 

amounts of krotovinas in a soil affect soil morphology, soil hydrology, and soil 

behavior. These soils are known to occur along the coastal plain of Texas 

where bioturbation is caused by crayfish. (Back to Aqualfs key)

Albaqualfs—These soils have ground water seasonally perched above a 

slowly permeable clay-enriched argillic horizon. Commonly, a light-colored, 

leached, albic horizon rests abruptly on the argillic subsoil horizon with 

virtually no transitional horizon between the two. There is a significant 

difference in clay content immediately above and below the top of the argillic 

horizon. In the United States, most Albaqualfs have a mesic, thermic, or 

hyperthermic temperature regime and, unless the soils are irrigated, the albic 

horizon is dry for short periods in summer in most years. The dryness seems 

essential to the genesis of these soils. Thus, Aqualfs in which the albic 

horizon is rarely dry are in great groups other than Albaqualfs. (Back to 

Aqualfs key)

Glossaqualfs—These soils have a glossic (degraded argillic) horizon. The 

glossic horizon is interpreted as evidence that the argillic horizon has been 

partly destroyed by the dissolution and removal of clay. Tubular intrusions of 

light-colored albic materials into the argillic subsoil horizon may be formed by 

the filling of burrows made by crayfish or traces of taproots. Characteristically, 

these soils are the Alfisols that have the most humid climates and the most 

water passing through the profile and that have a relatively low base 
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-18

saturation for soils of this order. Before the soils were cultivated, the 

vegetation was mostly deciduous hardwood forest. The parent materials are

largely basic or calcareous sediments of late-Pleistocene age. Slopes are 

nearly level or concave. Glossaqualfs are mostly in the most northern and 

southern parts of areas where Aqualfs occur. They generally are in the Great 

Lakes area and on the Gulf coast. A few are in the Pacific Northwest. Except 

where the temperature regime is frigid, most of these soils have been drained 

and are used for cultivated crops. (Back to Aqualfs key)

Epiaqualfs—These soils have a perched water table. Ground water is 

commonly perched on horizons below the top of the argillic horizon, but it 

does not saturate the lower part of the profile. The water table fluctuates from 

a level near the soil surface to below the argillic subsoil horizon and 

sometimes is not evident. Before cultivation, most Epiaqualfs supported either 

deciduous broadleaf or coniferous forest. Epiaqualfs are generally nearly 

level, and their parent materials are typically late-Pleistocene sediments. 

(Back to Aqualfs key)

Endoaqualfs—These soils are saturated throughout the profile for some time 

during the year. The ground water fluctuates from a level near the soil surface 

to below the argillic subsoil horizon and is sometimes below a depth of 200 

cm. Before cultivation, most Endoaqualfs supported either deciduous 

broadleaf or coniferous forest. They are generally nearly level, and their 

parent materials are typically late-Pleistocene sediments. (Back to Aqualfs 

key)
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-19

Cryalfs Great Groups

Cryalfs are the more or less freely drained Alfisols of cold regions. Nearly all 

of these soils have a cryic temperature regime and a udic moisture regime.

Cryalfs are not extensive. They formed in North America, Eastern Europe, and 

Asia above 49 degrees north latitude and in some high mountains south of 

that latitude. In the mountains, they tend to form at the lower elevations,

adjacent to higher-lying cold Spodosols or Inceptisols. Most Cryalfs are or 

have been under coniferous forest. In North America, they are mainly in 

forests because of their short, cool growing season.

Characteristically, Cryalfs have an organic surface layer (O horizon), a lightcolored, leached albic horizon, and a clay-enriched argillic subsoil horizon. In 

some areas, they have a thin, mineral A horizon at the surface. In regions of 

the least rainfall, they are neutral or slightly acid in all horizons and a calcium 

carbonate-enriched horizon may underlie the argillic horizon. In many of the 

more humid areas where they occur, the lower part of the albic horizon and 

the upper part of the argillic horizon are strongly or very strongly acid.

Cryalfs in the United States generally developed in Pleistocene deposits, 

mostly of Wisconsinan age.

Cryalfs

Palecryalfs

Glossocryalfs

Haplocryalfs
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-20

Key to Great Groups of Cryalfs (Back to key to suborders)

Cryalfs that have:

1. An argillic (clay accumulation), kandic (very low cation-exchange

capacity), or natric (high levels of illuvial clay and sodium) horizon 

beginning at a depth > 60 cm and a glossic (degraded argillic) 

horizon above ------------------------------------------------------------ Palecryalfs

Begin measuring below any mantle with vitric volcanic materials, if 

present.Some layer above the argillic, kandic, or natric horizon must have 

texture finer than loamy fine sand. A glossic horizon is a kind of degraded 

argillic horizon. The glossic criterion is waved if there is interfingering of 

albic materials into the subsoil. 

2. A glossic (degraded argillic) horizon and an argillic horizon within a 

depth of 60 cm -------------------------------------------------------- Glossocryalfs

3. Other simple Cryalfs, with no degradation of the argillic 

horizon --------------------------------------------------------------------- Haplocryalfs
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-21

Descriptions of Great Groups of Cryalfs

Palecryalfs—These soils have an albic (light-colored and leached) horizon 

over a glossic (degraded argillic) subsoil horizon. Some have interfingering of 

light-colored albic materials into the subsoil. The soils are thought to be 

restricted to relatively stable surfaces in the mountains, many of which are 

older than the Wisconsinan Glaciation. The stability may be the result of 

stoniness. The vegetation on these soils is mostly coniferous forest. The 

temperature regimes are mostly cryic. The moisture regimes are mostly udic.

(Back to Cryalfs key)

Glossocryalfs—These soils have a glossic (degraded argillic) subsoil horizon 

and typically have an argillic horizon that has an upper boundary within 60 cm 

of the mineral soil surface (unless there is either a sandy or sandy-skeletal 

particle-size class throughout the layers above) or there is a surface mantle or 

layer in the upper 75 cm consisting of slightly or moderately weathered 

pyroclastic materials. The glossic subsoil horizon commonly has tongue-like 

projections of light-colored albic materials extending into the argillic horizon. 

These projections are thought to indicate that the argillic horizon is being 

moved deeper into the soil. In areas where the soils are transitional between 

Spodosols and Alfisols, some Glossocryalfs have a cambic (minimal soil 

development) horizon that appears to be an incipient spodic horizon. The 

cambic subsoil horizon is commonly separated from the argillic horizon by an 

albic horizon. The vegetation is mostly coniferous trees. A few Glossocryalfs 

have been cleared and are used mostly as pasture. (Back to Cryalfs key)

Haplocryalfs—These are the simple Cryalfs with an argillic (clay 

accumulation) subsoil horizon that has an upper boundary within 60 cm of the 

mineral soil surface. The common sequence of horizons is an organic surface 

horizon (O), an albic (light-colored and leached) horizon, and an argillic

subsoil horizon. In the United States, Haplocryalfs are in the mountains of 

western States and have a cold (cryic) temperature regime. Most support 

coniferous forest vegetation. Virtually none of them are cultivated because 

their slopes are steep and the growing season is short and cool. In other 

countries, Haplocryalfs occur on mountains and also on plains nearly as far 

north as the line of continuous permafrost. Some of the associated soils on 

these landscapes are Gelisols (soils with permafrost) on north-facing slopes 

and Histosols (organic soils). (Back to Cryalfs key)
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-22

Ustalfs Great Groups

Ustalfs are the Alfisols of cold to warm, subhumid to semiarid regions. They 

have an ustic moisture regime (moisture is limited, but available, during 

portions of the growing season). Moisture moves through most of these soils 

to deeper layers only in some years. If there are carbonates in the parent 

materials or in the dust that settles on the surface, the soils tend to have a 

zone of calcium carbonate accumulation below or in the subsoil. The dry 

season or seasons are pronounced enough that trees are either deciduous or 

xerophytic. Many of these soils have or have had savanna vegetation, and 

some were grasslands. Most of the soils are used as cropland or for grazing. 

Some are used as irrigated cropland. Sorghum, wheat, and cotton are the 

most common crops. Droughts are common.

Ustalfs tend to form a belt between the Aridisols of arid regions and the 

Udalfs, Ultisols, Oxisols, and Inceptisols of humid regions. Ustalfs are 

extensive in the world, occurring in North America, South America, Africa, 

Australia, and Asia. In the United States, they generally are moderate in 

extent but are extensive in the southern part of the Great Plains.

Ustalfs may be in areas of erosional surfaces or deposits of late-Wisconsinan 

age, but a great many, and characteristically those of warm regions, are on 

old surfaces. In the Ustalfs on old surfaces, the minerals have been strongly 

weathered, possibly in an environment more humid than the present one. The 

clays in many of these older soils are kaolinitic. Their base saturation at 

present probably reflects additions of bases from dust and rain.
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-23

Ustalfs

Durustalfs

Plinthustalfs

Natrustalfs

Kandiustalfs

Kanhaplustalfs

Paleustalfs

Rhodustalfs

Haplustalfs
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-24

Profile of an Ustalf (specifically a Paleustalf) in Botswana. The soil 

has a thick clayey argillic horizon. Scale is in decimeters.

(Back to Ustalfs)
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-25

Key to Great Groups of Ustalfs (Back to key to suborders)

Ustalfs that have:

1. A duripan (layer cemented by silica) within a depth of 

100 cm ------------------------------------------------------------------------ Durustalfs

Begin measuring depth below any O horizon.

2. Plinthite making up > 50% of a layer within a depth of 

150 cm --------------------------------------------------------------------- Plinthustalfs

Begin measuring depth below any O horizon. Plinthite is a firm, iron oxiderich mass that irreversibly hardens after exposure to repeated wet-dry 

cycles.

3. A natric (high levels of illuvial clay and sodium) 

horizon ----------------------------------------------------------------------- Natrustalfs

4. A kandic (very low cation-exchange capacity) horizon in which 

clay content does not decrease significantly within a depth of 

150 cm --------------------------------------------------------------------- Kandiustalfs

Clay content must not decrease by > 20% of the maximum in the kandic 

horizon, unless skeletans (ped surfaces stripped of clay) are present in the 

layer and the clay content increases again by > 3% below. Kandiustalfs 

cannot have a root-limiting layer within a depth of 150 cm. Begin 

measuring depth below any O horizon.

5. A kandic (very low cation-exchange capacity) horizon with a 

significant decrease in clay content within a depth of

150 cm ------------------------------------------------------------------ Kanhaplustalfs

6. Either:

a) A petrocalcic (cemented by calcium carbonate) horizon within a 

depth of 150 cm, OR 

b) An argillic (clay accumulation) horizon in which the clay content 

does not decrease significantly within a depth of 150 cm and that has

hue of 7.5YR in some part of the lower half (and no root-limiting layer 

within a depth of 150 cm), OR

c) An abrupt increase in clay content to an argillic horizon with 

clayey texture (and no root-limiting layer within a depth of

50 cm) ------------------------------------------------------------------------ Paleustalfs

Begin measuring depth below any O horizon. See 12th edition of “Keys to 

Soil Taxonomy” for specific criteria regarding colors (item b above) and 

clay increase (item c above). For item b, clay content must not decrease 
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-26

by > 20% of the maximum in the argillic horizon, unless skeletans (ped 

surfaces stripped of clay) are present in the layer and the clay content 

increases again by > 3% below.

7. An argillic (clay accumulation) horizon with dominantly dark red 

colors ----------------------------------------------------------------------- Rhodustalfs

Specifically, hue is 2.5YR or 10R and value is < 3 moist (and < 1 unit of 

change dry). Colors make up > 50% and are required throughout the upper 

100 cm of the argillic horizon, or the entire argillic if < 100 cm thick.

8. An argillic (clay accumulation) subsoil horizon with a significant 

decrease in clay content within a depth of 150 cm ----------- Haplustalfs
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-27

Descriptions of Great Groups of Ustalfs

Durustalfs—These soils have a duripan (layer cemented by silica) that has its 

upper boundary within 100 cm of the mineral soil surface. They are not known 

to occur in the United States, but the great group has been provided for use in 

other countries. (Back to Ustalfs key)

Plinthustalfs—These soils have one or more horizons within 150 cm of the

mineral soil surface in which plinthite (firm, iron oxide-rich concentration that 

irreversibly hardens after exposure to repeated wet-dry cycles) either forms a 

continuous phase or constitutes one-half or more of the volume. There are no 

soil series in the United States that are presently classified in this great group, 

but the group is provided for use in other parts of the world. (Back to Ustalfs

key)

Natrustalfs—These soils have a natric (high levels of illuvial clay and sodium)

subsoil horizon. The epipedon is typically 10 to 20 cm thick. The natric horizon 

generally is underlain by a layer with calcium carbonate accumulations at a 

depth between about 25 and 40 cm. Except in a few areas of the Great Plains 

and mountain basins, Natrustalfs are not extensive in the United States. (Back 

to Ustalfs key)

Kandiustalfs—These soils have thick kandic subsoil horizons that have 

relatively high base saturation but a low cation-exchange capacity. Many of 

these soils occur on the older surfaces in warm, humid or semihumid areas. 

Kandiustalfs occur mostly in Africa and South America. They are commonly 

used as rangeland or cropland. (Back to Ustalfs key)

Kanhaplustalfs—These soils have relatively high base saturation but have a 

low cation-exchange capacity. They have a thin to moderately thick kandic 

subsoil horizon. Kanhaplustalfs commonly occur on the older surfaces in 

warm, humid or semihumid areas. They are used for livestock grazing or crop 

production. (Back to Ustalfs key)

Paleustalfs—These are the reddish or red Ustalfs that are on old surfaces. 

Many of them have some plinthite (iron-rich concentration that irreversibly 

hardens after exposure to repeated wet-dry cycles) in their lower horizons. 

Paleustalfs occur in relatively stable landscape positions, their slopes are 

mostly gentle, and their genesis began before the late Pleistocene. In the 

United States, they typically have a horizon with accumulations of calcium 

carbonate in or below the argillic horizon as a result of additions of 

atmospheric carbonates. Secondary lime commonly coats the surfaces of 

peds that have noncalcareous interiors, and the soils may be noncalcareous 

at a depth of less than 200 cm. A few of these soils, near the boundary where 

they join Aridisols, have received enough calcareous dust to have a 

petrocalcic (cemented by calcium carbonate) horizon. A few others, near the 
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-28

boundary where they join Udults or Udalfs, do not have accumulated

carbonates. Before cultivation, the vegetation on the Paleustalfs in the United 

States included a mixture of grasses and woody plants. These soils are 

moderately extensive in the southern part of the Great Plains in the United 

States, and they probably are extensive in Africa and southern Asia. (Back to 

Ustalfs key)

Rhodustalfs—These soils have dark red colors and a solum that is thinner

than that of the Paleustalfs. Rhodustalfs generally formed on erosional 

surfaces or in deposits of late-Pleistocene age. In the United States, the 

vegetation was mostly grass and scattered woody shrubs and trees before the

soils were were cultivated. The parent materials are basic. These soils are 

rare in the United States. (Back to Ustalfs key)

Haplustalfs—These soils have a relatively thin argillic (clay accumulation) 

subsoil horizon. Many of these soils are relatively thin, are reddish to yellowish 

brown, or have a significant decrease in clay content within a depth of 150 cm. 

Haplustalfs are commonly in areas of relatively recent erosional surfaces or 

deposits, most of them late Pleistocene in age. Some of the soils have a 

monsoon climate. Others have two more or less marked dry seasons during 

the year. (Back to Ustalfs key)
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-29

Xeralfs Great Groups

Xeralfs are the Alfisols in regions that have a Mediterranean-type climate. As 

their name implies, they have a xeric moisture regime. They are dry for 

extended periods in summer. In winter in some years, however, moisture 

moves through the soil to deeper layers. Small grain and other winter annuals 

are common crops in nonirrigated areas of Xeralfs. Grapes and olives also are 

common crops where the climate is thermic. With irrigation, a wide variety of 

crops can be grown.

Xeralfs formed not only in the area around the Mediterranean Sea but also in 

parts of South Africa, Chile, western and southern Australia, and western 

United States. The temperature regime is cold to warm. Although Xeralfs are 

not very extensive worldwide, in some local regions they are dominant. In the 

United States, they are moderately extensive in some areas. In areas of the 

warmest and driest Xeralfs, the vegetation before the soils were farmed was a 

mixture of annual grasses, forbs, and woody shrubs. In areas of the coolest 

and moistest Xeralfs, it was coniferous forest.

Xeralfs formed on surfaces of different ages. Some formed on erosional 

surfaces or in deposits of late-Wisconsinan age. Some, such as those in 

Australia, are on old surfaces and have characteristics that probably reflect an 

environment greatly different from the present one. In the oldest Xeralfs, the 

boundary between the surface and subsoil horizons commonly is very abrupt. 

The epipedon of some Xeralfs is hard and massive when dry.
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-30

Xeralfs

Durixeralfs

Natrixeralfs

Fragixeralfs

Plinthoxeralfs

Rhodoxeralfs

Palexeralfs

Haploxeralfs
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-31

Profile of a Xeralf (specifically a Haploxeralf) in California. This soil 

has a paralithic contact below a depth of about 40 inches.

Scale is in inches.
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-32

Key to Great Groups of Xeralfs (Back to key to suborders)

Xeralfs that have: 

1. A duripan (layer cemented by silica) within a depth of

100 cm ------------------------------------------------------------------------ Durixeralfs

Begin measuring depth below any O horizon.

2. A natric (high levels of illuvial clay and sodium) 

horizon ---------------------------------------------------------------------- Natrixeralfs

3. A fragipan (firm and brittle but not cemented layer) within a depth of 

100 cm ---------------------------------------------------------------------- Fragixeralfs

Begin measuring depth below any O horizon.

4. Plinthite making up > 50% of a layer within a depth of 

150 cm ------------------------------------------------------------------- Plinthoxeralfs

Begin measuring depth below any O horizon. Plinthite is a firm, iron oxiderich mass that irreversibly hardens after exposure to repeated wet-dry 

cycles.

5. An argillic (clay accumulation) or kandic (very low cation-exchange

capacity) horizon with dominantly dark red colors ------- Rhodoxeralfs

Specifically, hue is 2.5YR or 10R and value is < 3 moist (and < 1 unit of 

change dry). Colors make up > 50% and are required throughout upper 

100 cm of an argillic or kandic horizon, or the entire argillic or kandic if < 

100 cm thick.

6. Either:

a) A petrocalcic (cemented by calcium carbonate) horizon within a 

depth of 150 cm, OR 

b) An argillic (clay accumulation) or kandic (very low cation-exchange

capacity) horizon that extends to > 150 cm and a clay content that 

does not decrease significantly within a depth of 150 cm, OR

c) An abrupt increase in clay content to an argillic (clay 

accumulation) or kandic (very low cation-exchange capacity) horizon 

with clayey texture (and no root-limiting layer within a depth of 50 

cm) ---------------------------------------------------------------------------- Palexeralfs

Begin measuring depth below any O horizon. For item b, clay content must 

not decrease by > 20% of the maximum in the argillic or kandic horizon, 

unless skeletans (ped surfaces stripped of clay) are present in the layer

and the clay content increases again by > 3% below. See 12th edition of 
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-33

“Keys to Soil Taxonomy” for specific criteria regarding an abrupt clay 

increase (item c above).

7. An argillic (clay accumulation) or kandic (very low cation-exchange

capacity) subsoil horizon ------------------------------------------- Haploxeralfs
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-34

Descriptions of Great Groups of Xeralfs

Durixeralfs—These soils have a duripan (layer cemented by silica) that has 

an upper boundary within a depth of 100 cm but below an argillic (clay 

accumulation) or natric (high levels of illuvial clay and sodium) horizon. These 

soils are known to have formed in California, Idaho, Nevada, Oregon, Chile, 

and Italy, in areas of Pleistocene or earlier volcanism. Some of the Durixeralfs 

in the United States appear to be on very old surfaces and have complex 

horizons that indicate polygenesis. The oldest soils are commonly reddish and 

have kaolinitic mineralogy. Other Durixeralfs appear to have formed in latePleistocene sediments and are yellowish brown and have less weathered 

clays. The duripan commonly has an upper boundary about 50 cm below the 

soil surface and has very coarse polyhedrons, the tops and sides of which are 

coated with opal or chalcedony. In undisturbed areas of Durixeralfs in the 

United States, a microrelief of about 10 to 100 cm or more is common. The 

microrelief is mainly the result of differences in the thickness of horizons that 

are above the duripan. In the United States, Durixeralfs are moderately 

extensive only in California and Idaho. (Back to Xeralfs key)

Natrixeralfs—These soils have a natric (high levels of illuvial clay and 

sodium) subsoil horizon. They are not extensive. They support sparse 

vegetation that commonly consists of salt-tolerant grasses and forbs. (Back to 

Xeralfs key)

Fragixeralfs—These soils have a fragipan (firm and brittle but not cemented 

layer) with an upper boundary within a depth of 100 cm. Commonly, these 

soils have a brown argillic (clay accumulation) or kandic (very low cationexchange capacity) subsoil horizon, a gray and red mottled color pattern 

(redoximorphic features), and a perched water table that is seasonally above 

the pan. Above the pan, some Fragixeralfs have a thin eluvial horizon with 

peds that have gray clay depletions. Most Fragixeralfs in the United States are 

on gentle or moderate slopes and formed, at least in part, in silty or loamy 

late-Pleistocene deposits. Some are strongly sloping. Temperature regimes 

are mesic or frigid. In the United States, the native vegetation is primarily 

coniferous forest. Fragixeralfs are of small extent in Idaho and Oregon. (Back 

to Xeralfs key)

Plinthoxeralfs—These soils have one or more horizons within 150 cm of the 

mineral soil surface in which plinthite (firm, iron oxide-rich concentration that 

irreversibly hardens after exposure to repeated wet-dry cycles) either forms a 

continuous phase or constitutes one-half or more of the volume. There are 

few areas of these soils in the United States, but the soils are moderately 

extensive in other parts of the world. (Back to Xeralfs key)

Rhodoxeralfs—These are the more or less dark red Xeralfs that formed in 

areas of limestone, basalt, and other highly basic parent materials. As a 
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-35

group, these soils are remarkably uniform in virtually all properties, except

depth to bedrock. A few that receive carbonates may have a calcic (calcium 

carbonate accumulation) or petrocalcic (cemented by calcium carbonate) 

horizon below the argillic (clay accumulation) or kandic (very low cationexchange capacity) horizon. Rhodoxeralfs are rare in the United States. (Back 

to Xeralfs key)

Palexeralfs—These soils have a petrocalcic (cemented by calcium 

carbonate) subsoil horizon or an argillic (clay accumulation) or kandic (very 

low cation-exchange capacity) subsoil horizon that is thick or that has, at its 

upper boundary, both a clayey texture and a large increase in clay content. 

Many of these soils have some plinthite (firm, iron oxide-rich concentration

that irreversibly hardens after exposure to repeated wet-dry cycles) in their 

lower horizons, but this feature is rare in the United States. Palexeralfs are in 

relatively stable landscape positions on gentle slopes, and most began their 

genesis before the late Pleistocene. During pluvial periods of the Pleistocene, 

carbonates appear to have been almost completely removed from the argillic 

or kandic horizon of most of these soils, but some of the soils appear to have 

been recalcified later. Most Palexeralfs formed in acid or in moderately basic 

parent materials, but some formed in materials as basic as basalt. The native 

vegetation on the warmest Palexeralfs in the United States was a mixture of 

annual grasses, forbs, and woody shrubs. The native vegetation on the 

coolest Palexeralfs was mostly a coniferous forest. Palexeralfs are moderately 

extensive in the United States as well as in other parts of the world. (Back to 

Xeralfs key)

Haploxeralfs—These soils have an argillic (clay accumulation) or kandic 

(very low cation-exchange capacity) subsoil horizon that is relatively thin, has 

a clear or gradual upper boundary, or has a loamy particle-size class 

throughout. Many Haploxeralfs formed in late-Pleistocene deposits or on 

erosional surfaces of that age. Their parent materials may be either acidic or 

basic. These soils are extensive in the areas of Xeralfs in the western United 

States. (Back to Xeralfs key)
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-36

Udalfs Great Groups

Udalfs are the more or less freely drained Alfisols that have seasonally welldistributed precipitation (udic moisture regime) and cold to warm temperature 

regimes. These soils are principally but not entirely in areas of latePleistocene deposits and erosional surfaces of about the same age. Some of 

the Udalfs that are on the older surfaces are underlain by limestone or other 

calcareous sediments.

Udalfs are very extensive in the United States and Western Europe. All of 

them are believed to have supported forest vegetation at some time during 

development. Most Udalfs with a mesic or warmer temperature regime have 

or had deciduous forest vegetation, and many with a frigid temperature regime 

have or had mixed coniferous and deciduous forest vegetation.

Many Udalfs have been cleared of trees and are intensively farmed. As a 

result of erosion, many now have subsoil material exposed at the surface and 

incorporated into plow layers. Other Udalfs are on stable surfaces and retain 

most of their original surface layers. Normally, the undisturbed soils have a 

thin surface layer darkened by humus. A few Udalfs have a natric (high levels 

of illuvial clay and sodium) horizon. Others have a fragipan (firm and brittle but 

not cemented layer).
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-37

Udalfs

Natrudalfs

Ferrudalfs

Fraglossudalfs

Fragiudalfs

Kandiudalfs

Kanhapludalfs

Paleudalfs

Rhodudalfs

Glossudalfs

Hapludalfs
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-38

Profile of a Udalf (specifically a Hapludalf). Note the light-colored 

E horizon that extends to a depth of about 10 inches. Clay 

content of the argillic horizon decreases noticeably below a 

depth of about 3 feet.

(Back to Udalfs)
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-39

Key to Great Groups of Udalfs (Back to key to suborders)

Udalfs that have:

1. A natric (high levels of illuvial clay and sodium) 

horizon ------------------------------------------------------------------------ Natrudalfs

2. Both:

a) A glossic (degraded argillic) horizon AND

b) Iron-enriched nodules in the subsoil --------------------------- Ferrudalfs

The nodules are 2.5-30 cm in diameter, have some degree of cementation 

(at least an extremely weak class), and have exteriors that have redder 

hue or higher chroma as compared to interiors.

3. Both:

a) A glossic (degraded argillic) horizon AND

b) A fragipan (firm and brittle but not cemented layer) within a depth 

of 100 cm -------------------------------------------------------------- Fraglossudalfs

Begin measuring depth below any O horizon. 

4. A fragipan (firm and brittle but not cemented layer) within a depth of 

100 cm ----------------------------------------------------------------------- Fragiudalfs

Begin measuring depth below any O horizon. 

5. A kandic (very low cation-exchange capacity) horizon in which 

clay content does not decrease significantly within a depth of 

150 cm ---------------------------------------------------------------------- Kandiudalfs

Clay content must not decrease by > 20% of the maximum in the kandic 

horizon, unless skeletans (ped surfaces stripped of clay) are present in the 

layer and the clay content increases again by > 3% below. Kandiudalfs 

cannot have a root-limiting layer within a depth of 150 cm. Begin 

measuring depth below any O horizon.

6. A kandic (very low cation-exchange capacity) horizon with a 

significant decrease in clay content within a depth of

150 cm ------------------------------------------------------------------- Kanhapludalfs
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-40

7. All:

a) No root-limiting layer within a depth of 150 cm, AND

b) A clay content that does not decrease significantly within a depth 

of 150 cm, AND

c) Either an argillic (clay accumulation) horizon with hue or 7.5YR or 

redder colors in at least part OR, if the temperature regime is frigid, 

an argillic horizon (any color) that is overlain by a glossic (degraded 

argillic) horizon ------------------------------------------------------------ Paleudalfs

For items a and b, begin measuring depth below any O horizon. Clay 

content must not decrease by > 20% of the maximum in the argillic

horizon, unless skeletans (ped surfaces stripped of clay) are present in the 

layer and the clay content increases again by > 3% below. See 12th 

edition of “Keys to Soil Taxonomy” for specific details regarding required 

colors for the argillic horizon (item c). For frigid soils (item c) begin 

measuring below any O horizon or mantle with vitric volcanic materials, if 

present. Some layer above the argillic horizon must have texture finer than 

loamy fine sand. The glossic criterion is waved if there is interfingering of 

albic materials into the argillic horizon.

8. An argillic (clay accumulation) horizon with dominantly dark red 

colors ------------------------------------------------------------------------ Rhodudalfs

Specifically, hue is 2.5YR or 10R and value is < 3 moist (and < 1 unit of 

change dry). Colors make up > 50% and are required throughout upper 

100 cm of argillic horizon, or the entire argillic horizon if < 100 cm thick.

9. A glossic (degraded argillic) horizon ---------------------------- Glossudalfs

10.An argillic (clay accumulation) horizon with a significant decrease in 

clay content within a depth of 150 cm ----------------------------- Hapludalfs
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-41

Descriptions of Great Groups of Udalfs

Natrudalfs—These soils have a natric (high levels of illuvial clay and sodium)

subsoil horizon. Because the natric horizon is slowly permeable, perched 

ground water is common for short periods and the natric horizon commonly 

has redoximorphic features. Some of the soils were forested. Some supported 

open forests, and some supported grass. Most of the soils have been cleared 

and are used as cropland or pasture. These soils are not as productive as 

most Udalfs, but they generally are farmed if they occur in very small areas 

and if the associated soils are productive. These soils are of small extent in 

the United States. (Back to Udalfs key)

Ferrudalfs—These soils have both a glossic (degraded argillic) subsoil 

horizon and an argillic (clay accumulation) or kandic (very low cationexchange capacity) subsoil horizon that has been partially cemented by iron. 

In some areas, these soils formed under heath vegetation. The heath 

vegetation may have succeeded a forest following many years of collection 

and removal of forest litter. Under the heath, which is efficient in complexing 

iron and aluminum, the upper part of the argillic horizon was destroyed and 

fragments of this horizon became coated and weakly cemented with iron. 

Some Ferrudalfs have a fragipan (firm and brittle but not cemented layer) at 

some depth, commonly about 100 cm. Ferrudalfs are not known to occur in 

the United States and are rare elsewhere. (Back to Udalfs key)

Fraglossudalfs—These soils have both a fragipan (firm and brittle but not 

cemented layer) within a depth of 100 cm and a glossic (degraded argillic) 

subsoil horizon. They are not extensive in the United States. They are more 

extensive in Europe. (Back to Udalfs key)

Fragiudalfs—These soils have a fragipan (firm and brittle but not cemented 

layer) within a depth of 100 cm. They commonly have an argillic (clay 

accumulation) or cambic (minimal soil development) subsoil horizon above the 

fragipan. Redoximorphic features (gray and red mottled color pattern) are in 

many pedons, starting at a depth 50 to 100 cm. Ground water is perched 

seasonally above the fragipan, and a thin eluvial horizon commonly is directly 

above the fragipan. Most Fragiudalfs in the United States are on gentle slopes 

and formed, at least in part, in silty or loamy deposits. The deposits are largely 

of late-Pleistocene age. The duripan formed in an older buried soil in some 

areas. A duripan seems to form if the burial was to a depth of about 50 to 75 

cm. Temperature regimes are mostly cold to warm. In the United States, the 

native vegetation on these soils was primarily a broadleaf deciduous forest. 

(Back to Udalfs key)

Kandiudalfs—These soils have a thick kandic (very low cation-exchange 

capacity) subsoil horizon in which the content of clay does not decrease 

significantly from its maximum within a depth of 150 cm. These soils are 150 
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-42

cm or more deep to any root-limiting layer, such as a densic, lithic, paralithic, 

or petroferric contact. The base of the kandic horizon is commonly more than 

150 cm below the soil surface and in some areas is more than 200 cm below. 

Most of these soils have a thermic or warmer soil temperature regime. The 

natural vegetation was forest, mostly deciduous, but many of the soils are now 

cleared and used as cropland. (Back to Udalfs key)

Kanhapludalfs—These soils have a kandic (very low cation-exchange 

capacity) subsoil horizon in which the content of clay decreases significantly 

from its maximum amount within a depth of 150 cm. Most of these soils have 

a thermic or warmer soil temperature regime. The natural vegetation was 

forest, mostly deciduous, but many of the soils are now cleared and used as 

cropland. (Back to Udalfs key)

Paleudalfs—These soils have a thick solum. Some have an argillic (clay 

accumulation) subsoil horizon that shows evidence of destruction in the form 

of a glossic horizon. Paleudalfs are on relatively stable surfaces. Most of them 

are older than the Wisconsinan Glaciation. The time of soil formation dates 

from the Sangamon interglacial period or earlier. Base saturation commonly is 

lower than that in many other Alfisols. Before cultivation, most Paleudalfs in 

the United States had a vegetation of mixed deciduous hardwood forest. 

(Back to Udalfs key)

Rhodudalfs—These are the dark red Udalfs that have a solum that is thinner 

than that of Paleudalfs. Because their parent materials are mostly basic, they 

are relatively fertile soils. Rhodudalfs are rare in the United States. (Back to 

Udalfs key)

Glossudalfs—These soils have an argillic (clay accumulation) subsoil horizon

that shows evidence of destruction in the form of a glossic horizon. The 

glossic horizon extends through the argillic horizon in some of these soils. 

Glossudalfs are more extensive in Europe than in the United States. (Back to 

Udalfs key)

Hapludalfs—These soils have an argillic (clay accumulation) subsoil horizon

that typically extends to a depth of less than 50 cm. In many areas, the argillic 

horizon is at a depth of less than 100 cm. Where undisturbed, these soils 

generally have a thin, very dark brown A horizon that is 5 to 10 cm thick, over 

a lighter-colored brownish eluvial horizon. The eluvial horizon grades into a 

finer textured argillic horizon, generally at a depth of about 30 to 45 cm in 

loamy materials. Because these soils have been cultivated extensively, many 

of those on slopes have lost their eluvial horizons through erosion. Hapludalfs 

formed principally in late-Pleistocene deposits or on a surface of comparable 

age. Temperature regimes are mesic or thermic. Hapludalfs are extensive in 

the northeastern States, excluding New England, and in Europe, excluding 
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-43

most of Scandinavia. In the United States, the vegetation was deciduous 

broadleaf forest but these soils are now mostly farmed. (Back to Udalfs key)
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-44

Andisols Order

Andisols are relatively young soils, mostly of volcanic origin, and are 

characterized by unique minerals with poorly organized crystalline 

structure.

General Characteristics

The key distinguishing characteristic of the Andisols is the presence of andic 

soil properties within a significant portion of at least the upper 60 cm of the 

soil. In general, the soils with andic properties are characterized as either: 

1) weakly weathered soils with relatively high contents of volcanic glass and 

only low or moderate amounts of the minerals that result from weathering of 

the glass, or 2) moderately weathered soils that are rich in short-range order 

(poorly crystalline) weathering by-products and have lesser amounts of glass 

remaining. However, Alfisols can have a wide variety of surface and subsoil 

horizon types. 

Environment and Processes

Most Andisols formed in volcanic ejecta, such as volcanic ash, pumice, 

cinders, and lava. The dominant processes in most Andisols are the 

weathering and mineral transformation of primary alumino-silicate minerals 

(commonly glassy) that form short-range-order (poorly crystalline) minerals, 

such as allophane, imogolite, and ferrihydrite. These short-range-order 

minerals are responsible for andic soil properties. Some Andisols formed in 

non-volcanic parent materials. These soils formed in environments with 

relatively high amounts of soil organic matter, particularly in cool and moist

areas. These environments promote the formation of organo-metal 

compounds, which can also result in andic soil properties. 

Andisols are generally considered highly productive soils. Their unique 

mineral constituents typically result in high water-holding capacities. These 

soils are mostly very friable and have low bulk density. In areas that 

experience periodic volcanic eruptions, additions of fresh tephra can resupply 

nutrients and maintain favorable fertility. One negative aspect of the unique 

mineral make up of Andisols is a propensity to adsorb phosphorus, making it 

unavailable to plants.

Location

Andisols occur in a wide range of environments. They may have any soil 

moisture or temperature regime. They can occupy any position on the 

landscape and can occur at any elevation. Globally, Andisols are the leastextensive soil order and occupy only about 1% of the ice-free land area. They 

are concentrated along the Pacific Ring of Fire, a zone of active volcanoes 
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-45

that stretches along the western coast of North, Central, and South America;

across the Aleutian Islands; along the Russia’s east coast; through Japan, the 

Philippine Islands, and Indonesia; and across Papua New Guinea, the 

Solomon Islands, and Vanuatu and other Pacific Islands to New Zealand. 

Andisols also occur in other areas, including the Rift Valley of Africa, the west 

coast of Italy, the Hawaiian Islands, the West Indies, Iceland, the Canary 

Islands, and other islands. In the United States, they are concentrated mostly 

in the Pacific Northwest, Alaska, and Hawaii.

Profile of an Andisol.

(Back to Key to Soil Orders)
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-46

Andisols by suborder in the United States. Note: This map was 

produced before the introduction of the Gelands suborder. Some 

areas shown as Cryands in Alaska are now Gelands.

Global distribution of Andisols.
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-47

Andisols Suborders

Classification of Andisols

In the definitions of the suborders, emphasis is primarily given to soil moisture 

and temperature regimes (Aquands, Torrands, Xerands, Ustands, Udands, 

Gelands, and Cryands). However, in one of the suborders (Vitrands) emphasis 

is placed on a water-holding capacity that is low relative to that of most other 

Andisols.

The great group level reflects a combination of important properties, including 

the presence of a melanic (dark and humus-rich with andic properties)

epipedon, a placic (cemented by iron and organic matter) horizon, or other 

cemented layers; soil moisture or temperature regimes; patterns of soil 

saturation; and water-holding capacity. 

The eight suborders are:

1. Aquands—wet Andisols (aquic conditions in the upper part)

2. Gelands—very cold Andisols that lack permafrost (gelic temperature 

regime)

3. Cryands—cold Andisols (cryic temperature regime) 

4. Torrands—arid Andisols

5. Xerands—moderately dry Andisols (limited moisture that is supplied in 

winter and Mediterranean-type climate)

6. Vitrands—Andisols with a relatively low water-holding capacity

7. Ustands—moderately dry Andisols (limited moisture)

8. Udands—Andisols of humid regions with well-distributed rainfall
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-48

Andisols

Aquands

Gelands

Cryands

Torrands

Xerands

Vitrands

Ustands

Udands
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-49

Key to Suborders of Andisols

Andisols that have: 

1. A seasonal high water table within a depth of 50 cm ----------- Aquands

A histic epipedon (wet, organic surface layer) and/or redoximorphic 

features (gray and red mottled color pattern) are evidence of a seasonal 

high water table. See 12th edition of “Keys to Soil Taxonomy” for specific 

details regarding color, depth, and location of redoximorphic features in 

Aquands. Artificially drained sites are included in Aquands.

2. Very cold average soil temperature ------------------------------------ Gelands

These soils have a gelic soil temperature regime.

3. Cold average soil temperature ------------------------------------------ Cryands

These soils have a cryic soil temperature regime.

4. Inadequate soil moisture for crop growth -------------------------- Torrands

These soils have an aridic (torric) soil moisture regime.

5. A Mediterranean-type climate -------------------------------------------- Xerands

These soils have a xeric soil moisture regime (cool and moist in winter and 

warm and dry in summer).

6. A relatively low water-holding capacity within the upper 

60 cm ----------------------------------------------------------------------------- Vitrands

For Vitrands, water held at 1500 kPa tension is < 15% (by weight) on airdry samples and < 30% on undried samples. See 12th edition of “Keys to 

Soil Taxonomy” for specific details regarding location within the profile of 

this property.

7. Somewhat limited soil moisture available for crop 

growth ---------------------------------------------------------------------------- Ustands

These soils have an ustic soil moisture regime. Moisture is limited, but 

available, during portions of the growing season.

8. Seasonally well-distributed precipitation ----------------------------- Udands

These soils have a udic soil moisture regime.
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-50

Aquands Great Groups

Aquands are the Andisols that are saturated close to the surface by ground

water for a long enough period during the year to become devoid of oxygen 

(aquic conditions). Commonly, these soils have dark-colored, humus-rich 

surface horizons that meet the requirements for either a histic (wet, organic 

surface layer), umbric (humus-rich with low base saturation), or mollic (rich in 

humus and bases) epipedon. They occur in the lower landscape positions and 

support forest or grass vegetation. Some have been drained and are used for 

small grain or grass for livestock. Aquands occur in the Pacific Northwest and 

in other areas that have a volcanic influence.

Aquands

Gelaquands

Cryaquands

Placaquands

Duraquands

Vitraquands

Melanaquands

Epiaquands

Endoaquands
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-51

Key to Great Groups of Aquands (Back to key to suborders)

Aquands that have:

1. Very cold average soil temperature ------------------------------ Gelaquands

These soils have a gelic soil temperature regime.

2. Cold average soil temperature ------------------------------------- Cryaquands

These soils have a cryic soil temperature regime.

3. A placic (cemented by iron and organic matter) horizon within a 

depth of 100 cm -------------------------------------------------------- Placaquands

The placic horizon must be > 50% of the pedon. Begin measuring below 

any organic surface layer (O), unless the O horizon has andic soil 

properties.

4. A cemented layer within a depth of 100 cm ------------------- Duraquands

The cemented layer must be > 75% of the pedon. Begin measuring below 

any organic surface layer (O), unless the O horizon has andic soil 

properties.

5. A relatively low water-holding capacity within the upper 

60 cm ------------------------------------------------------------------------ Vitraquands

For Vitraquands, water held at 1500 kPa tension is < 15% (by weight) on 

air-dry samples and < 30% on undried samples. See 12th edition of “Keys 

to Soil Taxonomy” for specific details regarding location within the profile of 

this property.

6. A melanic (dark and humus-rich with andic properties)

epipedon --------------------------------------------------------------- Melanaquands

7. Episaturation (perched water table) ------------------------------ Epiaquands

8. Endosaturation (saturated throughout the profile) ------- Endoaquands
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-52

Descriptions of Great Groups of Aquands

Gelaquands—These are the more or less poorly drained Andisols of very 

cold regions. They are not extensive. These soils have a gelic temperature 

regime but lack permafrost within the soil profile. They tend to be in landscape 

positions that are subject to wide variations in summer high temperatures and 

winter low temperatures, which result in mean annual soil temperatures of 0 

degrees C or less, but without permafrost. (Back to Aquands key)

Cryaquands—These are the more or less poorly drained Andisols of cold 

regions. They are not extensive. These soils are mostly in the western part of 

North America and the northeastern part of Asia above 49 degrees north 

latitude and in mountains south of that latitude. Most of these soils formed 

under grassy meadow or coniferous forest vegetation. Characteristically, 

Cryaquands have a thin O horizon, a mollic (rich in humus and bases) or 

umbric (humus-rich with low base saturation) epipedon, and a cambic 

(minimal soil development) subsoil horizon with many redoximorphic 

concentrations (reddish mottled color pattern). In the United States, the 

Cryaquands generally developed in late-Pleistocene or Holocene deposits. 

(Back to Aquands key)

Placaquands—These are the more or less poorly drained Andisols that have, 

in half or more of each pedon, a placic (cemented by iron and organic matter) 

horizon within 100 cm of the mineral soil surface or of the upper boundary of 

an organic layer that has andic soil properties, whichever is shallower. Most of 

these soils formed under coniferous forest vegetation. Characteristically, 

Placaquands have a thin O horizon and an umbric (humus-rich with low base 

saturation) or histic (wet, organic surface layer) epipedon above the placic 

horizon. They are rare in the United States. (Back to Aquands key)

Duraquands—These are the more or less poorly drained Andisols that have, 

in 75% or more of each pedon, a cemented horizon (duripan or other 

cemented layer) that has its upper boundary within 100 cm of the mineral soil 

surface or of the top of an organic layer with andic soil properties, whichever 

is shallower. Duraquands are rare in the United States but known to occur in 

the Pacific Northwest. (Back to Aquands key)

Vitraquands—These are the more or less poorly drained Andisols with 

relatively coarse textures. Most of these soils have no slowly permeable layer 

that causes unsaturated layers to underlie saturated layers within a depth of 

200 cm. Many of these soils formed under grassy or brushy meadow 

vegetation, but some formed under coniferous forest vegetation. 

Characteristically, Vitraquands have a thin O horizon, a mollic (rich in humus 

and bases) or umbric (humus-rich with low base saturation) epipedon, and a 

cambic (minimal soil development) subsoil horizon with many redoximorphic 
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-53

concentrations (reddish mottled color pattern). In the United States, most 

Vitraquands developed in Holocene deposits. (Back to Aquands key)

Melanaquands—These are the more or less poorly drained Andisols with a 

melanic (dark and humus-rich with andic properties) epipedon. Most of these 

soils formed under forest vegetation. Characteristically, Melanaquands have a 

thin O horizon, a thick melanic (dark and humus-rich with andic properties) 

epipedon, and a cambic (minimal soil development) subsoil horizon with many 

redoximorphic concentrations (reddish mottled color pattern). (Back to 

Aquands key)

Epiaquands—These are the more or less poorly drained Andisols with a 

slowly permeable layer that results in saturated layers overlying unsaturated 

layers within a depth of 200 cm. Most of these soils formed under grassy 

meadow vegetation, but some formed under coniferous forest vegetation. 

Characteristically, Epiaquands have a thin O horizon, a mollic (rich in humus 

and bases) or umbric (humus-rich with low base saturation) epipedon, and a 

cambic (minimal soil development) subsoil horizon with many redoximorphic 

concentrations (reddish mottled color pattern). In the United States,

Epiaquands generally developed in late-Pleistocene or Holocene deposits. 

(Back to Aquands key)

Endoaquands—These are the more or less poorly drained Andisols that are 

saturated in all layers to a depth of 200 cm. Most of these soils formed under 

grassy meadow vegetation, but some formed under coniferous forest 

vegetation. Characteristically, Endoaquands have a thin O horizon, a mollic 

(rich in humus and bases) or umbric (humus-rich with low base saturation)

epipedon, and a cambic (minimal soil development) subsoil horizon with many 

redoximorphic concentrations (reddish mottled color pattern). In the United 

States, Endoaquands generally developed in late-Pleistocene or Holocene 

deposits. (Back to Aquands key)
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-54

Gelands Great Groups

Gelands are the Andisols of very cold regions. They have a gelic temperature 

regime but lack permafrost within the soil profile. These soils tend to be in 

landscape positions that are subject to wide variations in summer high 

temperatures and winter low temperatures, which result in mean annual soil 

temperatures of 0 degrees C or less, but without permafrost.

Gelands Vitrigelands
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-55

Key to Great Groups of Gelands (Back to key to suborders)

Gelands that have:

1. A relatively low water-holding capacity ------------------------- Vitrigelands

Currently, this is the only great group of Gelands, so all Gelands currently 

are classified as Vitrigelands. Specific criteria are not developed.
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-56

Descriptions of the Great Group of Gelands

Vitrigelands—Currently, this is the only great group of Gelands recognized.

These soils tend to be coarse in texture and have a relatively low waterholding capacity. (Back to Gelands key)
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-57

Cryands Great Groups

Cryands are the more or less well drained Andisols of cold regions. These 

soils are moderately extensive. They formed in the western part of North 

America and the northeastern part of Asia above 49 degrees north latitude 

and in mountains south of that latitude. Most of the soils formed under 

coniferous forest vegetation. Characteristically, Cryands have a thin O horizon 

and a cambic (minimal soil development) subsoil horizon. The epipedon is 

commonly the simple ochric (typically thin and/or light-colored) or melanic 

(dark and humus-rich with andic properties). In the United States, Cryands 

generally developed in late-Pleistocene or Holocene deposits. 

Cryands

Duricryands

Hydrocryands

Melanocryands

Fulvicryands

Vitricryands

Haplocryands
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-58

Key to Great Groups of Cryands (Back to key to suborders)

Cryands that have:

1. A cemented layer within a depth of 100 cm ------------------- Duricryands

The cemented layer must be > 75% of the pedon. Begin measuring below 

any organic surface layer (O), unless the O horizon has andic soil 

properties.

2. A very high water-holding capacity within the upper 

100 cm ------------------------------------------------------------------- Hydrocryands

For Hydrocryands, water held at 1500 kPa tension is > 100% (by weight)

on undried samples. See 12th edition of “Keys to Soil Taxonomy” for 

specific details regarding location within the profile of this property.

3. A melanic (dark and humus-rich with andic properties) 

epipedon -------------------------------------------------------------- Melanocryands

4. A melanic-like epipedon (but lighter in color) --------------- Fulvicryands

5. A relatively low water-holding capacity within the upper 

60 cm ------------------------------------------------------------------------ Vitricryands

For Vitraquands, water held at 1500 kPa tension is < 15% (by weight) on 

air-dry samples and < 30% on undried samples. See 12th edition of “Keys 

to Soil Taxonomy” for specific details regarding location within the profile of 

this property.

6. A cambic (minimal soil development) horizon ------------- Haplocryands
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-59

Descriptions of Great Groups of Cryands

Duricryands—These are Cryands that have, in 75% or more of each pedon, 

a cemented horizon (duripan or other cemented layer) that has its upper 

boundary within 100 cm of either the mineral soil surface or the top of an 

organic layer with andic soil properties, whichever is shallower. 

Characteristically, Duricryands have a thin O horizon and a cambic (minimal 

soil development) subsoil horizon above the cemented horizon. In the United 

States, Duricryands generally developed in late-Pleistocene or Holocene 

deposits. Most formed under coniferous forest vegetation. (Back to Cryands

key)

Hydrocryands—These soils have the capacity to absorb and store large 

amounts of water. On undried samples, they have 1500 kPa (wilting point) 

water retention of 100% (by weight) or more throughout 60% or more of the 

upper part of the pedon. Hydrocryands have not been recognized in the 

United States. Most formed under coniferous forest vegetation. (Back to 

Cryands key)

Melanocryands—These soils have a melanic (dark and humus-rich with 

andic properties) epipedon. Most have a cambic (minimal soil development) 

subsoil horizon. Melanocryands have not been recognized in the United 

States. (Back to Cryands key)

Fulvicryands—These soils have a surface layer that meets the depth, 

thickness, and organic-carbon requirements for a melanic epipedon but is 

lighter in color. Characteristically, Fulvicryands have a thin O horizon, an 

umbric (humus-rich with low base saturation) epipedon, and a cambic 

(minimal soil development) horizon. The Fulvicryands in the United States 

generally developed in late-Pleistocene or Holocene deposits. Most formed 

under coniferous forest vegetation. (Back to Cryands key)

Vitricryands—These soils have relatively coarse textures and a low waterholding capacity. They have 1500 kPa water retention of less than 15% (by 

weight) on air-dried samples and less than 30% on undried samples 

throughout 60% or more of the upper 60 cm of the andic materials. 

Characteristically, Vitricryands have a thin O horizon, an ochric (typically thin 

and/or light-colored), mollic (rich in humus and bases) or umbric (humus-rich 

with low base saturation) epipedon, and a cambic (minimal soil development) 

subsoil horizon. The Vitricryands in the United States generally developed in 

Holocene deposits. Most formed under coniferous forest vegetation. (Back to 

Cryands key)

Haplocryands—Characteristically, Haplocryands have a thin O horizon, an 

umbric (humus-rich with low base saturation) epipedon, and a cambic 

(minimal soil development) subsoil horizon. The Haplocryands in the United 
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-60

States generally developed in late-Pleistocene or Holocene deposits. Most 

formed under coniferous forest vegetation. (Back to Cryands key)
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-61

Torrands Great Groups

Torrands are the more or less well drained Andisols of dry regions. These 

soils are not extensive. Some formed in the western part of North America, 

and some are known to occur in Hawaii and other Pacific Islands. Most of the 

soils formed under grassy or shrub vegetation. Characteristically, Torrands 

have an ochric (typically thin and/or light-colored) or mollic (rich in humus and 

bases) epipedon and a cambic (minimal soil development) subsoil horizon. 

Some have a duripan (layer cemented by silica) or a petrocalcic (cemented by 

calcium carbonate) horizon in the subsoil. In the United States, the Torrands

generally developed in late-Pleistocene or Holocene deposits.

Torrands

Duritorrands

Vitritorrands

Haplotorrands
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-62

Key to Great Groups of Torrands (Back to key to suborders)

Torrands that have:

1. A cemented horizon within a depth of 100 cm --------------- Duritorrands

The cemented layer must be > 75% of the pedon. Begin measuring below 

any organic surface layer (O), unless the O horizon has andic soil 

properties.

2. A relatively low water-holding capacity within the upper 

60 cm ----------------------------------------------------------------------- Vitritorrands

For Vitritorrands, water held at 1500 kPa tension is < 15% (by weight) on 

air-dry samples. See 12th edition of “Keys to Soil Taxonomy” for specific 

details regarding location within the profile of this property.

3. A cambic (minimal soil development) horizon ------------ Haplotorrands
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-63

Descriptions of Great Groups of Torrands

Duritorrands—These soils have, in 75% or more of each pedon, a cemented 

horizon (duripan or other cemented layer) that has its upper boundary within 

100 cm of either the mineral soil surface or the top of an organic layer with 

andic soil properties, whichever is shallower. Characteristically, these soils 

have an ochric (typically thin and/or light-colored) or mollic (rich in humus and 

bases) epipedon and a cambic (minimal soil development) subsoil horizon

above the cemented horizon. The Duritorrands in the United States generally 

developed in late-Pleistocene deposits. Most formed under grass or shrub 

vegetation, and some have widely spaced trees. (Back to Torrands key)

Vitritorrands—These soils have relatively coarse texture and a low waterholding capacity. They have, on air-dried samples, a 1500 kPa (wilting point) 

water retention of less than 15% (by weight) throughout 60% or more of the 

thickness within 60 cm of the mineral soil surface or the top of an organic 

layer with andic soil properties, whichever is shallower. Characteristically, 

Vitritorrands have an ochric (typically thin and/or light-colored) or mollic (rich in 

humus and bases) epipedon and a cambic (minimal soil development) subsoil 

horizon. The Vitritorrands in the United States generally developed in 

Holocene deposits. Most formed under grass or shrub vegetation, and some 

have widely spaced trees. (Back to Torrands key)

Haplotorrands—Characteristically, Haplotorrands have an ochric (typically 

thin and/or light-colored) or mollic (rich in humus and bases) epipedon and a 

cambic (minimal soil development) subsoil horizon. The Haplotorrands in the 

United States generally developed in late-Pleistocene deposits. Most formed 

under grass or shrub vegetation, and some have widely spaced trees. (Back 

to Torrands key)
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-64

Xerands Great Groups

Xerands are in regions that have a Mediterranean-type climate. As their name 

implies, these soils have a xeric moisture regime. They are dry for extended 

periods in summer. In winter, in some if not most years, moisture moves 

through the soils to the deeper layers. Most of the Xerands in the United 

States are in Oregon, Washington, Idaho, and California, formed under 

coniferous forest vegetation, and have cool or moderate average soil 

temperatures. Some formed under grass or shrub vegetation. 

Characteristically, Xerands have an ochric (typically thin and/or light-colored) 

or mollic (rich in humus and bases) epipedon and a cambic (minimal soil 

development) subsoil horizon. Some have a melanic (dark and humus-rich 

with andic properties) epipedon. Most of the Xerands in the United States 

developed in late-Pleistocene or Holocene deposits.

Xerands

Vitrixerands

Melanoxerands

Haploxerands
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-65

Profile of a Xerand in Turkey. This soil is specifically a Vitrixerand and formed 

in volcanic ash deposits.

Back to Xerands
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-66

Key to Great Groups of Xerands (Back to key to suborders)

Xerands that have:

1. A relatively low water-holding capacity within the upper 

60 cm ------------------------------------------------------------------------ Vitrixerands

For Vitrixerands, water held at 1500 kPa tension is < 15% (by weight) on 

air-dry samples and < 30% on undried samples. See 12th edition of “Keys 

to Soil Taxonomy” for specific details regarding location within the profile of 

this property.

2. A melanic (dark and humus-rich with andic properties) 

epipedon -------------------------------------------------------------- Melanoxerands

3. A cambic (minimal soil development) horizon ------------- Haploxerands
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-67

Descriptions of Great Groups of Xerands

Vitrixerands—These soils have relatively coarse textures and a low waterholding capacity. They are relatively young soils that, in many areas, occur 

near volcanoes. These soils are moderately extensive in the United States 

and occur mostly in Oregon, Washington, and Idaho. They formed mainly 

under coniferous forest vegetation. Commonly, Vitrixerands have an ochric 

(typically thin and/or light-colored) or mollic (rich in humus and bases) 

epipedon and a cambic (minimal soil development) subsoil horizon. Most of 

the Vitrixerands in the United States developed in Holocene or latePleistocene deposits. (Back to Xerands key)

Melanoxerands—These soils have a melanic (dark and humus-rich with 

andic properties) epipedon and commonly have a cambic (minimal soil 

development) subsoil horizon. Most of the Melanoxerands in the United States 

developed in late-Pleistocene or Holocene deposits. Most formed under forest 

or savanna vegetation. (Back to Xerands key)

Haploxerands—These soils have a thin O horizon, an ochric (typically thin 

and/or light-colored) or mollic (rich in humus and bases) epipedon, and a 

cambic (minimal soil development) subsoil horizon. The Haploxerands in the 

United States generally developed in late-Pleistocene or Holocene deposits. 

Most formed under coniferous forest vegetation, but some formed under grass 

or shrub vegetation. (Back to Xerands key)
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-68

Vitrands Great Groups

Vitrands are the more or less well drained, coarse textured Andisols with a 

relatively low water-holding capacity. These are relatively young soils that 

occur mostly near volcanoes. Most of the Vitrands in the United States are in 

Oregon, Washington, and Idaho and formed mainly under coniferous forest 

vegetation. Characteristically, Vitrands have an ochric (typically thin and/or 

light-colored), mollic (rich in humus and bases), umbric (humus-rich with low 

base saturation), or melanic (dark and humus-rich with andic properties) 

epipedon and a cambic (minimal soil development) subsoil horizon. Most of 

the Vitrands in the United States developed in Holocene deposits.

Vitrands

Ustivitrands

Udivitrands
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-69

Profile of a Vitrand (specifically an Ustivitrand) in

the Philippines. This soil has an umbric epipedon.

Scale is in cm.

(Back to Vitrands)
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-70

Key to Great Groups of Vitrands (Back to key to suborders)

Vitrands that have:

1. Somewhat limited soil moisture available for plant 

growth ---------------------------------------------------------------------- Ustivitrands

These soils have an ustic soil moisture regime. Moisture is limited, but 

available, during portions of the growing season.

2. Seasonally well-distributed precipitation ----------------------- Udivitrands

These soils have an udic soil moisture regime.
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-71

Descriptions of Great Groups of Vitrands

Ustivitrands—These soils have an ustic moisture regime. Moisture is limited, 

but available, during portions of the growing season. Characteristically, 

Ustivitrands have an ochric (typically thin and/or light-colored) epipedon and a 

cambic (minimal soil development) subsoil horizon. Ustivitrands are rare in the 

United States. (Back to Vitrands key)

Udivitrands—These soils have a udic moisture regime. Precipitation is 

distributed throughout the year. Characteristically, these soils have an ochric 

(typically thin and/or light-colored) epipedon and a cambic (minimal soil 

development) subsoil horizon. Some have an argillic (clay accumulation) 

subsoil horizon. Most of the Udivitrands in the United States developed in 

Holocene deposits under coniferous forest vegetation. (Back to Vitrands key)
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-72

Ustands Great Groups

Ustands are the more or less well drained Andisols of subhumid to semiarid 

regions. These soils are of relatively small extent. They are mostly in Mexico, 

the western part of the United States, the Pacific Islands, and the eastern part 

of Africa. Most Ustands in the United States are in Hawaii, Arizona, and New 

Mexico. Most formed under grass, shrub, or forest vegetation. 

Characteristically, Ustands have an ochric (typically thin and/or light-colored) 

or mollic (rich in humus and bases) epipedon and a cambic (minimal soil 

development) subsoil horizon. Some have a duripan. The Ustands in the 

United States generally developed in late-Pleistocene or Holocene deposits.

Ustands

Durustands

Haplustands
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-73

Profile of an Ustand (specifically a Haplustand) in Chile. Scale is in 

decimeters. Arrows indicate horizon boundary locations.

Back to Ustands
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-74

Key to Great Groups of Ustands (Back to key to suborders)

Ustands that have:

1. A cemented horizon within a depth of 100 cm ---------------- Durustands

The cemented layer must be > 75% of the pedon. Begin measuring below 

any organic surface layer (O), unless the O horizon has andic soil 

properties.

2. A cambic (minimal soil development) horizon --------------- Haplustands
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-75

Descriptions of Great Groups of Ustands

Durustands—These soils have, in 75% or more of each pedon, a cemented 

horizon (duripan or other cemented layer) with its upper boundary within 100 

cm of either the mineral soil surface or of the top of an organic layer with andic 

soil properties, whichever is shallower. Characteristically, Durustands have an 

ochric (typically thin and/or light-colored) epipedon and a cambic (minimal soil 

development) subsoil horizon above the cemented horizon. Some have a 

mollic (rich in humus and bases) epipedon. The Durustands in the United 

States generally developed in late-Pleistocene or Holocene deposits. Most 

formed under coniferous/grass or shrub vegetation. (Back to Ustands key)

Haplustands—These soils have an ochric (typically thin and/or light-colored) 

epipedon and a cambic (minimal soil development) subsoil horizon. Some 

have a mollic (rich in humus and bases) epipedon. The Haplustands in the 

United States generally developed in late-Pleistocene or Holocene deposits. 

Most formed under coniferous vegetation or grass or shrub vegetation. (Back 

to Ustands key)
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-76

Udands Great Groups

Udands are the more or less well drained Andisols of humid climates. These 

soils are moderately extensive. They are mostly on the Pacific Rim, mainly in 

the western part of North America and in Japan, New Zealand, the 

Philippines, and Indonesia. Most of the Udands in the United States are in 

Washington and Oregon, but some are in Hawaii. Most Udands formed under 

forest vegetation. Characteristically, Udands have an ochric (typically thin 

and/or light-colored) or umbric (humus-rich with low base saturation) epipedon

and a cambic (minimal soil development) subsoil horizon. Some have a 

melanic (dark and humus-rich with andic properties) epipedon, and some 

have a duripan (layer cemented by silica) or placic (cemented by iron and 

organic matter) horizon. Most of the Udands in the United States developed in 

late-Pleistocene or Holocene deposits.

Udands

Placudands

Durudands

Melanudands

Hydrudands

Fulvudands

Hapludands
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-77

Profile of a Udand (specifically a Hapludand) in Japan. 

This soil formed in several ash deposits and contains

a series of paleosols. Scale is in decimeters.

(Back to Udands)
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-78

Key to Great Groups of Udands (Back to key to suborders)

Udands that have: 

1. A placic (cemented by iron and organic matter) horizon within a 

depth of 100 cm ---------------------------------------------------------- Placudands

The placic horizon must be > 50% of the pedon. Begin measuring below 

any organic surface layer (O), unless the O horizon has andic soil 

properties.

2. A cemented horizon within a depth of 100 cm ----------------- Durudands

The cemented layer must be > 75% of the pedon. Begin measuring below 

any organic surface layer (O), unless the O horizon has andic soil 

properties.

3. A melanic (dark and humus-rich with andic properties) 

epipedon ----------------------------------------------------------------- Melanudands

4. A very high water-holding capacity within the upper 

100 cm ---------------------------------------------------------------------- Hydrudands

For Hydrudands, water held at 1500 kPa tension is > 100% (by weight) on 

undried samples. See 12th edition of “Keys to Soil Taxonomy” for specific 

details regarding location within the profile of this property.

5. A melanic-like epipedon (but lighter in color) ----------------- Fulvudands

6. A cambic (minimal soil development) horizon ---------------- Hapludands
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-79

Descriptions of Great Groups of Udands

Placudands—These soils have, in half or more of each pedon, a placic 

(cemented by iron and organic matter) horizon within 100 cm of either the 

mineral soil surface or the top of an organic layer with andic soil properties, 

whichever is shallower. Characteristically, Placudands have a thin O horizon 

and an ochric (typically thin and/or light-colored) or umbric (humus-rich with 

low base saturation) epipedon above the placic horizon. Placudands are rare 

in the United States. Most formed under forest vegetation. (Back to Udands

key)

Durudands—These soils have, in 75% or more of each pedon, a cemented 

horizon that has its upper boundary within 100 cm of the mineral soil surface 

or of the upper boundary of an organic layer that has andic soil properties, 

whichever is shallower. Characteristically, Durudands have a thin O horizon, 

an ochric (typically thin and/or light-colored) or umbric (humus-rich with low 

base saturation) epipedon, a cambic (minimal soil development) subsoil 

horizon, and a cemented horizon (duripan or other cemented layer). The 

Durudands in the United States generally developed in late-Pleistocene 

deposits. Most formed under coniferous forest vegetation. (Back to Udands

key)

Melanudands—These soils have a melanic (dark and humus-rich with andic 

properties) epipedon and a cambic (minimal soil development) subsoil 

horizon. The Melanudands in the United States generally developed in latePleistocene deposits. Most formed under forest or savanna vegetation. (Back 

to Udands key)

Hydrudands—These soils have the capacity to absorb and store large 

amounts of water. On undried samples, they have a 1500 kPa (wilting point) 

water retention of 100% or more, by weighted average, throughout the major 

part of the andic materials. Commonly, Hydrudands have a thin O horizon, an 

ochric (typically thin and/or light-colored) or umbric (humus-rich with low base 

saturation) epipedon, and a cambic (minimal soil development) subsoil 

horizon. Most of the Hydrudands in the United States developed in latePleistocene deposits under forest vegetation. (Back to Udands key)

Fulvudands—These soils have a surface layer that meets the depth, 

thickness, and organic-carbon requirements for a melanic epipedon but is 

lighter in color. Characteristically, Fulvudands have a thin O horizon, an 

umbric (humus-rich with low base saturation) epipedon, and a cambic 

(minimal soil development) subsoil horizon. The Fulvudands in the United 

States generally developed in late-Pleistocene deposits. Most formed under 

coniferous forest vegetation. (Back to Udands key)
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-80

Hapludands—Commonly, Hapludands have a thin O horizon, an ochric 

(typically thin and/or light-colored) or umbric (humus-rich with low base 

saturation) epipedon, and a cambic (minimal soil development) subsoil 

horizon. Most of the Hapludands in the United States developed in latePleistocene or Holocene deposits. Most formed under coniferous forest 

vegetation. (Back to Udands key)
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-81

Aridisols Order

Aridisols are the dry soils of deserts.

General Characteristics

Aridisols have very limited soil moisture available for the growth of plants. 

Typically, these soils have a simple ochric (typically thin and/or light-colored)

epipedon for a surface layer. In some places, this surface layer has many 

small, rounded viscular pores that formed as a result of the trapping of air 

during periodic intense rains. The surface layer of some Aridisols commonly is 

blanketed by a thin physical or biological crust.

Aridisols have at least one of several possible forms of subsoil horizons that 

are, for the most part, indicative of a low leaching environment, where various 

salts and other water-soluble minerals accumulate in the subsoil. These forms 

commonly include calcic (calcium carbonate accumulation), gypsic (gypsum 

accumulation), and salic (high salts) horizons as well as the highly developed 

and cemented petrocalcic (cemented by calcium carbonate) horizon, 

petrogypsic (cemented by gypsum) horizon, and duripan (layer cemented by 

silica). In addition, some Aridisols have a cambic (minimal soil development), 

argillic (clay accumulation), or natric (high levels of illuvial clay and sodium) 

subsoil horizon.

Environment and Processes

As the name suggests, Aridisols are found in desert environments where 

evaporation greatly exceeds precipitation and therefore water is not available 

to mesophytic plants for long periods of time. Since the limited rainfall only 

percolates to shallow depths before it evaporates or is absorbed by plants and 

other soil organisms, the water-soluble salts that are carried downward by the 

water tend to accumulate in the soil to the depth of wetting. In areas with 

higher amounts of rainfall, these salts would be removed fairly easily from the 

profile by leaching. Due to the imbalance between evapotranspiration and 

precipitation, they accumulate in Aridisols. 

A few very salty Aridisols have a high water table. In these Aridisols, salts that 

are carried by evaporating water tend to move upward toward the surface,

where they accumulate. These “wet Aridisols” are characteristic of some 

playas in the western United States as well as the sabkhas of North Africa and 

the Middle East. Due to the salt-induced high osmotic potential, only salttolerant halophytes can obtain this water. The water is held at tensions too 

high for other plants. These effectively “dry” soils are also included in the 

Aridisols.
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-82

Location

Aridisols are found throughout the world in desert environments. Globally, they 

occupy about 12% of the ice-free land area. They occur in North America, 

South America, Africa, the Middle East, Asia, and Australia. These 

environments include both hot and cold deserts and cover a wide range in 

both latitude and elevation. In the United States, Aridisols are common in the 

western deserts. 

Profile of an Aridisol (specifically a Haplocalcid) in the

United Arab Emirates. White concentrations are 

calcium carbonate masses. Scale is in cm. (Photo 

courtesy of Dr. Craig Ditzler)

(Back to Key to Soil Orders)
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-83

Aridisols by suborder in the United States.

Global distribution of Aridisols.
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-84

Aridisols Suborders

Classification of Aridisols

In the definition of the suborders, emphasis is given to the redistribution of 

soluble materials and their accumulation in the subsoil or surface layer. Four 

of the seven suborders are defined on the basis of the composition and 

accumulation of the soluble fraction (Salids, Durids, Gypsids, and Calcids). 

Weathering and clay translocation also take place in Aridisols. Two suborders 

reflect these processes (Argids and Cambids). One suborder reflects very cold 

temperature (Cryids). 

The great group level reflects the degree of expression of the horizons of 

accumulation and/or the results of other processes that are considered 

subordinate to the particular suborder. The defining element is the degree of 

expression of the diagnostic horizon.

The seven suborders are:

1. Cryids—Aridisols in cold areas

2. Salids—Aridisols with an accumulation of salts more soluble than gypsum

3. Durids—Aridisols with an accumulation of and cementation by silica

4. Gypsids—Aridisols with an accumulation of gypsum

5. Argids—Aridisols with an accumulation of clay

6. Calcids—Aridisols with an accumulation of carbonates

7. Cambids—Aridisols with translocation and/or transformation of material 

Aridisols

Cryids

Salids

Durids

Gypsids

Argids

Calcids

Cambids
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-85

Key to the Suborders of Aridisols

Aridisols that have:

1. Cold average soil temperature --------------------------------------------- Cryids

These soils have a cryic soil temperature regime.

2. A salic (high content of salts) horizon within a depth of 

100 cm ------------------------------------------------------------------------------ Salids

3. A duripan (layer cemented by silica) within a depth of 

100 cm ------------------------------------------------------------------------------ Durids

4. A gypsic (gypsum accumulation) or petrogypsic (cemented by 

gypsum) horizon within a depth of 100 cm -------------------------- Gypsids

5. An argillic (clay accumulation) or natric (high levels of illuvial clay 

and sodium) horizon ---------------------------------------------------------- Argids

These soils do not have a petrocalcic (cemented by calcium carbonate)

horizon within a depth of 100 cm.

6. A calcic (calcium carbonate accumulation) or petrocalcic 

(cemented by calcium carbonate) horizon within a depth of 

100 cm ----------------------------------------------------------------------------- Calcids

7. A cambic (minimal soil development) horizon -------------------- Cambids
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-86

Cryids Great Groups

Cryids are the Aridisols of cold deserts. Short growing seasons combined 

with arid conditions limit the use of these soils. The soils are characteristically 

at high elevations, dominantly in the mountain and basin areas in the United 

States and Asia and in other parts of the world.

The cold deserts commonly have undergone alternating periods of colder and 

warmer climates, which have resulted in the expansion and contraction of 

alpine glaciers in the adjacent mountains. The consequent variations in the 

sediment load carried by mountain streams have resulted in relict landforms, 

such as pediments, terraces, and alluvial fans. Cryids commonly show 

evidence of periglacial features, such as ice-wedged casts, ground wedges, 

and mounds.

Cryids may have a duripan (layer cemented by silica) or an argillic (clay 

accumulation), calcic (calcium carbonate accumulation), cambic (minimal soil 

development), gypsic (gypsum accumulation), natric (high levels of illuvial clay 

and sodium), petrocalcic (cemented by calcium carbonate), petrogypsic 

(cemented by gypsum), or salic (high content of salts) horizon. These horizons 

are the basis for the great groups. For example, Haplocryids are characterized 

by minimal development. 

Cryids

Salicryids

Petrocryids

Gypsicryids

Argicryids

Calcicryids

Haplocryids
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-87

Key to Great Groups of Cryids (Back to key to suborders)

Cryids that have:

1. A salic (high content of salts) horizon within a depth of 

100 cm ------------------------------------------------------------------------- Salicryids

2. A cemented horizon within a depth of 100 cm ----------------- Petrocryids

This may be either a petrogypic horizon, petrocalcic horizon, or duripan.

3. A gypsic (gypsum accumulation) horizon within a depth of 

100 cm ---------------------------------------------------------------------- Gypsicryids

4. An argillic (clay accumulation) or natric (high levels of illuvial clay 

and sodium) horizon ----------------------------------------------------- Argicryids

5. A calcic (calcium carbonate accumulation) horizon within a depth of 

100 cm ----------------------------------------------------------------------- Calcicryids

6. A cambic (minimal soil development) horizon ---------------- Haplocryids
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-88

Descriptions of Great Groups of Cryids

Salicryids—These soils have a salic (high content of salts) horizon. They can 

have other diagnostic horizons and characteristics, but the presence of 

soluble salts is considered important to interpretations. These soils occur in 

extremely arid, cold regions of the world. (Back to Cryids key)

Petrocryids—These soils have a cemented layer (a duripan or a petrocalcic 

or petrogypsic horizon). An argillic (clay accumulation), cambic (minimal soil 

development), or natric (high levels of illuvial clay and sodium) horizon may 

occur above the cemented layer. These soils occur in the mountains of Idaho 

and possibly Wyoming. (Back to Cryids key)

Gypsicryids—These soils have a gypsic (gypsum accumulation) horizon. A 

cambic (minimal soil development) horizon commonly occurs above the 

gypsic horizon. These soils formed in parent materials rich in gypsum. They 

are rare in the world. (Back to Cryids key)

Argicryids—These soils have an argillic (clay accumulation) or natric (high 

levels of illuvial clay and sodium) horizon in which silicate clays have 

accumulated. In general, the Argicryids without a natric horizon formed in 

areas of late-Pleistocene or older sediments or surfaces. Many of these soils 

receive inputs from dust, which may be a source of clay-sized particles. 

Argicryids may be on gentle or steep slopes. These soils are not extensive

worldwide. (Back to Cryids key)

Calcicryids—These soils are derived from parent materials that have a high 

content of calcium carbonate or that have had calcium carbonate added as 

dust. Precipitation is unable to remove the calcium carbonate to substantial 

depths. These soils typically have a thin surface layer and a calcic (calcium

carbonate accumulation) horizon. Some have a cambic (minimal soil 

development) horizon overlying the calcic horizon. (Back to Cryids key)

Haplocryids—These soils have a cambic (minimal soil development) subsoil 

horizon. A thin ochric (typically thin and/or light-colored) epipedon is common. 

Other diagnostic horizons may occur below a depth of 100 cm. These soils 

commonly have accumulations of calcium carbonate below the cambic 

horizon. They are rare in the world. (Back to Cryids key)
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-89

Salids Great Groups

Salids are most commonly in depressions (playas) in deserts or in closed 

basins in the wetter areas bordering deserts. In North Africa and the Near 

East, such depressions are referred to as sabkhas or chotts, depending on 

the presence or absence of surface water for prolonged periods. 

In an arid environment with hot temperatures, accumulation of salts commonly 

results when there is a supply of water soluble salts and a net upward 

movement of water in the soil. In some areas, a salic (high content of salts) 

horizon has formed in salty parent materials without the presence of ground 

water. The most common form of salt is sodium chloride (halite), but sulfates 

(thenardite, mirabilite, and hexahydrite) and other salts may also occur. 

The concept of Salids is based on the accumulation of an excessive amount 

of salts that are more soluble than gypsum. As a rule, Salids are unsuitable for 

agricultural use, unless the salts are leached out. Leaching the salts is an 

expensive undertaking, particularly if there is no natural outlet for drainage 

water. 

Two great groups are recognized. Aquisalids are saturated with water for 1 

month or more during the year. Haplosalids are drier.

Salids

Aquisalids

Haplosalids
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-90

Profile of a Salid (specifically an Aquisalid). Salts are moving 

upward from the water table and accumulating in the salic 

horizon in the upper part of the profile and on the surface 

as a white salt crust.

(Back to Salids)
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-91

Key to Great Groups of Salids (Back to key to suborders)

Salids that:

1. Are saturated with water within a depth of 100 cm for 1 month or 

more annually -------------------------------------------------------------- Aquisalids

2. Are more or less freely drained ------------------------------------ Haplosalids
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-92

Descriptions of Great Groups of Salids

Aquisalids—These are salty soils in wet areas of deserts where capillary rise 

and evaporation of water concentrate the salts near the surface. Some of 

these soils have red and gray mottled color patterns (redoximorphic features) 

that result from saturation, reduction, and oxidation. In other soils,

redoximorphic features may not be evident because of a high pH, which 

inhibits iron and manganese reduction. Aquisalids occur dominantly in 

depressional areas where they are saturated by ground water at least part of 

the year. The vegetation generally is sparse and consists of salt-tolerant 

shrubs, grasses, and forbs. Although these soils may be moist, the dissolved 

salt content makes them physiologically dry. (Back to Salids key)

Haplosalids—These soils have a high concentration of salts but do not have 

the saturation that is associated with Aquisalids. Haplosalids may be 

saturated for shorter periods than Aquisalids or may have had a water table 

associated with a past climate. In the Four Corners area of the United States, 

salic horizons have formed without the influence of a water table in saline 

parent materials. (Back to Salids key)
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-93

Durids Great Groups

Durids are the Aridisols that have a layer cemented by silica (duripan) that 

has an upper boundary within a depth of 100 cm. In many areas, the duripan 

is within a depth of 50 cm. These soils occur dominantly on gentle slopes and 

formed in sediments that contain pyroclastics. The duripan is cemented partly 

with opal or chalcedony. The soils commonly have accumulations of calcium 

carbonate. The duripan is a barrier to both roots and water.

Some Durids have an argillic (clay accumulation) or natric (high levels of 

illuvial clay and sodium) horizon above the duripan. These horizons are the 

basis for the great groups. Most of these soils are used for grazing. The 

amount of forage is low where the duripan is shallow. 

Durids occur in the western part of the United States, particularly in Nevada. 

Durids

Natridurids

Argidurids

Haplodurids
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-94

Profile of a Durid (specifically a Haplodurid). The duripan 

begins at a depth of about 70 cm. Scale in decimeters.

(Back to Durids)
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-95

Key to Great Groups of Durids (Back to key to suborders)

Durids that have:

1. A natric (high levels of illuvial clay and sodium) horizon above the 

duripan ---------------------------------------------------------------------- Natridurids

2. An argillic (clay accumulation) horizon above the 

duripan ----------------------------------------------------------------------- Argidurids

3. A cambic (minimal soil development) horizon above the 

duripan --------------------------------------------------------------------- Haplodurids

A cambic horizon is common, but not required, in Haplodurids.
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-96

Descriptions of Great Groups of Durids

Natridurids—These soils have a natric (high levels of illuvial clay and sodium) 

horizon above the silica-cemented duripan. Commonly, the duripan is within a 

depth of 50 cm. In many areas it is plugged by calcium carbonate. These soils 

are commonly on gently sloping landscapes and formed in materials derived 

from pyroclastics. They are not extensive and are used mostly for grazing. 

(Back to Durids key)

Argidurids—These soils have an argillic (clay accumulation) subsoil horizon

above the duripan. Commonly, the duripan is within a depth of 50 cm. These 

soils are close to volcanic areas or formed in eolian or alluvial sediments 

derived from pyroclastics. (Back to Durids key)

Haplodurids—These soils have a cambic (minimal soil development) subsoil 

horizon above the silica-cemented duripan. Some have no diagnostic subsoil 

layer other than the duripan. These soils formed in materials with a 

pyroclastics influence. Most Haplodurids are used for grazing. (Back to Durids 

key)
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-97

Gypsids Great Groups

Gypsids are the Aridisols that have a gypsic (gypsum accumulation) or 

petrogypsic (cemented by gypsum) horizon within a depth of 100 cm. 

Accumulation of gypsum occurs initially as crystal aggregates in the voids of 

the soil. These aggregates grow by accretion, displacing the enclosing soil 

material. When the gypsic horizon occurs as a cemented impermeable layer, it 

is recognized as the petrogypsic horizon. Each of these forms of gypsum 

accumulation implies processes in the soil, and each presents a constraint to 

soil use. One of the largest constraints is caused by the dissolution of 

gypsum, which which negatively affects structures, roads, and irrigation 

delivery systems.

The presence of a gypsic or petrogypsic horizon (or both), with or without 

other diagnostic horizons, defines the great groups of Gypsids. Gypsids occur 

in Iraq, Syria, Saudi Arabia, Iran, the United Arab Emirates, Somalia, West 

Asia, and arid areas in the western part of the United States. They occur on 

many segments of the landscape. Some of them have calcic (calcium

carbonate accumulation) or related horizons that overlie the gypsic horizon.

Gypsids

Petrogypsids

Natrigypsids

Argigypsids

Calcigypsids

Haplogypsids
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-98

Profile of a Gypsid in New Mexico. This soil formed in a 

gypsum-rich parent material. The white color is due 

primarily to the presence of gypsum throughout the 

profile. Soil is a Haplogypsid. Scale is in cm. (Photo 

courtesy of Dr. David Weindorf)

Return to Gypsids
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-99

Key to Great Groups of Gypsids (Back to key to suborders)

Gypsids that have:

1. A cemented horizon that has its upper boundary within a depth of 

100 cm -------------------------------------------------------------------- Petrogypsids

The cemented layer may be either petrogypsic or petrocalcic.

2. A natric (high levels of illuvial clay and sodium) horizon within a 

depth of 100 cm -------------------------------------------------------- Natrigypsids

3. An argillic (clay accumulation) horizon within a depth of 

100 cm ---------------------------------------------------------------------- Argigypsids

4. A calcic (calcium carbonate accumulation) horizon within a depth of 

100 cm --------------------------------------------------------------------- Calcigypsids

5. A cambic (minimal soil development) horizon above the gypsic

horizon ------------------------------------------------------------------- Haplogypsids

A cambic horizon is common, but not required, in Haplogypsids.
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Descriptions of Great Groups of Gypsids

Petrogypsids—These soils have a cemented layer (a petrogypsic or 

petrocalcic horizon) that has its upper boundary within a depth of 100 cm. 

These soils occur in very arid areas of the world where the parent material has 

a high content of gypsum. When the petrogypsic horizon is close to the 

surface, crusting forms pseudohexagonal patterns on the soil surface. 

Petrogypsids occupy old surfaces. In Syria and Iraq, they are on the highest 

terraces along the Tigris and Euphrates Rivers. They are also known to occur 

in western Texas and southern New Mexico. (Back to Gypsids key)

Natrigypsids—These soils have a natric (high levels of illuvial clay and 

sodium) subsoil horizon within a depth of 100 cm. The gypsic horizon is 

commonly below the natric horizon. These soils formed in parent materials 

that had high contents of gypsum and sodium, such as sedimentary materials 

that were deposited in a marine environment or in closed basins. They are 

rare but are known to occur in the Four Corners area in the western part of the 

United States (northwestern New Mexico, northeastern Arizona, southeastern 

Utah, and southwestern Colorado). These soils are used primarily for grazing. 

(Back to Gypsids key)

Argigypsids—These soils have an argillic (clay accumulation) subsoil horizon

above the gypsic horizon. They are known to occur in the Four Corners area 

in the western part of the United States and in New Mexico and Texas. They 

are used primarily for grazing. (Back to Gypsids key)

Calcigypsids—These soils have a calcic (calcium carbonate accumulation)

subsoil horizon. Commonly, the calcic horizon is above the gypsic horizon 

because of differences in the solubility of gypsum and calcium carbonate. 

These soils are known to occur in Arizona, New Mexico, and Texas. Most 

Calcigypsids are used for grazing. (Back to Gypsids key)

Haplogypsids—Some Haplogypsids have a cambic (minimal soil 

development) subsoil horizon overlying the gypsic horizon. Others have only a 

gypsic horizon. Haplogypsids are commonly very pale in color. They are not 

extensive in the United States. The largest concentrations in the United States 

are in Arizona, Nevada, New Mexico, and Texas. These soils are more 

common in other parts of the world. (Back to Gypsids key)
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Argids Great Groups

Argids are the Aridisols that have an argillic (clay accumulation) or natric 

(high levels of illuvial clay and sodium) horizon. In many Aridisols, the low 

water flux and high concentration of salts hinder clay illuviation. The presence 

of an argillic horizon is therefore commonly attributed to a moister 

paleoclimate (although there is evidence that clay illuviation occurred during 

the Holocene in arid soils). In semiarid areas (where the soil moisture regime 

grades to ustic or xeric), clay translocation commonly is more evident. Most 

Argids occur in North America. A few have been recognized in the deserts of 

North Africa and the Near East.

Argids

Petroargids

Natrargids

Paleargids

Gypsiargids

Calciargids

Haplargids
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-102

Profile of an Argid (specifically a Haplargid) in Australia. This

soil has a relatively thin argillic horizon (darker color) in the 

upper part of the profile. Scale is in feet.

Return to Argids
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-103

Key to Great Groups of Argids (Back to key to suborders)

Argids that have:

1. A cemented layer within a depth of 150 cm -------------------- Petroargids

The cemented layer may be a petrocalcic horizon, petrogypsic horizon, or 

duripan.

2. A natric (high levels of illuvial clay and sodium) horizon --- Natrargids

3. Either: 

a) An abrupt increase in clay content at the contact with, or within, 

the argillic horizon; OR

b) A thick argillic (clay accumulation) horizon in which clay does not 

decrease significantly within a depth of 50 cm and that has mostly 

hue of 7.5YR or redder color in some part ----------------------- Paleargids

For item a, the clay increase is > 15% (absolute) over 2.5 cm. For item b, 

clay does not decrease by > 20% relative to the maximum clay amount 

above. See 12th edition of “Keys to Soil Taxonomy” for specific criteria 

regarding color. Paleargids have no root-limiting layer within a depth of 50 

cm.

4. A gypsic (gypsum accumulation) horizon within a depth of 

150 cm ---------------------------------------------------------------------- Gypsiargids

5. A calcic (calcium carbonate accumulation) horizon within a depth of 

150 cm ----------------------------------------------------------------------- Calciargids

6. Only an argillic (clay accumulation) horizon -------------------- Haplargids
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Descriptions of Great Groups of Argids

Petroargids—These soils have a cemented layer (a duripan or a petrocalcic 

or petrogypsic horizon) that has its upper boundary at a depth between 100

and 150 cm. They occur on stable landscapes in the western part of the 

United States, in southern Argentina, and in western South Africa. (Back to 

Argids key)

Natrargids—These soils have a natric (high levels of illuvial clay and sodium) 

horizon that commonly has prismatic or columnar structure. They generally 

contain carbonates, soluble salts, or both. These soils formed in sediments 

that range in age from Holocene to late-Pleistocene. Most of them are nearly 

level to gently sloping. Natrargids occur in the western part of the United 

States and on the western edge of the Great Plains. (Back to Argids key)

Paleargids—These soils are on stable land surfaces and have an abrupt 

textural change or a clay distribution in the argillic horizon that does not 

decrease significantly with increasing depth. Most of these soils formed in 

sediments appreciably older than late Pleistocene. If calcareous dust is 

present, some of these soils may be calcareous in all horizons. Slopes are 

typically gentle. (Back to Argids key)

Gypsiargids—These soils have a gypsic (accumulation of gypsum) subsoil 

horizon within a depth of 150 cm. Most of these soils are on late-Pleistocene 

surfaces. In the United States, Gypsiargids are of minor extent and are known 

to occur in Arizona, New Mexico, and Texas. (Back to Argids key)

Calciargids—These soils have, below the clay-enriched argillic horizon, a 

calcic (calcium carbonate accumulation) horizon within a depth of 150 cm. 

Where the argillic horizon is calcareous, it is generally thought to have been 

recharged with calcium carbonate, possibly from dust, after the argillic horizon 

formed. Calciargids are commonly on late-Pleistocene erosional surfaces or 

on gentle to steep slopes. (Back to Argids key)

Haplargids—These soils have only an argillic (clay accumulation) horizon. 

They commonly have some calcium carbonate accumulations within or below 

the argillic horizon. Haplargids commonly occur on late-Pleistocene surfaces 

or sediments. (Back to Argids key)
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Calcids Great Groups

Calcids are the Aridisols with accumulations of calcium carbonate that was in 

the parent materials or was added as dust, or both. Precipitation is insufficient 

to leach or move the carbonates to great depths. The upper boundary of the 

calcic (calcium carbonate accumulation) or petrocalcic (cemented by calcium 

carbonate) horizon is typically within a depth of 50 cm. If these soils are 

irrigated and cultivated, they typically have micronutrient deficiencies. Calcids

are extensive in the western part of the United States and in other arid regions 

of the world.

Calcids

Petrocalcids

Haplocalcids
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-106

Profile of a Calcid (specifically a Haplocalcid). The white color 

below a depth of about 30 cm is due to accumulations of 

calcium carbonate. Scale is in decimeters.

(Back to Calcids)
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Key to Great Groups of Calcids (Back to key to suborders)

Calcids that have:

1. A petrocalcic (cemented by calcium carbonate) horizon within a 

depth of 100 cm --------------------------------------------------------- Petrocalcids

2. A calcic (calcium carbonate accumulation) horizon ------- Haplocalcids
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Descriptions of Great Groups of Calcids

Petrocalcids—These soils have a petrocalcic (cemented by calcium 

carbonate) subsoil horizon that has its upper boundary within a depth of 100 

cm. Typcially, the upper boundary of the petrocalcic horizon is close to the soil 

surface. Some of these soils show evidence of former argillic (clay 

accumulation) horizons that are now engulfed with carbonates. Generally, 

Petrocalcids are on gentle slopes that have been stable for long time periods. 

They occur on old landscapes in the southwestern part of the United States 

and in other deserts of the world. (Back to Calcids key)

Haplocalcids—These soils have a calcic (calcium carbonate accumulation)

subsoil horizon with its upper boundary within a depth of 100 cm. Some of 

these soils have a cambic (minimal soil development) horizon above the calcic 

horizon. Haplocalcids are extensive worldwide. (Back to Calcids key)
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Cambids Great Groups

Cambids are the Aridisols with the least degree of soil development. These 

soils have a cambic horizon within a depth of 100 cm. They may have other 

diagnostic horizons, such as petrocalcic (cemented by calcium carbonate), 

gypsic (gypsum accumulation), or calcic (calcium carbonate accumulation) 

horizons, but the upper boundary of these horizons is below a depth of 100 

cm. Cambids are the most common Aridisols in the United States.

Cambids

Aquicambids

Petrocambids

Haplocambids
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-110

Profile of a Cambid (specifically a Haplocambid) in the United Arab 

Emirates. The cambic horizon extends from a depth of 10 cm to 

about 50 cm. Scale is in cm. (Photo courtesy of Dr. Craig Ditzler)

(Back to Cambids)
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Key to Great Groups of Cambids (Back to key to suborders)

Cambids that have:

1. Saturation with water within a depth of 100 cm for > 1 month 

annually ------------------------------------------------------------------ Aquicambids

Aquicambids may also be irrigated and have saturation and reducing 

conditions at a depth of < 100 cm.

2. A cemented layer within a depth of 150 cm ----------------- Petrocambids

The cemented layer may be a duripan, petrocalcic horizon, or petrogypsic 

horizon.

3. A cambic (minimal soil development) horizon ------------ Haplocambids
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Descriptions of Great Groups of Cambids

Aquicambids—These soils are saturated with water for short periods in most 

years. They commonly are adjacent to playas and have accumulations of 

salts. Aquicambids commonly have high pH values, which inhibit the formation 

of red and gray mottled color patterns characteristic of wetness 

(redoximorphic features). (Back to Cambids key)

Petrocambids—These soils have a cemented layer (a duripan or a 

petrocalcic or petrogypsic horizon) that has its upper boundary at a depth 

between 100 and 150 cm. These soils are not extensive because most 

Aridisols have these diagnostic horizons at shallower depths. However, 

because Petrocambids indicate water movement, which is important in 

determining interpretations, classes for these types of soils are provided. 

(Back to Cambids key)

Haplocambids—Haplocambids are the most commonly occurring Cambids. 

These soils are characterized by minimal horizon expression. Most 

Haplocambids have a redistribution of carbonates below the cambic horizon. 

The amount of carbonates, however, is insufficient to meet the definition of a 

calcic horizon, or the upper boundary of the cambic horizon is more than 100 

cm below the soil surface. A few of these soils have an anthropic (humanmodified) epipedon. Haplocambids occur on a variety of landscapes, 

commonly on those younger than late Pleistocene. (Back to Cambids key)
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Entisols Order

Entisols are young soils with little or no soil profile development.

General Characteristics

Entisols typically have little or no development of soil horizons, other than a 

slightly darkened ochric (typically thin and/or light-colored) epipedon as a 

surface layer. Therefore, these soils are characterized not by the kinds of 

horizons that have formed but rather by their minimal degree of soil 

development. 

Environment and Processes

In many landscape positions where Entisols occur, the soil material has not 

been in place long enough for soil-forming processes to create distinctive 

horizons. Typical landscape settings include steep, actively eroding slopes, 

flood plains that receive new deposits of alluvium at frequent intervals, and 

shifting sand dunes. In general, Entisols are in landscape settings where 

processes of erosion or deposition are happening at rates faster than those 

needed for the formation of soil horizons. In some areas, Entisols occur in 

more stable landscape positions. In these areas, the soils consist mostly of 

quartz or other minerals that are resistant to the weathering needed to form 

soil horizons or the soil-forming processes are hindered by extreme 

environmental conditions.

Location

Globally, Entisols occupy about 16% of the ice-free land area. They occur 

throughout the world in a wide range of ecological settings, ranging from the 

warm tropics into the cool higher latitudes. Entisols range from wet to dry 

areas and are common in many deserts of the world, especially in dune fields 

and on sand sheets. Entisols, especially those in rocky, coarse textured 

materials, are also on many steep mountain slopes, and they are found in 

shallow water areas, where they are permanently submerged (subaqueous 

soils). They have a wide range of parent materials and support many kinds of 

vegetation. 
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Profile of an Entisol.

(Back to Key to Soil Orders)
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-115

Entisols by suborder in the United States. Note: This map was produced 

before the introduction of the Wassents suborder and the elimination of the 

Arents suborder. Wassents are predominantly in shallow subtidal coastal 

bays and estuaries and in shallow (< 2.5 meters of overlying water) 

freshwater areas, such as lakes and ponds. Areas shown as Arents are 

now primarily included in Orthents.
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Global distribution of Entisols.
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-117

Entisols Suborders

Classification of Entisols

In the definitions of the suborders, emphasis is divided between permanent 

inundation by water (Wassents); near surface saturation (Aquents); sandy 

texture (Psamments); formation in recent stratified alluvial sediments 

(Fluvents); and a virtual lack of horizon formation, mostly in areas of young 

erosional surfaces (Orthents).

The great groups of Wassents reflect those soils inundated by fresh water, 

those dominated by sandy textures, those containing sulfidic materials, those 

with a very low bearing capacity (high n value), those with an irregular

decrease/increase in carbon content with increasing depth, and those with 

minimal evidence of soil development. The great groups within the other 

suborders reflects a combination of important properties, including moisture 

regime, cold soil temperature, patterns of soil saturation, and a dominance of 

resistant minerals making up the sand grains in sandy soils. 

The five suborders are:

1. Wassents—Entisols that are permanently inundated with water

2. Aquents—wet Entisols (aquic conditions in the upper part)

3. Psamments—Entisols dominated by sandy textures

4. Fluvents—Entisols that formed in stratified alluvial sediments as evidenced 

by the irregular decrease/increase of carbon content with increasing depth

5. Orthents—other Entisols lacking in pedogenic development

Entisols

Wassents

Aquents

Psamments

Fluvents

Orthents
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Key to Suborders of Entisols

Entisols that:

1. Are permanently inundated by water -------------------------------- Wassents

Wassents in tidal areas can be exposed to the atmosphere < 3 hours 

every day. Water is shallow enough to permit rooted vegetation to grow 

(generally less than 2.5 m).

2. Have a seasonal high water table within a depth of 50 cm ---- Aquents

Artificially drained soils are included in Aquents. See 12th edition of “Keys 

to Soil Taxonomy” for specific details regarding required depths, presence 

of sulfidic materials, presence of a reduced matrix (soil exhibits color 

change upon exposure to air), and various texture/color combinations 

characteristic of Aquents.

3. Have sandy texture throughout the upper 100 cm ----------- Psamments

Psamments have < 35% rock fragments. Required depth of sandy textures

is less in profiles with bedrock above 100 cm.

4. Consist of stratified alluvial deposits and have an an irregular 

decrease/increase in carbon content with increasing

depth ----------------------------------------------------------------------------- Fluvents

Fluvents do not have 50 cm or more of human-transported material on the 

surface.

5. Other simple Entisols (not included above) ------------------------ Orthents
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Wassents Great Groups

Wassents are Entisols that formed in shallow, permanently flooded 

environments. These soils are considered to have a subaqueous drainage 

class where there is a positive water pressure at the soil surface for more than 

21 hours of each day in all years. The 21-hour minimum allows for short daily 

exposure of the soil surface in areas with large tidal fluctuations, such as 

coastal Maine in the United States. Some of these soils formed in coastal 

saltwater environments, while others formed in freshwater deposits. Most are 

known to occur in Holocene-age depositional environments from recent 

sedimentary deposits. Wassents have been found in all climates warmer than 

cryic and under submersed aquatic vegetation. These soils are important for 

wildlife habitat and aquaculture.

Wassents

Frasiwassents

Psammowassents

Sulfiwassents

Hydrowassents

Fluviwassents

Haplowassents
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Profile of a Wassent exposed in a soil 

core from Rhode Island. Scale is 

in centimeters.
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Eelgrass and other aquatic vegetation growing on a subaqueous soil.

(Back to Wassents)
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Key to Great Groups of Wassents (Back to key to suborders)

Wassents that have:

1. Low salinity levels -------------------------------------------------- Frasiwassents

The upper 100 cm has electrical conductivity of less than 0.2 dS/m in a 

5:1, by volume, mixture (not extract) of water and soil. 

2. Sandy textures to a depth of 100 cm -------------------- Psammowassents

Texture is loamy fine sand or coarser, and rock fragments make up < 35%.

3. Sulfidic materials (acid-producing, oxidizable sulfur compounds) 

within a depth of 50 cm -------------------------------------------- Sulfiwassents

See 12th edition of “Keys to Soil Taxonomy” regarding depth and 

thickness requirements for sulfidic materials.

4. A low load-bearing capacity (flows when 

compressed) --------------------------------------------------------- Hydrowassents

See 12th edition of “Keys to Soil Taxonomy” for specific criteria regarding 

this property (including location in profile, clay content, and n value).

5. Layers consisting of stratified alluvial deposits and an irregular 

decrease/increase in carbon content with increasing 

depth --------------------------------------------------------------------- Fluviwassents

Fluviwassents cannot have human-transported material that is 50 cm or 

more thick on the surface.

6. Moderate to high salinity and loamy or clayey 

textures ---------------------------------------------------------------- Haplowassents
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Descriptions of Great Groups of Wassents

Frasiwassents—These soils formed in fresh water, in both natural and manmade water bodies. They have a low electrical conductivity (< 0.2 dS/m) in a 

1:5 (by volume) soil:water ratio throughout the upper meter. Many of these 

soils have high levels of nutrients that support a large number of native and 

non-native vegetation. These soils are believed to be of large extent in the 

United States and are most extensive in mid to northern latitudes. (Back to 

Wassents key)

Psammowassents—These soils formed in salt or brackish water and have 

sandy textures of loamy fine sand or coarser throughout the upper meter. 

Psammowassents are commonly derived from Holocene sand on flood-tidal 

deltas and washover fans of coastal lagoons and shore face complexes. The 

fluidity class is nonfluid throughout the profile. Vegetation includes native 

algae, eelgrass, and widgeon grass. The soils are believed to be of large 

extent in the United States and are important for aquaculture. (Back to 

Wassents key)

Sulfiwassents—These soils formed in salt or brackish water and have sulfidic 

materials (acid-producing, oxidizable sulfur compounds) within the upper 50 

cm. They typically have, in place, neutral pH values but, when exposed to air,

are potentially extremely acid or ultra acid. Most Sulfiwassents will develop a 

sulfuric (highly acid due to oxidation and production of sulfuric acid) horizon if 

they are artificially drained to an extent that oxygen is allowed to reach the 

sulfidic materials. Sulfiwassents are commonly derived from loamy and 

organic material in lagoon bottoms, barrier and mainland coves, shoals, and 

submerged wave-built terraces and are subject to minimal currents and wave 

action. Sulfiwassents are believed to be of large extent in the United States 

and are important for wildlife habitat and aquaculture. (Back to Wassents key)

Hydrowassents—These soils formed in salt or brackish water and have a low 

load-bearing capacity, as evidenced by their slightly fluid or higher fluidity 

class, low bulk density, and clay content of more than 8%. The water content 

is high, and the soil strength is low. These soils are commonly derived from 

loamy and organic-rich mineral material in lagoon bottoms, barrier and 

mainland coves, and submerged stream valleys. They are subject to minimal 

currents and wave action. Hydrowassents are believed to be of large extent in 

the United States and are important for wildlife habitat and aquaculture. (Back 

to Wassents key)

Fluviwassents—These soils formed in salt or brackish water and have either 

an organic-carbon content of 0.2% or more at a depth of 125 cm or an 

irregular decrease/increase in organic carbon with increasing depth. 

Fluviwassents are commonly derived from Holocene sand in salt or brackish 

water on washover fans, submerged-upland tidal marsh, and submerged 
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wave-built terraces. These soils are subject to severe storm events and strong 

wave action. Vegetation includes native algae, eelgrass, and widgeon grass. 

Fluviwassents are believed to be of large extent in the United States and are 

used as wildlife habitat, for aquaculture, and for recreation. (Back to Wassents 

key)

Haplowassents—These soils formed in salt or brackish water and have 

loamy textures finer than loamy fine sand. These soils are commonly derived 

from coarse textured deposits of sand and gravel. Haplowassents have a high 

load-bearing capacity and are potential ground-water discharge areas. 

Vegetation includes native algae and eelgrass. These soils are believed to be 

of large extent in the United States and are important to aquaculture. (Back to 

Wassents key)
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Aquents Great Groups

Aquents are the wet Entisols. They are saturated close to the surface for a 

long enough period during the year to become devoid of oxygen (aquic 

conditions). Some are artificially drained. Aquents commonly occur in 

wetlands, such as in tidal marshes, on deltas, on the margins of lakes (where 

the soils are continuously saturated with water), on flood plains along streams 

(where the soils are saturated at some time of the year), and in areas of wet, 

sandy deposits. Many Aquents have bluish or grayish colors and 

redoximorphic features (gray and red color patterns). They may have any 

temperature regime. Most of them formed in recent sediments and support 

vegetation that tolerates permanent or periodic wetness.

Aquents

Sulfaquents

Hydraquents

Gelaquents

Cryaquents

Psammaquents

Fluvaquents

Epiaquents

Endoaquents
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Key to Great Groups of Aquents (Back to key to suborders)

Aquents that have:

1. Sulfidic materials (acid-producing, oxidizable sulfur compounds) 

within a depth of 50 cm ----------------------------------------------- Sulfaquents

See 12th edition of “Keys to Soil Taxonomy” for information regarding 

depth and thickness requirements for sulfidic materials.

2. A low load-bearing capacity (fluid when compressed) --- Hydraquents

See 12th edition of “Keys to Soil Taxonomy” for specific criteria regarding 

this property (such as location in profile, clay content, and n value).

3. Very cold average soil temperature ------------------------------- Gelaquents

These soils have a gelic soil temperature regime.

4. Cold average soil temperature -------------------------------------- Cryaquents

These soils have a cryic soil temperature regime.

5. Sandy textures to a depth of 100 cm ------------------------ Psammaquents

Texture is loamy fine sand or coarser throughout the upper meter, and 

rock fragments make up < 35%.

6. Layers consisting of stratified alluvial deposits, with an 

irregular decrease/increase in carbon content with increasing 

depth ------------------------------------------------------------------------ Fluvaquents

Fluvaquents do not have 50 cm or more of human-transported material on 

the surface. Slope is < 25%. 

7. Episaturation (perched water table) ------------------------------- Epiaquents

8. Endosaturation (saturated throughout the profile) -------- Endoaquents
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Descriptions of Great Groups of Aquents

Sulfaquents—These soils have an appreciable amount of sulfides close to 

the soil surface and have few or no carbonates. The sulfides are close enough 

to the surface for the soils to become extremely acid and virtually sterile if 

drained. Sulfaquents are largely restricted to coastal marshes where the water 

is brackish. Tidal marshes at the mouth of rivers normally have areas of these 

soils unless the river sediments are rich in carbonates. Coastal marshes that 

are not near rivers also may have areas of these soils if storms periodically 

flood them with salt water. Sulfaquents are permanently saturated at or near 

the surface, and many are nearly neutral in reaction. They commonly have a 

low load-bearing capacity because they tend to behave like a fluid when 

compressed (the n value typically is high, commonly more than 1 in most 

horizons). A histic epipedon (wet, organic surface layer) is common. 

Sulfaquents commonly are not used as cropland, but some are used for the 

production of rice. The areas used for rice are kept flooded at all times, except 

for a very brief period while the crop is harvested. Sulfaquents are extensive 

along coasts and bays, ranging from the Equator to high latitudes. They are 

moderately extensive on the east coast of the United States, where they 

border the Atlantic Ocean in areas not protected by barrier islands. (Back to 

Aquents key)

Hydraquents—These are loamy and clayey soils that formed in sediments 

that were deposited under water and are now permanently saturated with 

water. Hydraquents never have been dry. Consequently, the bulk density is 

low (commonly about 0.6 g/cc) and the water content is very high (commonly 

more than 100% of the dry weight). Because of the high water content, soil 

strength is commonly too low to support grazing animals. If the soils are 

drained, the loss of water is irreversible and the bulk density increases as 

water is withdrawn. The soils typically have a reduced matrix (exhibit color 

change on exposure to air), and their colors mostly are bluish gray to greenish 

gray and change to shades of brown when the soils are exposed to air. A 

color change normally can be seen within a matter of seconds or a few 

minutes but may not be complete until some weeks have passed. 

Hydraquents are extensive in tidal marshes. In the United States, they are 

largely confined to the Atlantic and Gulf coasts, although a few are on the 

northern Pacific coast and near San Francisco Bay (Back to Aquents key)

Gelaquents—These are the Aquents of very cold regions. They have a gelic 

soil temperature regime but lack permafrost within the soil profile. They are 

subject to wide variations in summer high temperatures and winter low

temperatures, which result in mean annual soil temperatures of 0 degrees C 

or less, but without permafrost. These soils tend to have a high content of rock 

fragments, and some are shallow to bedrock. (Back to Aquents key)
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Cryaquents—These are the cold, wet soils in high mountains or of high 

latitudes. Cryaquents are on flood plains, in depressional areas, and in coastal 

marshes. Many are grayish and stratified. Those in coastal marshes 

commonly have greenish to bluish hues. Because they are both cold and wet, 

Cryaquents have a low potential for farming. These soils formed in recent 

sediments. The major areas of these soils in the United States are in Alaska.

In Alaska, Cryaquents are extensive south of the permafrost zone and occur

in coastal marshes, on outwash plains, and on flood plains. Most Cryaquents 

support mixed forest, shrub, or grassy vegetation. Many are nearly level, and 

their parent materials are typically Holocene or late-Pleistocene sediments.

(Back to Aquents key)

Psammaquents—These soils have a sandy texture and commonly are gray, 

with or without redoximorphic concentrations (reddish to black accumulations 

of iron-manganese oxides). Most of these soils formed in late-Pleistocene to 

recent sediments. Most do not have distinctive features, but a few have a 

weakly developed subsoil horizon that is similar to the one in Spodosols. 

Others show some darkening and accumulation of organic matter in the 

surface layer. (Back to Aquents key)

Fluvaquents—These are primarily the stratified, wet soils on flood plains and 

deltas of mid and low latitudes. The stratification reflects the deposition of 

sediments under changing currents and in shifting channels. The sediments 

are of Holocene age. Fluvaquents have a relatively high content of organic 

carbon at a considerable depth when compared with many other wet, mineral 

soils. Because the materials have dried or have partially dried from time to 

time as they accumulated, they are not fluid when wet. Fluvaquents are 

extensive along large rivers, particularly in humid areas. Generally, these soils

are nearly level. Many support either a deciduous or coniferous forest. Some 

have been cleared and protected from flooding and are used as cropland or 

pasture. (Back to Aquents key)

Epiaquents—These soils have ground water that is perched during some 

periods and fluctuates from a level near or above the soil surface in wet 

seasons. Many Epiaquents support either a deciduous or coniferous forest. 

Some have been cleared and are used as cropland or pasture. Epiaquents 

are generally nearly level, and their parent materials are typically latePleistocene or Holocene sediments. (Back to Aquents key)

Endoaquents—These soils are saturated throughout the profile for some time 

during the year. Many Endoaquents support either a deciduous or coniferous 

forest. Some have been cleared and are used as cropland or pasture. 

Endoaquents are generally nearly level, and their parent materials are 

typically late-Pleistocene or Holocene sediments. (Back to Aquents key)
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Psamments Great Groups

Psamments are the sandy Entisols with < 35% rock fragments. They are 

sandy in all layers, generally to a depth of 100 cm or more. Some formed in 

poorly graded (well sorted) sands on shifting or stabilized sand dunes, in 

cover sands, or in sandy parent materials that were sorted in an earlier 

geologic cycle. Some formed in sands that were sorted by water and are on 

outwash plains, lake plains, natural levees, or beaches. A few Psamments 

formed in material weathered from sandstone or granitic bedrock. Psamments 

occur in any climate, but they cannot have permafrost within a depth of 100 

cm. They can have any vegetation and are on surfaces of virtually any age,

from recent historic to Pliocene or older. The Psamments on old stable 

surfaces commonly consist of quartz sand. The depth to ground water 

typically is more than 50 cm and commonly much more.

Psamments have a relatively low water-holding capacity. Those that are bare 

and become dry are subject to soil blowing and drifting and cannot easily 

support wheeled vehicles. Because very gravelly sands do not have the two 

qualities just described, they are excluded from Psamments and are grouped 

with Orthents. Thus, not all Entisols that have a sandy texture are 

Psamments.

Psamments

Cryopsamments

Torripsamments

Quartzipsamments

Ustipsamments

Xeropsamments

Udipsamments
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Profile of a Psamment. This soil is specifically a Quartzipsamment 

that has lamellae in the lower part. Scale is in decimeters. 

(Photo courtesy of John Kelley)

(Back to Psamments)
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Key to Great Groups of Psamments (Back to key to suborders)

Psamments that have:

1. Cold average soil temperature ------------------------------- Cryopsamments

These soils have a cryic soil temperature regime.

2. Inadequate soil moisture available for crop 

growth ---------------------------------------------------------------- Torripsamments

These soils have an aridic (torric) soil moisture regime.

3. Sand grains dominated by quartz and other minerals that are highly 

resistant to weathering -------------------------------------- Quartzipsamments

These soils have > 90% (by weighted average) resistant minerals. 

4. Somewhat limited soil moisture available for crop

growth ----------------------------------------------------------------- Ustipsamments

These soils have an ustic soil moisture regime. Moisture is limited, but 

available, during portions of the growing season.

5. A Mediterranean-type climate -------------------------------- Xeropsamments

These soils have a xeric soil moisture regime (cool and moist in winter and

warm and dry in summer).

6. Seasonally well-distributed precipitation ------------------ Udipsamments

These soils have a udic soil moisture regime.
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Descriptions of Great Groups of Psamments

Cryopsamments—These are the Psamments of cold areas. In the United 

States, they are primarily in the high mountains of the West and in Alaska. 

Generally, they have, or had before they were disturbed, coniferous forest 

vegetation. They are of small extent. Most of them formed in material 

weathered from granitic rocks or in deposits of late-Pleistocene or recent age. 

Some formed in recent volcanic deposits. (Back to Psamments key)

Torripsamments—These are the cool to hot Psamments of arid climates. 

They have an aridic (or torric) moisture regime. Many of these soils are on 

stable surfaces, some are on dunes, some are stabilized, and some are 

moving. Torripsamments consist of quartz, mixed sands, volcanic glass, or 

even gypsum and may have any color. Generally, they are neutral or 

calcareous and are nearly level to steep. The vegetation consists mostly of 

xerophytic shrubs, grasses, and forbs. Many of these soils support more 

vegetation than other soils with an aridic moisture regime, presumably 

because rapid infiltration results in little or no precipitation being lost to runoff. 

Some of the soils on dunes support a partial cover of xerophytic and 

ephemeral plants. The shifting dunes may be devoid of plants in most years. 

Most of the deposits are of late-Pleistocene or younger age. These soils are 

used mainly for grazing. They are extensive in many deserts around the 

world, including areas in the western United States. (Back to Psamments key)

Quartzipsamments—These are the freely drained Psamments that have 

more than 90% resistant minerals. They are in humid to semiarid, cool to hot 

regions. These soils have a high content of quartz sand and are white or 

stained in shades of brown, yellow, or red. Because they have virtually no 

minerals that can weather, they can occur on some extremely old land 

surfaces. They also occur on late-Pleistocene and younger surfaces. The 

vegetation on Quartzipsamments varies widely depending on climate. In

cultivated areas, supplemental water and nutrient needs can be high. 

Quartzipsamments are extensive on the coastal plains of the United States. 

(Back to Psamments key)

Ustipsamments—These are the Psamments that have an ustic moisture 

regime. Although moisture is limited, it is generally available during portions of 

the growing season. Temperatures range from cool to hot. These soils 

support mostly grass or savanna vegetation. A few are in drought-tolerant 

forests of small, scattered trees. Many support as much or more vegetation as 

other soils with an ustic moisture regime, perhaps because rapid infiltration 

results in little or no precipitation being lost to runoff. Ustipsamments are used 

mainly for grazing. Few of them are cultivated because they are subject to soil 

blowing if cultivated. Ustipsamments are extensive on the Great Plains of the 

United States. (Back to Psamments key)
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Xeropsamments—These are the Psamments of Mediterranean-type

climates, where they are moist in winter and dry in summer. Temperatures

range from cool to hot. Most Xeropsamments formed in deposits of lateWisconsinan or more recent age. Some are on terraces and glacial outwash 

plains. Others are on dunes. Because the supply of winter moisture is reliable, 

few of the dunes are shifting. Some of these soils have supported coniferous 

vegetation, but most have supported and still support a mixture of grasses 

and xerophytic shrubs or trees. Xeropsamments are moderately extensive in 

parts of the western United States. (Back to Psamments key)

Udipsamments—These are the Psamments of humid regions. Precipitation is 

distributed throughout the year. These soils are predominantly in areas of latePleistocene or more recent deposits and are mostly brownish and freely 

drained. Most have supported forest vegetation, but a few have been 

cultivated ever since the sands were deposited. Others have been cultivated 

for a very long time. Udipsamments are extensive in the United States. Many 

are used as forest. Large areas of these soils, mostly where they have 

moderate to hot temperature regimes, have been cleared and are used as 

cropland or pasture. (Back to Psamments key)
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Fluvents Great Groups

Fluvents are mostly brownish to reddish soils that formed in recent waterdeposited sediments, mainly on flood plains, fans, and deltas of rivers and 

small streams but not in backswamps, where drainage is poor. An irregular 

decrease/increase of carbon content with increasing depth is the basis for the 

definition of Fluvents. Many Fluvents are frequently flooded unless they are 

protected by dams or levees. Stratification of the materials is typical. Former 

surface layers, now covered by more recent sediments, generally have higher 

contents of organic carbon than layers above or below. Most of the alluvial 

sediments are derived from eroding upland soils or streambanks and contain 

an appreciable amount of organic carbon, which is mainly in the clay fraction. 

Strata of clayey or loamy materials commonly have more organic carbon than 

the overlying more sandy strata. Thus, the percentage of organic carbon of 

Holocene age decreases irregularly with increasing depth if the materials are 

stratified. Because the deposits generally are loamy and recent, however, the 

percentage of carbon in the deep layers of Fluvents is higher than that in 

nearby soils that formed in parent materials other than alluvium. In humid 

regions, the age of the sediments is commonly a few years or decades or a 

very few hundred years. In arid regions, it may be somewhat greater.

Fluvents

Gelifluvents

Cryofluvents

Xerofluvents

Ustifluvents

Torrifluvents

Udifluvents
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-135

Profile of a Fluvent (specifically a Udifluvent) in China. The 

soil is comprised of many thin water-deposited strata 

and has a buried surface layer (black) in the lower part. 

Scale is in decimeters.

.

(Back to Fluvents)
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Key to Great Groups of Fluvents (Back to key to suborders)

Fluvents that that have:

1. Very cold average soil temperature ------------------------------ Gelifluvents

These soils have a gelic soil temperature regime.

2. Cold average soil temperature ------------------------------------ Cryofluvents

These soils have a cryic soil temperature regime.

3. A Mediterranean-type climate ------------------------------------- Xerofluvents

These soils have a xeric soil moisture regime (cool and moist in winter and

warm and dry in summer).

4. Somewhat limited soil moisture available for crop

growth ---------------------------------------------------------------------- Ustifluvents

These soils have an ustic soil moisture regime. Moisture is limited, but 

available, during portions of the growing season.

5. Inadequate soil moisture available for crop growth ------- Torrifluvents

These soils have an aridic (torric) soil moisture regime.

6. Seasonally well-distributed precipitation ------------------------ Udifluvents

These soils have a udic soil moisture regime.
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-137

Descriptions of Great Groups of Fluvents

Gelifluvents—These are the Fluvents of very cold regions. They have a gelic 

soil temperature regime. They tend to have a high content of rock fragments, 

and some may be shallow to bedrock. They are subject to wide variations in 

summer high temperatures and winter low temperatures, which result in mean 

annual soil temperatures of 0 degrees C or less, but without permafrost. In the 

United States, Gelifluvents occur in Alaska. (Back to Fluvents key)

Cryofluvents—These are the cold Fluvents. The growing season is short and 

cool. The vegetation typically is a coniferous or mixed forest. In the United 

States, these soils are mostly in Alaska but some are in mountain valleys in 

the West. Cryofluvents are moderately extensive. (Back to Fluvents key)

Xerofluvents—These soils have a Mediterranean-type climate (xeric moisture 

regime) where they are moist in winter and dry in summer. Temperatures 

range from cool to hot. These soils are on flood plains along rivers or streams 

or on alluvial fans. Flooding is most common in winter. Some of the soils are 

flooded in spring because of melting snow in the nearby mountains. The 

vegetation is commonly mixed forest or grass and shrubs. In the United 

States, Xerofluvents are mostly in California and the northwestern States. The 

soils are moderately extensive. (Back to Fluvents key)

Ustifluvents—These soils have an ustic moisture regime. Although moisture 

is limited, it is generally available during portions of the growing season. 

Temperatures range from cool to hot. These soils are on flood plains along 

rivers and streams in areas of mid or low latitudes. Flooding can occur in any 

season but is most common in summer in the mid latitudes and during the 

rainy season in the Tropics. A few of the soils are flooded regularly in summer, 

even though the summer is rainless, because of melting snow in high 

mountains. (Back to Fluvents key)

Torrifluvents—These are the Fluvents of arid climates. They have an aridic 

(or torric) moisture regime. Temperatures range from cool to hot. Most 

Torrifluvents have a high pH value and are calcareous, and a few are 

somewhat salty. They are subject to flooding, but most are not flooded 

frequently or for long periods. They commonly are irrigated where they occur 

as larger areas, have a favorable topography, and are close to a source of 

water. The natural vegetation on Torrifluvents in the United States consisted 

mostly of grasses, xerophytic shrubs, and cacti. In some parts of the world, 

however, the only vegetation on these soils has been irrigated crops because 

the sediments accumulated while the soils were being cultivated. (Back to 

Fluvents key)

Udifluvents—These soils have a udic moisture regime with precipitation 

distributed throughout the year. Temperatures range from cool to hot. 
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-138

Udifluvents are on flood plains along streams and rivers, and they may be 

flooded during almost any season. There is little or no evidence of alteration of 

the fine stratification in the alluvium, although in some Udifluvents, especially 

those with finer textures, identifying the stratification may be difficult. Some 

Udifluvents formed under forest vegetation, but many have had no vegetation 

other than pasture or cultivated crops because the sediments in which the 

soils formed were deposited while the soils were being used. Udifluvents are 

extensive in the United States. (Back to Fluvents key)
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-139

Orthents Great Groups

Orthents are primarily the Entisols on recent erosional surfaces. The erosion 

may have been geologic or been human-induced by cultivation, mining, or 

other activities. Any former soil that was on the landscape before accelerated 

erosion began has been completely removed or so truncated that the 

diagnostic horizons for all other soil orders do not occur. A few Orthents are in 

areas of recent loamy or fine eolian deposits, in areas of solifluction or glacial 

deposits, or in areas of debris from recent landslides and mudflows. Orthents 

occur in a wide range of climates and under any vegetation.

Orthents

Gelorthents

Cryorthents

Torriorthents

Xerorthents

Ustorthents

Udorthents
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-140

Profile of an Orthent (specifically a Udorthent). This 

soil formed in gravelly, cobbly, and stony 

colluvium. Other than the ochric epipedon, this 

soil has no other diagnostic horizons. Scale is in

inches.

(Back to Orthents)
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-141

Key to Great Groups of Orthents (Back to key to suborders)

Orthents that have:

1. Very cold average soil temperature ------------------------------- Gelorthents

These soils have a gelic soil temperature regime.

2. Cold average soil temperature ------------------------------------- Cryorthents

These soils have a cryic soil temperature regime.

3. Inadequate soil moisture available for crop growth ------- Torriorthents

These soils have an aridic (torric) soil moisture regime.

4. A Mediterranean-type climate --------------------------------------- Xerorthents

These soils have a xeric soil moisture regime (cool and moist in winter and 

warm and dry in summer).

5. Somewhat limited soil moisture available for crop

growth ----------------------------------------------------------------------- Ustorthents

These soils have an ustic soil moisture regime. Moisture is limited, but 

available, during portions of the growing season.

6. Seasonally well-distributed precipitation ------------------------ Udorthents

These soils have a udic soil moisture regime.
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-142

Descriptions of Great Groups of Orthents

Gelorthents—These are the Orthents of very cold regions. They have a gelic 

soil temperature regime but lack permafrost within the soil profile. They tend to 

have a high content of rock fragments, and some may be shallow to bedrock. 

They are subject to wide variations in summer high temperatures and winter 

low temperatures, which result in mean annual soil temperatures of 0 degrees 

C or less, but without permafrost. (Back to Orthents key)

Cryorthents—These are the Orthents of high mountains or high latitudes. 

Most of them are coarse in texture or are on slopes where depth to bedrock is 

very shallow or in areas of recent solifluction or volcanic deposits. The 

vegetation may be coniferous forest, tundra, or the sparse vegetation of very 

cold, dry areas. Few areas are cultivated. These soils are of small extent in 

high mountains and in areas of tundra vegetation in the United States. Some 

of the soils are cold and dry, have a thin efflorescence of salt on the surface 

during summer, and may be dry in all horizons during some periods. Most of 

the Cryorthents in the United States still support their native vegetation. (Back 

to Orthents key)

Torriorthents—These are the dry Orthents of cool to hot, arid regions. They 

have an aridic (or torric) moisture regime. Generally, they are neutral or 

calcareous and are on moderate to very steep slopes. A few are on gentle 

slopes. Many of the gently sloping soils are on rock pediments, are very 

shallow, have gravelly textures, or are salty. Others are on fans where 

sediments are recent but have little organic carbon. The vegetation on 

Torriorthents commonly is sparse and consists mostly of xerophytic shrubs 

and ephemeral grasses and forbs. The vegetation on a few of the soils is 

saltgrass. Torriorthents are used mainly for grazing. They are extensive in the 

western United States. (Back to Orthents key)

Xerorthents—These are the Orthents of Mediterranean-type climates (xeric 

moisture regime) where they are moist in winter and dry in summer. 

Temperatures range from cool to hot. The soils are generally neutral to 

moderately alkaline, but some are acid. Slopes are mostly moderate to steep 

but are gentle in a few areas. Xerorthents commonly are coarse in texture or 

occur in areas of very recently exposed regolith (such as loess or till), in areas 

of weakly cemented rocks (such as shale), or in areas of very thin regolith 

over hard rocks. The vegetation is commonly trees or shrubs, or the soils are 

used as pasture. (Back to Orthents key)

Ustorthents—These are the Orthents of cool to hot regions. They have an 

ustic moisture regime. Although moisture is limited, it is generally available 

during portions of the growing season. The soils are generally neutral to 

calcareous, but some are acid. Slopes are mostly moderate to steep but are 

gentle in a few areas. Ustorthents commonly occur in areas of very recently 
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-143

exposed regolith, mostly weakly cemented sedimentary deposits, or in areas 

of thin regolith over hard rocks. The vegetation in warm regions commonly is a 

deciduous forest or savanna. The soils that have a cooler temperature regime 

commonly support scattered grasses mixed with xerophytic shrubs. 

Ustorthents are extensive in the United States, particularly on the Great 

Plains. (Back to Orthents key)

Udorthents—These are the Orthents that have a udic moisture regime with 

precipitation distributed throughout the year. Temperatures range from cool to 

hot. These soils are generally acid to neutral, but some are calcareous. 

Slopes generally are moderate to steep but are gentle in a few areas. 

Udorthents commonly occur in areas of very recently exposed regolith (such 

as loess or till), in areas of weakly cemented rocks (such as shale), or in areas 

of thin regolith over hard rocks. Many of the gently sloping soils are the result 

of mining or other earth-moving activities. The vegetation is commonly a 

deciduous forest, or the soils are used as pasture. Udorthents are extensive 

on steep slopes in the humid parts of the United States. (Back to Orthents 

key)
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-144

Geolisols Order

Gelisols are very cold soils with permafrost in the subsoil.

General Characteristics

The distinguishing characteristics of Gelisols are very cold soil temperatures

and a permanently frozen part (permafrost) within a depth of 200 cm. Periodic 

freezing and thawing above the permafrost can result in intense frost churning 

(cryoturbation) which can retard the development of soil horizons in this active 

zone and may result in irregular or broken horizons, involutions, or organic 

matter accumulation above the permafrost. Where cryoturbation is less 

intense, ochric (typically thin and/or light-colored), mollic (rich in humus and 

bases), umbric (humus-rich with low base saturation), and histic (wet, organic 

surface layer) epipedons and argillic (clay accumulation), salic (high salts), 

gypsic (gypsum accumulation), and calcic (calcium carbonate accumulation) 

subsoil horizons have been observed.

Environment and Processes

The most important environmental factor in the formation of Gelisols is the

very cold temperature. Because of the very cold temperature, weathering 

processes proceed very slowly. Gelisols form in a variety of parent materials,

including glacial deposits, mountain colluvium, residuum, and loess, but they 

also can form in organic materials. Gelisols is the only soil order that includes 

both organic soils and mineral soils. Cryoturbation is an important dynamic 

physical process in many Gelisols. Unique “patterned ground” land surface 

features are characteristic of some areas of Gelisols, especially those with 

thick permafrost. These features include sorted and unsorted rock circles and 

stripes, large polygonal surface patterns, ice wedges, frost boils, and pingos. 

The presence of permafrost presents significant challenges to engineering 

projects, such as road building and maintenance, housing development, and 

construction of pipelines. Gelisols are sensitive to disturbance, and problems 

may result when the permafrost melts and the soils become unstable. An 

important environmental consequence of the combined very cold 

temperatures and frost churning has been the accumulation of organic matter 

within and above the permafrost zone. There is a potential for significant 

contributions to global warming if the permafrost melts and the organic matter 

decays, causing the evolution of carbon dioxide and methane.

Location

Gelisols are found in tundra, taiga, and boreal forests at high latitudes and 

occur in both the northern and southern circumpolar regions. While they occur 

mostly in moist areas, Gelisols have also been recognized in the very dry 
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-145

valleys of Antarctica. They also may occur at very high altitudes in alpine 

areas on some mountains in the lower latitudes, although these are not known 

to be extensive. They occur in much of Alaska as well as in Canada, 

Greenland, Northern Europe, Russia, China, Mongolia, and Antarctica. 

Globally, Gelisols occupy about 9% of the ice-free land area, but because of 

the extreme environments in which they are found, they only support about 

0.4% of the world’s population.

Profile of a Gelisol (specifically an Aquiturbel) in Alaska. 

Permafrost is at a depth of about 50 cm. Scale is 

in cm. (Photo courtesy of Dr. David Weindorf)

(Back to Key to Soil Orders)
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-146

Gelisols by suborder in the United States.

Global distribution of Gelisols.
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-147

Gelisols Suborders 

Classification of Gelisols 

In the definitions of the suborders, emphasis is divided between significant 

accumulations of organic matter (Histels), evidence of cryoturbation or frost 

churning (Turbels), and little evidence of cryoturbation and a lack of significant 

amounts of organic matter (Orthels).

The great group level reflects a combination of important properties, including 

the presence of diagnostic horizons, the degree of seasonal saturation, the 

presence of significant amounts of organic soil material and their degree of 

decomposition, sandy textures, the presence of massive ice in the soil, and 

very dry conditions.

The three suborders are:

1. Histels—Gelisols dominated by organic soil materials

2. Turbels—Gelisols with morphology characteristic of cryoturbation

3. Orthels—other simple Gelisols

Gelisols

Histels

Turbels

Orthels
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-148

Key to Suborders of Gelisols

Gelisols that:

1. Have organic soil material making up > 40 cm (within the upper 

50 cm) ------------------------------------------------------------------------------ Histels

See 12th edition of “Keys to Soil Taxonomy” for specific details regarding 

required thickness. 

2. Have evidence of cryoturbation (intense frost churning) within the 

profile ----------------------------------------------------------------------------- Turbels

Evidence includes irregular, broken, or distorted horizon boundaries;

involutions; the accumulation of organic matter on top of the permafrost;

ice or sand wedges; and oriented rock fragments.

3. Have horizons of mineral soil material and are not 

cryoturbated --------------------------------------------------------------------- Orthels
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Histels Great Groups

Histels are the Gelisols with large amounts of organic carbon that commonly 

accumulate under wet, anaerobic conditions. In some cases, Histels are well 

drained and very coarse textured with organic matter filling voids between

rock fragments. Cold temperatures contribute to the accumulation of organic 

matter.

Histels

Folistels

Glacistels

Fibristels

Hemistels

Sapristels
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-150

Profile of a Histel (specifically a Fibristel) in Alaska. The organic soil material

(brown and black) ranges from about 20 to 40 cm in thickness and is 

intermingled with the underlying mineral soil material due to cryoturbation. 

Permafrost is at a depth of about 40 cm. Scale is in decimeters.

(Back to Histels)
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-151

Key to Great Groups of Histels (Back to key to suborders)

Histels that:

1. Are freely drained ------------------------------------------------------------ Folistels

These soils are saturated < 30 days (cumulative) each year and are not 

artificially drained.

2. Have a glacic layer (> 75% ice) within a depth of 100 cm ---- Glacistels

These soils have < 75% sphagnum fibers in the upper 50 cm. The ice layer 

is > 30 cm thick.

3. Are predominantly composed of fibric (slightly decomposed) soil 

material to a depth of 50 cm or more -------------------------------- Fibristels

4. Are predominantly composed of hemic (moderately decomposed) 

soil material to a depth of 50 cm or more -------------------------- Hemistels

5. Are predominantly composed of sapric (highly decomposed) soil 

material to a depth of 50 cm or more ------------------------------- Sapristels
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Descriptions of Great Groups of Histels

Folistels—These are the more or less freely drained Histels that consist 

primarily of organic layers (O horizons) derived from plant litter resting directly 

on rock, or they formed on very coarse materials that consist of gravel or 

larger rock fragments in which the voids are mostly filled with organic 

materials. Plant roots grow in the organic materials. These soils are rare in the 

world. (Back to Histels key)

Glacistels—These are the wet Histels that have 30 cm or more of ice within a 

depth of 100 cm. They are saturated with water for > 30 total days each year. 

Disturbance of the surface-insulating layer causes the ice layer to melt and 

the soils to collapse. Glacistels occur in Alaska, Canada, and Siberia. (Back to 

Histels key)

Fibristels—These are the wet Histels in which the organic materials are 

slightly decomposed. Most of the fiber is not destroyed by rubbing between 

the thumb and fingers. The botanical origin of much of the material can be 

readily determined. Many of these soils have ground water near the surface 

nearly all the time. Fibristels occur in depressional or level areas, commonly 

adjacent to water bodies. They are of small extent and occur in Alaska, 

Canada, and Siberia. (Back to Histels key)

Hemistels—These are the wet Histels in which the organic materials are 

moderately decomposed. The botanical origin of the organic material can only 

be determined for some of the material. The fiber content is about one-sixth to 

two-thirds after rubbing between the thumb and fingers. Water is at or very 

close to the surface of these soils much of the time, unless artificial drainage 

is provided. The level of ground water may fluctuate but seldom drops much 

below the surface. Hemistels occur in depressional areas or adjacent to water 

bodies. These soils are known to occur in Siberia, Canada, and Alaska. (Back 

to Histels key)

Sapristels—These are the wet Histels in which the organic materials are well 

decomposed. The botanical origin of the organic material is difficult to 

determine. The fiber content of most of the organic material is less than onesixth after rubbing between the the thumb and fingers. Sapristels occur in 

areas where ground water tables tend to fluctuate within the soil or where the 

soil was better aerated during drier periods in the past, which led to increased 

organic matter decomposition. (Back to Histels key)
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Turbels Great Groups

Turbels are the Gelisols that have one or more horizons with evidence of 

cryoturbation (intense frost churning) in the form of irregular, broken, or 

distorted horizon boundaries; involutions; the accumulation of organic matter 

on top of the permafrost; ice or sand wedges; or oriented rock fragments. 

Cryoturbation occurs only in soils that have sufficient moisture for the 

formation of ice crystals. Soils that have cryoturbated horizons and are dry for 

most of the year were probably moister in the past. Turbels are the dominant 

suborder of Gelisols. They account for about half of the Gelisols worldwide. 

These soils are common in the High and Middle Arctic Vegetation Regions of 

North America and Eurasia, at latitudes of 65 degrees north or more.

Turbels

Histoturbels

Aquiturbels

Anhyturbels

Molliturbels

Umbriturbels

Psammoturbels

Haploturbels
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-154

Profile of a Turbel (specifically a Haploturbel) in Alaska. Involutions of soil 

material (brown) are mixed into the underlying soil by cryoturbation.

(Back to Turbels)
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-155

Key to Great Groups of Turbels (Back to key to suborders)

Turbels that have:

1. More than 40% organic soil materials in the upper 

50 cm ----------------------------------------------------------------------- Histoturbels

Organic soil materials are in > 30% of the pedon. Histoturbels cannot have 

a folistic epipedon (freely drained, organic surface layer).

2. A seasonal high water table within a depth of 50 cm ------- Aquiturbels

Redoximorphic features (gray and red color patterns) are evidence of a 

seasonal high water table. Artificially drained sites are included in 

Aquiturbels. See 12th edition of “Keys to Soil Taxonomy” for specific 

criteria regarding color of redoximorphic features.

3. Very dry soil moisture conditions (anhydrous 

conditions) ---------------------------------------------------------------- Anhyturbels

Water is held at > 1500 kPa tension for much of the time when soil 

temperature is > 0

oC. See 12th edition of “Keys to Soil Taxonomy” for 

specific criteria for anhydrous conditions.

4. A mollic (rich in humus and bases) epipedon ----------------- Molliturbels

5. An umbric (humus-rich with low base saturation) 

epipedon ----------------------------------------------------------------- Umbriturbels

6. Sandy texture throughout the upper 100 cm ------------- Psammoturbels

Psammoturbels have < 35% rock fragments. Required depth of sandy

textures is less in profiles with bedrock above a depth of 100 cm.

7. An ochric (typically thin and/or light-colored) epipedon and a 

cryoturbated subsoil ------------------------------------------------- Haploturbels
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Descriptions of Great Groups of Turbels

Histoturbels—These soils have organic soil materials making up much, or all, 

of the surface soil to a depth of 50 cm or more. Commonly, the organic 

materials are thicker in the micro-low positions on the landscape and thinner 

in the micro-high positions. These soils are commonly saturated at or near the 

surface for some time during most years. They commonly have redoximorphic 

features (gray and red color patterns). (Back to Turbels key)

Aquiturbels—These soils are saturated with water close to the surface. 

Saturation commonly occurs in spring, when water perches on the permafrost 

and temperatures are warm enough for oxygen depletion and iron reduction. 

These soils can have mollic (rich in humus and bases), umbric (humus-rich 

with low base saturation), or ochric (typically thin and/or light-colored) 

epipedons. They occur in depressional areas in Alaska, Canada, and Eurasia. 

(Back to Turbels key)

Anhyturbels—These soils have anhydrous conditions (very dry) and may 

have dry permafrost (i.e., insufficient moisture for interstitial ice). The cold 

deserts where Anhyturbels occur commonly receive less than 30 mm of 

precipitation annually. These soils support little or no vegetation. Anhyturbels 

are limited in extent but occur in continental Antarctica, northern Greenland, 

and the cold, dry mountains of Eurasia at elevations of more than 3,700 

meters. These soils have experienced climatic periods in the past when more 

moisture was available. (Back to Turbels key)

Molliturbels—These soils have a mollic (rich in humus and bases) epipedon. 

They are known to occur in Alaska, Canada, and Siberia. On the North Slope 

of the Brooks Range in Alaska, they support vegetation that differs from that 

on the more acid Umbriturbels. (Back to Turbels key)

Umbriturbels—These soils have an umbric (humus-rich with low base 

saturation) epipedon. Umbriturbels are known to occur in Alaska, Canada, 

Greenland, and Siberia. On the North Slope of the Brooks Range in Alaska, 

they support vegetation that differs from that on Molliturbels. (Back to Turbels 

key)

Psammoturbels—These soils have less than 35% (by volume) rock 

fragments and a texture of loamy fine sand or coarser in all layers to a depth 

of 100 cm, or to a root-limiting layer. They commonly have an ochric (typically 

thin and/or light-colored) epipedon. (Back to Turbels key)

Haploturbels—These soils have an ochric (typically thin and/or light-colored) 

epipedon and have sufficient moisture for cryoturbation. Commonly, the 

cryoturbation is not well expressed. These soils occur in Alaska, Canada, and 

Siberia. (Back to Turbels key)
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Orthels Great Groups

Orthels are the Gelisols that show little or no evidence of cryoturbation 

(intense frost churning) and are the second most abundant suborder of 

Gelisols. These soils occur primarily within the zone of widespread permafrost 

or in areas of coarse textured materials in the continuous zone of permafrost. 

Orthels are generally drier than Turbels and Histels. They occur in the 

southern Andes and the high latitudes of the Northern Hemisphere.

Orthels

Historthels

Aquorthels

Anhyorthels

Mollorthels

Umbrorthels

Argiorthels

Psammorthels

Haplorthels
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-158

Key to Great Groups of Orthels (Back to key to suborders)

Orthels that have:

1. A histic epipedon (wet, organic surface layer) ----------------- Historthels

2. A seasonal high water table within a depth of 50 cm -------- Aquorthels

Redoximorphic features (gray and red color patterns) are evidence of a 

seasonal high water table. Artificially drained sites are included in 

Aquiturbels. See 12th edition of “Keys to Soil Taxonomy” for specific 

criteria regarding color of redoximorphic features.

3. Very dry soil moisture conditions (anhydrous 

conditions) ---------------------------------------------------------------- Anhyorthels

Water is held at > 1500 kPa tension for much of the time when soil 

temperature is > 0

oC. See 12th edition of “Keys to Soil Taxonomy” for

specific criteria for anhydrous conditions.

4. A mollic (rich in humus and bases) epipedon ------------------ Mollorthels

5. An umbric (humus-rich with low base saturation) 

epipedon ------------------------------------------------------------------ Umbrorthels

6. An argillic (clay accumulation) subsoil horizon within a depth of 

100 cm ----------------------------------------------------------------------- Argiorthels

7. Sandy texture between depths of 25 and 100 cm -------- Psammorthels

Psammorthels have < 35% rock fragments. Required depth of sandy

textures is less in profiles with bedrock above a depth of 100 cm.

8. An ochric (typically thin and/or light-colored) epipedon and a cambic 

(minimal soil development) subsoil horizon ------------------- Haplorthels

Some Haplorthels do not have a cambic horizon.
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Descriptions of Great Groups of Orthels

Historthels—These soils have organic materials at the surface. Commonly, 

the organic materials are thicker in the micro-low positions on the landscape 

and thinner in the micro-high positions. These soils are commonly saturated at 

or near the surface for some time during most years and have redoximorphic 

features (gray and red color patterns). (Back to Orthels key)

Aquorthels—These soils are saturated and depleted of oxygen (reduced) 

close to the surface for at least part of the year. They have redoximorphic

depletions (gray areas with loss of iron/manganese oxides and/or clay) with

chroma of 2 or less. These soils are common in the center of large-scale, lowcentered polygons of patterned ground along the North Slope of the Brooks 

Range in Alaska. (Back to Orthels key)

Anhyorthels—These soils have anhydrous conditions (very dry). They 

commonly have dry permafrost (i.e., insufficient moisture for interstitial ice). 

The cold deserts where Anhyorthels are found commonly receive less than 30 

mm of precipitation annually. These soils support little or no vegetation. They

are limited in extent but are known to occur in continental Antarctica, the High 

Arctic (northern Greenland and Ellesmere Island), and the cold, dry mountains 

of Eurasia at elevations of more than 3,700 meters. (Back to Orthels key)

Mollorthels—These soils have a mollic (rich in humus and bases) epipedon. 

They are more or less well drained or moderately well drained. They are 

known to occur in Alaska, Canada, and Siberia. On the North Slope of the 

Brooks Range in Alaska, they support vegetation that differs from that on the 

more acid Umbrorthels. (Back to Orthels key)

Umbrorthels—These soils have an umbric (humus-rich with low base 

saturation) epipedon. They are more or less well drained or moderately well 

drained. They are known to occur in Alaska, Canada, and Siberia. On the 

North Slope of the Brooks Range in Alaska, they support vegetation that 

differs from that on Mollorthels. (Back to Orthels key)

Argiorthels—These soils have an argillic (clay accumulation) subsoil horizon. 

They are typically well drained or moderately well drained. The argillic horizon 

in these soils commonly is weakly expressed. Argiothels are rare in the world 

but are known to occur in northern Siberia. (Back to Orthels key)

Psammorthels—These soils have less than 35% (by volume) rock fragments 

and a texture of loamy fine sand or coarser in all layers between depths of 25 

and 100 cm, or to a root-limiting layer. They are more or less well drained or

moderately well drained. These soils commonly have an ochric (typically thin 

and/or light-colored) epipedon. (Back to Orthels key)
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Haplorthels—These soils have an ochric (typically thin and/or light-colored) 

epipedon and commonly a cambic (minimal soil development) subsoil horizon

but have insufficient moisture prior to freezing for cryoturbation. They are 

typically well drained or moderately well drained. They occur in Alaska, 

Canada, and Siberia. (Back to Orthels key)
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-161

Histosols Order

Histosols are soils that formed in decaying organic material.

General Characteristics

Histosols formed in thick accumulations of organic matter from decaying plant 

material. The organic-dominated layers are typically at least 40 cm thick and 

commonly much thicker. They have a minimum of 12 to 18% organic carbon,

by weight (depending on clay content), and most have significantly more than 

this. Histosols do not exhibit the kinds of horizons common to mineral soils but 

rather have layers, or tiers, that vary in color, botanical origin of the organic 

material, amount of mixed-in mineral soil material, degree of decomposition, 

and other properties. Histosols generally have significantly lower bulk density 

and higher nutrient- and water-holding capacities than most mineral soils.

Environment and Processes

Histosols occur at all latitudes, ranging from the tropics to temperate areas 

and extending to the boreal forests of high latitudes. They form in 

environments where inputs of organic matter exceed losses due to 

decomposition. These conditions are generally in cool, wet environments 

where precipitation exceeds evaporation. Histosols are common in low-lying 

areas with water-logging and anaerobic (oxygen-depleted) conditions. Some 

are on hillsides where local ground-water seeps are prevalent. However, 

some Histosols are more or less well drained and formed in cool upland 

positions. These Histosols are characterized by organic soil layers over 

shallow bedrock, or they are very gravelly to extremely bouldery and have

organic soil material filling the voids between rock fragments. Histosols also 

occur in permanently submerged areas as subaqueous soils. The general 

properties of the Histosols in a given area are reflective of the general types of 

plants that supplied the organic matter (for example, peat, sedges, rushes, or 

woody plants), pH, activity of microbes, and the hydrology of the area. In 

general, Histosols in areas of ground-water discharge tend to have a higher 

nutrient status than those in areas of ground-water recharge. 

Location

Globally, Histosols occupy about 1% of the ice-free land area. They are 

particularly extensive in the northern boreal areas of North America, Northern 

Europe, and Russia. In the United States, Histosols are fairly extensive in

Alaska, along the southeast coast, and in the north-central States. They can 

be found in many locations where cool, wet conditions promote the 

accumulation of organic soil deposits.
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Profile of a Histosol.

(Back to Key to Soil Orders)
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-163

Histosols by suborder in the United States. Note: This map was 

produced before the introduction of the Wassists suborder.

Wassists are predominantly in shallow-water coastal bays and 

estuaries and may also occur in some inland freshwater areas.

Global distribution of Histosols.
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-164

Histosols Suborders

Classification of Histosols

In the definitions of the suborders, emphasis is placed on drainage (Folists),

permanent submergence (Wassists), and the general level of organic matter 

decomposition (Fibrists, Hemists, and Saprists). 

The great group level reflects a combination of important properties, including 

cold soil temperature; soil moisture regimes (in the Folists); the presence of a 

sulfuric (highly acid due to oxidation and production of sulfuric acid) horizon or 

sulfidic materials (acid-producing, oxidizable sulfur compounds) as well as the 

presence of humilluvic material (illuviated humus); and the dominance of 

Sphagnum (with its unique water-holding capacity) as the botanical source of 

the organic material. In addition, the great groups of the Wassists are 

separated based on inundation by fresh, brackish, or salty water.

The five suborders are:

1. Folists—freely drained Histosols (saturated less than 30 days)

2. Wassists—permanently submerged Histosols

3. Fibrists—wet Histosols exhibiting the least decomposition

4. Saprists—wet Histosols exhibiting the most decomposition

5. Hemists—wet Histosols exhibiting intermediate decomposition

Histosols

Folists

Wassists

Fibrists

Saprists

Hemists
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Key to Suborders of Histosols

Histosols that are:

1. Freely drained ------------------------------------------------------------------- Folists

These soils are saturated < 30 days (cumulative) each year and are not

artificially drained.

2. Continuously and permanently inundated by water ------------ Wassists

Wassists include soils in tidal areas that are exposed to the atmosphere 

< 3 hours each day. Water is shallow enough (≤ 2.5 m) to permit rooted, 

often submerged vegetation to grow.

3. Predominantly (> 50%) composed of fibric (slightly decomposed) soil 

material between depths of 30 and 90 cm ---------------------------- Fibrists

If a mineral soil layer > 40 cm thick begins within a depth of 90 cm, then 

fibric soil material must be dominant from the soil surface to the top of the 

mineral layer. Fibrists do not have a sulfuric (highly acid due to oxidation 

and production of sulfuric acid) horizon within a depth of 50 cm or sulfidic 

materials (acid-producing, oxidizable sulfur compounds) within a depth of 

100 cm.

4. Predominantly (> 50%) composed of sapric (highly decomposed) soil 

material between depths of 30 and 90 cm --------------------------- Saprists

If a mineral soil layer > 40 cm thick begins within a depth of 90 cm, then 

sapric soil material must be dominant from the soil surface to the top of the 

mineral layer. 

5. Predominantly (> 50%) composed of hemic (moderately decomposed) 

soil material between depths of 30 and 90 cm --------------------- Hemists

If a mineral soil layer > 40 cm thick begins within a depth of 90 cm, then 

hemic soil material must be dominant from the soil surface to the top of the 

mineral layer. 
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Folists Great Groups

Folists are the more or less freely drained Histosols that consist primarily of O 

horizons derived from leaf litter, twigs, and branches resting on rock or on 

fragmental materials (gravel, cobbles, stones, and boulders) in which the 

interstices are filled or partly filled with organic materials. Plant roots grow only 

in the organic materials. Many of these soils are in very humid climates,

ranging from the Tropics to high latitudes and high elevations, but some are 

semiarid or arid areas. Most of the Folists in the United States are in Hawaii 

and Alaska. There are some Folists in the mountains in the western part of the 

United States, in the northern Lake States, and in the northeastern part of the 

United States.

Folists

Cryofolists

Torrifolists

Ustifolists

Udifolists
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Key to Great Groups of Folists (Back to key to suborders)

Folists that have:

1. Cold average soil temperature --------------------------------------- Cryofolists

These soils have a cryic soil temperature regime.

2. Inadequate soil moisture available for crop growth ---------- Torrifolists

These soils have an aridic (or torric) soil moisture regime.

3. Somewhat limited soil moisture available for crop

growth ------------------------------------------------------------------------- Ustifolists

These soils have either an ustic or xeric soil moisture regime.

4. Seasonally well-distributed precipitation -------------------------- Udifolists

These soils have a udic soil moisture regime.
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Descriptions of Great Groups of Folists

Cryofolists—These are the cold Folists. In the United States, they occur 

mostly in southeastern Alaska, where the climate is cool, oceanic, and very 

humid. They are on gentle to very steep slopes. Their vegetation is primarily 

coniferous forests, and the only part of the soils in which there are plant roots 

is the O horizon. Beneath the O horizon, there may be a few centimeters of 

mineral soil material, or bedrock, or there may be fragmental materials in 

which the interstices are filled or partly filled with organic materials. (Back to

Folists key)

Torrifolists—These are the Folists of arid regions. They occur in Hawaii, 

chiefly on lava flows in areas that receive little rainfall. The vegetation is 

mostly fountaingrass mixed with shrubs. (Back to Folists key)

Ustifolists—These are the Folists with somewhat limited soil moisture 

available for plant growth. They have either an ustic moisture regime 

(moisture is limited, but available, during periods of plant growth) or a xeric 

moisture regime (cool and moist in winter and warm and dry in summer).

These soils have an O horizon that overlies, or is in, fragmental materials or 

that directly overlies bedrock that is less than 50 cm from the surface. (Back to 

Folists key)

Udifolists—These are the Folists in humid areas with seasonally welldistributed precipitation. They have a udic (or perudic) soil moisture regime. 

They occur in Hawaii, chiefly on lava flows in areas that receive very high 

amounts of well-distributed rainfall. The vegetation is mostly forest mixed with 

tree ferns. (Back to Folists key)
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Wassists Great Groups

Wassists are the Histosols with a subaqueous drainage class. They formed in 

shallow, permanently submerged environments and have a positive water 

pressure at the soil surface for more than 21 hours each day in all years. The 

21-hour minimum allows for short daily exposure of the soil surface in areas 

with large tidal fluctuations, such as the coastal areas of eastern Maine in the 

United States. Some Wassists formed in saltwater environments, while others 

formed in freshwater lakes and ponds with low salinity. Most Wassists are 

known to occur in Holocene-age depositional environments. The organic 

materials are derived from wood, grasses, sedges, submerged aquatic 

vegetation, or any combination of these materials. Wassists are known to 

occur in any climate warmer than cryic. These soils are important for wildlife 

habitat.

Wassists

Frasiwassists

Sulfiwassists

Haplowassists
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Key to Great Groups of Wassists (Back to key to suborders)

Wassists that have:

1. Little or no salinity --------------------------------------------------- Frasiwassists

To a depth of 100 cm, electrical conductivity is < 0.2 dS/m (measured in a 

5:1, by volume, mixture of water and soil).

2. Sulfidic materials (acid-producing, oxidizable sulfur compounds) 

within a depth of 50 cm --------------------------------------------- Sulfiwassists

The layer containing sulfidic materials is > 15 cm thick. 

3. Significant salinity levels ----------------------------------------- Haplowassists

To a depth of 100 cm, electrical conductivity is > 0.2 dS/m (measured in a 

5:1, by volume, mixture of water and soil).
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Descriptions of Great Groups of Wassists

Frasiwassists—These are the Wassists that formed in fresh water. They 

have a low electrical conductivity (< 0.2 dS/m) in a 1:5 (by volume) soil:water

ratio throughout the upper meter. These soils are derived from freshwater 

organic and loamy materials and occur in natural and man-made water 

bodies. Many of these soils have high levels of nutrients that support a large 

number of native and non-native vegetation. They are believed to be of large 

extent in the United States and are most extensive in mid to northern 

latitudes. (Back to Wassists key)

Sulfiwassists—These are the Wassists that formed in salt or brackish water 

and have horizons with sulfidic materials totaling 15 cm or more within the 

upper 50 cm. They are potentially extremely acid or ultra acid. These soils are 

of small extent in the United States and occur mainly along the Atlantic and 

Gulf coasts. (Back to Wassists key)

Haplowassists—These are the Wassists that formed in salt or brackish water 

and that do not have a thick layer of sulfidic materials near the surface. They

are of small extent in the United States, occurring mainly along the Atlantic 

and Gulf coasts. (Back to Wassists key)
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Fibrists Great Groups

Fibrists are the wet Histosols in which the organic materials are only slightly 

decomposed. A significant amount of the soil consists of fibers that remain 

after rubbing between the thumb and fingers. The botanic origin of the 

materials can be readily determined. The bulk density is commonly less than 

0.1 g/cm3. Many Fibrists have ground water near the soil surface nearly all the 

time. A few areas of Fibrists are artificially drained. The level of the ground 

water fluctuates but seldom drops much below about 30 cm.

Fibrists are of relatively small extent but occur from the Equator to the high 

latitudes. They are in closed depressions and in broad flat areas, such as 

coastal plains. Most are under natural vegetation.

Fibrists

Cryofibrists

Sphagnofibrists

Haplofibrists
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Key to Great Groups of Fibrists (Back to key to suborders)

Fibrists that have:

1. Cold average soil temperature -------------------------------------- Cryofibrists

These soils have a cryic soil temperature regime.

2. Sphagnum composing > 3/4 of the volume to a depth of 

90 cm ------------------------------------------------------------------ Sphagnofibrists

See 12th edition of “Keys to Soil Taxonomy” for specific criteria regarding 

depth of Sphagnum-dominated material.

3. Vegetation of which more than 1/4 of the volume is something other 

than Sphagnum -------------------------------------------------------- Haplofibrists

Vegetation includes wood, grasses, sedges, mosses, and other 

herbaceous plants or some combination of these.
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Descriptions of Great Groups of Fibrists

Cryofibrists—These are the cold Fibrists. The fibers may be derived from any 

plant, woody or herbaceous. These soils may freeze during the winter, or they 

may have a climate in which they do not freeze during winter in most years 

but are cold in summer. In either situation, the low temperatures limit the use 

of the soils. Most of these soils support native vegetation. (Back to Fibrists 

key)

Sphagnofibrists—These Fibrists are derived mainly from the various species 

of Sphagnum and associated herbaceous plants. At least three-fourths of the 

fibers, by volume, are Sphagnum in at least the upper 90 cm of the soil or 

throughout the soil if there is a root-limiting layer within a depth of 90 cm. Most 

of these soils have cool temperatures. The structure of Sphagnum, with its 

water-holding cells, makes these soils unique. For this reason, these soils are 

called Sphagnofibrists. (Back to Fibrists key)

Haplofibrists—These are the Fibrists in which more than one-fourth of the 

fiber volume is derived from wood, grasses, sedges, mosses, and other 

herbaceous plants or from some combination of these. If these soils are 

drained and cultivated using present technology, the organic materials will 

decompose either slowly or rapidly, depending on the management used and 

temperatures. Most of these soils in the United States support native 

vegetation. (Back to Fibrists key)
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Saprists Great Groups

Saprists are the wet Histosols in which the organic materials are well 

decomposed. The botanic origin of the organic material is difficult to determine 

in most of these soils. The fiber content is less than one-sixth after rubbing 

between the thumb and fingers. Most of these soils have a bulk density of 

more than 0.2 g/cm3. 

Saprists occur in areas where the ground-water table tends to fluctuate within 

the soil or in areas where the soil was aerobic during drier periods in the past. 

They consist of the residue that remains after the aerobic decomposition of 

organic matter. When drained, fibric and hemic soil materials commonly 

decompose and form sapric soil materials. If the organic materials are deep 

and are drained either artificially or naturally, Fibrists and Hemists are 

converted after some decades to Saprists.

Saprists

Sulfosaprists

Sulfisaprists

Cryosaprists

Haplosaprists
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Profile of a Saprist (specifically a Haplosaprist) in Malaysia. This soil 

consists of well decomposed organic soil material.

(Back to Saprists)
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Key to Great Groups of Saprists (Back to key to suborders)

Saprists that have:

1. A sulfuric (highly acid due to oxidation and production of sulfuric 

acid) horizon within a depth of 50 cm -------------------------- Sulfosaprists

2. Sulfidic materials (acid-producing, oxidizable sulfur compounds) 

within a depth of 100 cm --------------------------------------------- Sulfisaprists

3. Cold average soil temperature ------------------------------------ Cryosaprists

These soils have a cryic soil temperature regime.

4. Organic materials originating from woody, mossy, grassy, or 

herbaceous botanical sources ---------------------------------- Haplosaprists
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Descriptions of Great Groups of Saprists

Sulfosaprists—These are the acid sulfate soils that consist of organic soil 

materials. They have a sulfuric horizon that formed as a result of draining soil

which contain sulfidic materials. They are extremely acid and are toxic to most 

plants. They are mainly in drained coastal marshes or deltas near the mouths 

of rivers that carry sediments containing few or no carbonates. Most of these 

soils have an appreciable amount of mineral material within the control 

section. (Back to Saprists key)

Sulfisaprists—These are the potentially acid sulfate soils that consist of 

organic soil materials. They have sulfidic materials within a depth of 100 cm

and have not been drained. They occur mainly in coastal marshes near the 

mouths of rivers or in the deltas of rivers that carry sediments with a low 

content of carbonates. These soils are locally extensive in the coastal 

marshes and deltas of some large rivers that drain humid regions. (Back to 

Saprists key)

Cryosaprists—These are the cold Saprists that do not have a sulfuric horizon 

with its upper boundary within a depth of 50 cm and do not have sulfidic 

materials within a depth of 100 cm. They have a cryic temperature regime. 

These soils formed from many kinds of plant materials, including wood, moss, 

grass, and herbaceous materials. Some of these soils freeze during winter, 

and some do not. Those that do not freeze are insulated by snow cover or 

have a marine climate in which winters are mild and summers are very cool. If 

these soils are used as cropland, low soil temperatures in summer limit their 

suitablility for crops. Most of these soils support native coniferous forest 

vegetation. (Back to Saprists key)

Haplosaprists—These Saprists are composed of organic materials derived 

from many kinds of plant materials, including wood, moss, grass, and 

herbaceous materials. If these soils are drained and cultivated using present 

technology, the organic materials decompose and disappear slowly or rapidly, 

depending on the management used and temperatures. Eventually, within 

some decades, the Haplosaprists that are drained and cultivated will convert

to mineral soils. This conversion has been observed in the United States. 

Many Haplosaprists support native vegetation, mostly forest plants or shrubs 

and grass-like plants. Many areas are cleared, drained, and used as cropland. 

Some Haplosaprists are saline due to their proximity to coastal areas that are 

occasionally inundated by the ocean. (Back to Saprists key)
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Hemists Great Groups

Hemists are the wet Histosols in which the organic materials are moderately 

decomposed. The botanic origin of much of the organic material cannot be 

readily determined. The fiber content of much of the organic material is 

between one-sixth and two-thirds after rubbing between the thumb and 

fingers. The bulk density commonly is between 0.1 and 0.2 g/cm3. Ground 

water is at or very close to the soil surface much of the time unless artificial 

drainage is provided. The level of ground water may fluctuate but seldom 

drops below about 30 cm.

Hemists occur in areas ranging from the Equator to the high latitudes. They 

are in closed depressions and in broad flat areas, such as coastal plains and 

outwash plains. Most Hemists are under natural vegetation and are used as 

woodland, rangeland, or wildlife habitat. Some large areas of Hemists are 

cleared, drained, and used as cropland.

Hemists

Sulfohemists

Sulfihemists

Luvihemists

Cryohemists

Haplohemists
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Key to Great Groups of Hemists (Back to key to suborders)

Hemists that have:

1. A sulfuric (highly acid due to oxidation and production of sulfuric 

acid) horizon within a depth of 50 cm -------------------------- Sulfohemists

2. Sulfidic materials (acid-producing, oxidizable sulfur compounds) 

within a depth of 100 cm --------------------------------------------- Sulfihemists

3. A horizon > 2 cm thick composed of > 50% humilluvic material 

(illuviated humus) ------------------------------------------------------ Luvihemists

4. Cold average soil temperature ------------------------------------ Cryohemists

These soils have a cryic soil temperature regime.

5. Organic materials originating from herbaceous, moss, grass, or 

woody botanical sources ------------------------------------------ Haplohemists
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Descriptions of Great Groups of Hemists

Sulfohemists—These are the Hemists that have a sulfuric (highly acid due to 

oxidation and production of sulfuric acid) horizon within a depth of 50 cm. The 

sulfuric horizon formed as a consequence of draining soil containing sulfidic 

materials. Sulfohemists are extremely acid and are toxic to most plants. Most 

are nearly black and have straw-colored concentrations of iron sulfate 

(jarosite) within a depth of 50 cm. These soils are mainly in drained coastal 

marshes or deltas near the mouths of rivers that carry sediments having a low 

content of carbonates. Most have an appreciable amount of mixed-in mineral 

material. They support sparse vegetation, mostly forbs and grass-like plants. 

They are used mainly as recreational areas or wildlife habitat. (Back to 

Hemists key)

Sulfihemists—These are the Hemists that have sulfidic materials (acidproducing, oxidizable sulfur compounds) within a depth of 100 cm. They are 

potentially extremely acid or ultra acid. Most Sulfihemists will develop a 

sulfuric horizon and become Sulfohemists if they are artificially drained to an 

extent that oxygen is allowed to reach the sulfidic materials. These soils are 

permitted to have more fiber than other Hemists. They are locally extensive 

and occur mainly in coastal marshes near the mouths of rivers or in the deltas 

of rivers that carry sediments having a low content of carbonates. Most 

Sulfihemists support native vegetation, mainly forbs and grass-like plants. 

(Back to Hemists key)

Luvihemists—These soils are not known to occur in the United States, but 

the great group is provided tentatively for use in other countries. These are 

the Hemists that have a horizon 2 cm or more thick in which humilluvic 

material (illuviated humus) constitutes one-half or more of the volume. 

Because Luvihemists cannot be studied in the United States, a precise 

description of these soils is not provided. It should be noted, however, that 

these soils are typically acid and have been cultivated for a long time. (Back to 

Hemists key)

Cryohemists—These are the cold Hemists that have a cryic temperature 

regime. The fibers in these soils are from many kinds of plant materials, 

including wood, moss, grass, and herbaceous materials. Some of these soils 

freeze during winter, and some do not. Those that do not freeze are insulated 

by snow cover or have a marine climate in which winters are mild and 

summers are very cool. If these soils are used as cropland, low soil 

temperatures in summer limit their suitability for crops. Most of these soils 

support native coniferous forest vegetation. (Back to Hemists key)

Haplohemists—Most Haplohemists are saturated for considerably longer 

than 30 cumulative days per year. The organic materials in these soils are 

from many kinds of plant materials, including wood, moss, grass, and 
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-182

herbaceous materials. If these soils are drained and cultivated using present 

technology, the organic materials decompose and disappear slowly or rapidly, 

depending on the management used and temperatures. Eventually, within 

some decades, the Haplohemists that are drained and cultivated will convert 

to Saprists and then to mineral soils. Most Haplohemists support native 

vegetation, mostly forest plants or shrubs and grass-like plants. Some large 

areas are cleared, drained, and used as cropland. (Back to Hemists key)
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-183

Inceptisols Order

Inceptisols are youthful soils with a weak, but noticeable, degree of 

profile development. 

General Characteristics

Inceptisols have profiles that are more strongly developed than those of the 

Entisols but are too weakly developed to meet the criteria for any of the other 

soil orders. As a consequence, Inceptisols include a diverse collection of soils. 

The feature common to all Inceptisols is a relatively weak degree of 

development. Inceptisols are most commonly characterized by a soil profile 

with an ochric (typically thin and/or light-colored) epipedon and a cambic 

(minimal soil development) subsoil horizon. However, there are many 

exceptions. Inceptisols can have a wide range in kinds of surface and subsoil 

horizons. These horizons commonly include, but are not limited to, an umbric 

(humus-rich with low base saturation) epipedon, a histic (wet, organic surface 

layer) epipedon, a calcic (calcium carbonate accumulation) horizon, a 

petrocalcic (cemented by calcium carbonate) horizon, a duripan (layer 

cemented by silica), a fragipan (firm and brittle but not cemented layer), and

sulfuric (highly acid due to oxidation and production of sulfuric acid) subsoil 

horizons. 

Environment and Processes

In general, Inceptisols occur on relatively young geomorphic surfaces that are 

sufficiently stable to allow weak but noticeable profile development. Typical 

settings include upland slopes, flood plains, stream terraces, and glacial till 

and outwash plains. Inceptisols are very poorly drained to excessively 

drained. They are found in diverse environmental settings, ranging from 

tropics to tundra and from humid to semiarid climates. However, Inceptisols by 

definition cannot have an aridic soil moisture regime or permafrost and thus

are excluded from most deserts and many very cold regions. 

Location

Globally, Inceptisols occupy about 17% of the ice-free land area. They are 

found throughout the world and in all regions of the United States, with the 

abovementioned exclusions (deserts and permafrost-affected areas).
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Profile of an Inceptisol.

(Back to Key to Soil Orders)
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-185

Inceptisols by suborder in the United States. Note: This map was 

produced before the introduction of the Gelepts suborder and the 

deletion of the Anthrepts suborder. Some areas shown as 

Cryepts in Alaska are now Gelepts. Anthrepts are now distributed 

among the other suborders.

Global distribution of Inceptisols.
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Inceptisols Suborders

Classification of Inceptisols

In the definitions of the suborders, emphasis is placed on soil climate in the 

form of soil wetness (Aquepts), cold soil temperature (Gelepts and Cryepts), 

and soil moisture regime (Ustepts, Xerepts, and Udepts). 

The great group level reflects a combination of important properties, including 

the presence of a diagnostic horizon (other than the minimally developed 

ochric epipedon or cambic subsoil horizon), cold soil temperature, patterns of 

soil saturation, presence of plinthite (iron-rich concentration that irreversibly 

hardens after exposure to repeated wet-dry cycles), significant levels of 

sodium and other soluble salts, base saturation level, and intense bioturbation 

from earthworms or other organisms.

The six suborders are:

1. Aquepts—wet Inceptisols (aquic conditions in the upper part)

2. Gelepts—very cold Inceptisols that lack permafrost (gelic temperature 

regime)

3. Cryepts—cold Inceptisols (cryic temperature regime)

4. Ustepts—moderately dry Inceptisols (limited moisture)

5. Xerepts—moderately dry Inceptisols (limited moisture that is supplied in 

winter and Mediterranean-type climate)

6. Udepts—Inceptisols of humid regions with well-distributed rainfall

Inceptisols

Aquepts

Gelepts

Cryepts

Ustepts

Xerepts

Udepts
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-187

Key to Suborders of Inceptisols

Inceptisols that have:

1. Either:

a. A seasonal high water table within a depth of 50 cm,

This item requires either a histic epipedon (wet, organic surface layer), a 

sulfuric (highly acid due to oxidation and production of sulfuric acid)

horizon within a depth of 50 cm, or redoximorphic features (gray and red 

color patterns) within a depth of 50 cm. See 12th edition of “Keys to Soil 

Taxonomy” for specific color requirements).

OR

b. A seasonal high water table within a depth of 100 cm and high 

levels of sodium within a depth of 50 cm ---------------------------- Aquepts

This item requires an exchangeable sodium percentage of > 15 or a 

sodium adsorption ratio of > 13.

2. Very cold average soil temperature ------------------------------------- Gelepts

These soils have a gelic soil temperature regime.

3. Cold average soil temperature ------------------------------------------- Cryepts

These soils have a cryic soil temperature regime.

4. Somewhat limited soil moisture available for plant 

growth ---------------------------------------------------------------------------- Ustepts

These soils have an ustic soil moisture regime. Moisture is limited, but 

available, during portions of the growing season.

5. A Mediterranean-type climate --------------------------------------------- Xerepts

These soils have a xeric soil moisture regime (cool and moist in winter and

warm and dry in summer).

6. Seasonally well-distributed precipitation ------------------------------ Udepts

These soils have a udic soil moisture regime. 
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Aquepts Great Groups

Aquepts are the wet Inceptisols. These soils have poor or very poor natural 

drainage. If the soils are not artificially drained, ground water is at or near the 

soil surface at some time during most years but typically not during all 

seasons. These soils are saturated long enough to become devoid of oxygen 

(aquic conditions). They generally have a gray to black surface horizon and a 

gray subsurface horizon with redoximorphic concentrations (reddish to black 

accumulations of iron-manganese oxides) that begins at a depth of less than 

50 cm. A few of the soils have a brownish surface horizon that is less than 50 

cm thick. Most Aquepts formed in late-Pleistocene or younger deposits in 

depressions, on nearly level plains, or on flood plains. They occur in areas 

ranging from the Equator to latitudes with discontinuous permafrost. The 

common features of most of these soils are the redoximorphic features (gray 

and red color patterns) at a depth of 50 cm or less and, unless the soils have 

been artificially drained, shallow ground water. Most of the soils have a 

cambic (minimal soil development) horizon, and some have a fragipan (firm 

and brittle but not cemented layer). Some may have a human-made plaggen 

epipedon.
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Aquepts

Sulfaquepts

Petraquepts

Halaquepts

Fragiaquepts

Gelaquepts

Cryaquepts

Vermaquepts

Humaquepts

epiaquepts

Endoaquepts
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-190

Profile of an Aquept from Maine. The soil is specifically a 

Humaquept and has an umbric epipedon and a gray 

cambic horizon with redoximorphic features. Scale is in 

feet.

(Back to Aquepts) 
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-191

Key to Great Groups of Aquepts (Back to key to suborders)

Aquepts that have:

1. A sulfuric (highly acid due to oxidation and production of sulfuric 

acid) horizon within a depth of 50 cm ---------------------------- Sulfaquepts

Begin measuring depth below any O horizon.

2. A cemented horizon (or plinthite) within a depth of 

100 cm ---------------------------------------------------------------------- Petraquepts

The cemented portion (or plinthite) is > 50% of the horizon. Plinthite is an

iron-rich concentration that irreversibly hardens after exposure to repeated 

wet-dry cycles. Begin measuring depth below any O horizon.

3. A salic (high salinity) horizon or a high sodium content within a depth 

of 50 cm --------------------------------------------------------------------- Halaquepts

These soils either have a salic (high content of salts) horizon or have an 

exchangeable sodium percentage of > 15 (or a sodium absorption ratio of 

> 13) in a layer > 25 cm thick. Begin measuring depth below any O 

horizon.

4. A fragipan (firm and brittle but not cemented layer) within a depth of 

100 cm -------------------------------------------------------------------- Fragiaquepts

Begin measuring depth below any O horizon.

5. Very cold average soil temperature ------------------------------- Gelaquepts

These soils have a gelic soil temperature regime.

6. Cold average soil temperature--------------------------------------- Cryaquepts

These soils have a cryic soil temperature regime.

7. Significant evidence of bioturbation within a depth of 

100 cm -------------------------------------------------------------------- Vermaquepts

One or more layers > 25 cm thick (cumulative) must have > 25%, by 

volume, evidence of bioturbation. Begin measuring depth below any O 

horizon.

8. A thick, dark-colored, humus-rich, surface horizon ------- Humaquepts

These soils have either a histic (wet, organic surface layer), melanic (dark 

and humus-rich with andic properties), mollic (rich in humus and bases), or 

umbric (humus-rich with low base saturation) epipedon.

9. Episaturation (perched water table) ------------------------------- Epiaquepts

10.Endosaturation (saturated throughout the profile) -------- Endoaquepts
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Descriptions of Great Groups of Aquepts

Sulfaquepts—These are acid sulfate soils that have been drained and 

oxidized at some time. They are extremely acid and toxic to most plants. They 

are mostly dark gray and have straw-colored mottles of iron sulfate (jarosite) 

within a depth of 50 cm. They are mainly in drained coastal marshes near the 

mouths of rivers that carry sediments that are free of carbonates or have a low 

content of carbonates. They can have any texture, but most are loamy or 

clayey. These soils contain an appreciable amount of organic carbon (of 

Holocene age). They are rare in the United States but occur elsewhere in a 

few areas that have been used mainly for the production of rice. (Back to 

Aquepts key)

Petraquepts—These soils have one or more horizons, within a depth of 100 

cm, in which plinthite (firm, iron oxide-rich concentration) or a cemented or 

indurated diagnostic horizon either forms a continuous phase or constitutes 

one-half or more of the volume. The layer of plinthite and the placic (cemented 

by iron and organic matter) horizon are the only horizons known to occur in 

the Petraquepts in the United States. Some Petraquepts may have a duripan 

(layer cemented by silica) or a petrocalcic (cemented by calcium carbonate)

horizon. Petraquepts have a ground water table that fluctuates considerably 

during the year, between a level at or near the soil surface during the rainy 

season to a much lower level during less wet seasons. A histic (wet, organic 

surface layer) or umbric (humus-rich with low base saturation) epipedon and a 

cambic (minimal soil development) subsoil horizon may be present. In some of 

the soils, the placic horizon is so close to the soil surface that there is no 

cambic horizon above it. Petraquepts are in areas of very high rainfall. The 

vegetation may be rainforest, Sphagnum, or other water-loving plants. 

Temperatures range from cold to very warm. Slopes generally are such that 

water does not pond on the surface, but high rainfall keeps some of the soils 

continuously wet. Petraquepts are rare in the world. (Back to Aquepts key)

Halaquepts—These are saline or sodic soils that have both a seasonal high 

water table and a period in which capillary rise and evapotranspiration bring 

sodium or other salts to or near the soil surface. Salt efflorescence on the soil 

surface is common in dry seasons. Halaquepts typically have grayish colors, 

and some have redoximorphic concentrations (reddish to black accumulations 

of iron-manganese oxides) from near the soil surface downward. Nearly all are 

level and formed in Holocene alluvium. The native vegetation is mostly 

sedges and salt-tolerant grasses and shrubs, but some of the soils have been

artificially drained and are used as irrigated cropland. Halaquepts are not 

extensive in the United States. (Back to Aquepts key)

Fragiaquepts—These soils have a fragipan (firm and brittle but not cemented 

layer) that is within a depth of 100 cm and commonly at a depth of 30 to 50 

cm. Typically, the water table is perched on the fragipan. The horizons above 
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-193

the pan are grayish and are saturated with water during some period in most 

years. Most of these soils have forest vegetation, but a few areas have been 

cleared. The trees have a shallow root system and are particularly subject to 

windthrow. A distinct microrelief of 50 to 60 cm or more is very common above 

the pan. The upper surface of the pan generally is smooth. In many areas of 

these soils, the horizons above the fragipan appear to have been mixed by 

the uplift of the roots of fallen trees. In some areas of mounds, the horizons 

above the pan consist of an organic O horizon, an ochric (typically thin and/or 

light-colored) epipedon, and an intermittent cambic (minimal soil development) 

subsoil horizon. In other areas, the cambic horizon is continuous. Most 

Fragiaquepts are nearly level or gently sloping and developed in Pleistoceneage sediments. The Fragiaquepts in the United States are mostly in areas 

with cool to moderate temperatures. They are moderately extensive in parts of 

the northeastern States, and a few occur in Oregon and Washington. (Back to 

Aquepts key)

Gelaquepts—These are the Aquepts of very cold regions. They have a gelic 

soil temperature regime but lack permafrost within the soil profile. They tend to 

have a high content of rock fragments, and some are shallow to bedrock. 

They are subject to wide variations in summer high temperatures and winter 

low temperatures, which result in mean annual soil temperatures of 0 degrees 

C or less, but without permafrost. (Back to Aquepts key)

Cryaquepts—These are the cold Aquepts. They typically have an ochric 

(typically thin and/or light-colored) or histic (wet, organic surface layer) 

epipedon over a cambic (minimal soil development) subsoil horizon. 

Cryaquepts are on flood plains, in depressional areas, and on plains. Most 

have grayish subsoils, and some are stratified. Cryaquepts formed mostly in 

late-Pleistocene or recent sediments south of the continuous permafrost zone. 

Most support mixed forest, shrub, or grassy vegetation. Many are nearly level, 

but some in areas of high precipitation have strong slopes. Because 

Cryaquepts are both cold and wet, they have low potential for cropping. They 

are of moderate extent in the high mountains and subarctic regions of North 

America and Eurasia. The major areas of the Cryaquepts in the United States 

are on the outwash plains and flood plains in Alaska. (Back to Aquepts key)

Vermaquepts—These soils have recognizable bioturbation, such as filled 

animal burrows, wormholes, or casts. Krotovinas (filled animal burrows)

restrict water movement because they are dense, massive, compact, and 

stratified. Significant amounts of krotovinas affect soil morphology, soil 

hydrology, and soil behavior. Vermaquepts are known to occur along the 

coastal plain of Texas and in other southeastern States. (Back to Aquepts 

key)

Humaquepts—These soils have a thick, dark-colored, humus-rich surface 

layer. They have either a histic (wet, organic surface layer), melanic (dark and 
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-194

humus-rich with andic properties), mollic (rich in humus and bases), or umbric 

(humus-rich with low base saturation) epipedon. The ground water commonly 

fluctuates from a level near the soil surface to below a depth of 50 cm. These 

soils have cool to warm soil temperatures. Before they were cultivated, most 

Humaquepts supported forest vegetation. Humaquepts are generally nearly 

level, and their parent materials are typically late-Pleistocene or younger 

sediments. These soils are of small extent in the United States. (Back to 

Aquepts key)

Epiaquepts—These soils have one or more layers in the upper part of the 

profile that are saturated for part of the year and underlain by unsaturated 

layers (a perched water table). They have cool to warm soil temperatures. 

Before cultivation, most Epiaquepts supported forest vegetation. Epiaquepts 

are generally nearly level or gently sloping, and their parent materials are 

typically late-Pleistocene or younger sediments. (Back to Aquepts key)

Endoaquepts—These soils are saturated in all horizons throughout the profile 

for some time during the year (endosaturation). The ground water commonly 

fluctuates from a level near the soil surface to below a depth of 50 cm. These 

soils have cool to warm soil temperatures. Before they were cultivated, most 

Endoaquepts supported forest vegetation. Endoaquepts are generally nearly 

level, and their parent materials are typically late-Pleistocene or younger 

sediments. (Back to Aquepts key)
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-195

Gelepts Great Groups

Gelepts are the Inceptisols of very cold regions. These soils have a gelic soil 

temperature regime but lack permafrost within the soil profile. They tend to be 

in landscape positions that are subject to wide variations in summer high 

temperatures and winter low temperatures, which result in mean annual soil 

temperatures of 0 degrees C or less, but without permafrost.

Gelepts

Humigelepts

Dystrogelepts

Haplogelepts
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-196

Key to Great Groups of Gelepts (Back to key to suborders)

Gelepts that have:

1. A thick, dark-colored, humus-rich, surface horizon -------- Humigelepts

These soils have an umbric (humus-rich with low base saturation) or mollic 

(rich in humus and bases) epipedon.

2. Less than 50% base saturation within the upper 

50 cm --------------------------------------------------------------------- Dystrogelepts

Base saturation is determined by NH4OAc lab method. Begin measuring

depth below any O horizon. See 12th edition of “Keys to Soil Taxonomy”

for detailed criteria regarding thickness and depth requirements.

3. An ochric (typically thin and/or light-colored) epipedon and base 

saturation > 50% within a depth of 50 cm --------------------- Haplogelepts

Base saturation is determined by NH4OAc lab method. Begin measuring

depth below any O horizon.
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Descriptions of Great Groups of Gelepts

Humigelepts—These soils have a dark-colored, thick mollic (rich in humus 

and bases) or umbric (humus-rich with low base saturation) epipedon. The 

vegetation is mostly conifers or mixed conifers and hardwoods. Humigelepts 

may have formed in loess, drift, or alluvium or in solifluction deposits, mostly 

late Pleistocene or Holocene in age. They commonly have a thick, dark 

surface layer with a high content of organic matter and a brownish cambic 

(minimal soil development) subsoil horizon. Most have a high content of 

gravel in at least the lower part of the profile, and some have bedrock within a 

depth of 100 cm. (Back to Gelepts key)

Dystrogelepts—These soils have relatively low base saturation, and as a 

result, low natural fertility. The vegetation is mostly conifers or mixed conifers 

and hardwoods. Dystrogelepts may have formed in loess, drift, or alluvium or 

in solifluction deposits, mostly late Pleistocene or Holocene in age. They

commonly have a thin, dark brownish ochric (typically thin and/or light-colored) 

epipedon and a brownish cambic (minimal soil development) subsoil horizon. 

Most have a high content of gravel in at least the lower part of the profile, and 

some have bedrock within a depth of 100 cm. (Back to Gelepts key)

Haplogelepts—These soils have an ochric (typically thin and/or light-colored)

epipedon. They have relatively high base saturation, and as a result, high 

natural fertility. The vegetation is mostly conifers or mixed conifers and 

hardwoods. Haplogelepts may have formed in loess, drift, or alluvium or in 

solifluction deposits, mostly late Pleistocene or Holocene in age. They

commonly have a thin, dark brownish ochric (typically thin and/or light-colored) 

epipedon and a brownish cambic (minimal soil development) subsoil horizon. 

Most have a high content of gravel in at least the lower part of the profile, and 

some have bedrock within a depth of 100 cm. (Back to Gelepts key)
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-198

Cryepts Great Groups

Cryepts are the cold Inceptisols of high-elevation mountains or high latitudes. 

The vegetation is mostly coniferous forest, subalpine meadow, shrub steppe,

or tundra. Most of these soils are not cultivated. Cryepts formed in late 

Pleistocene or younger drift, colluvium, or alluvium on mountain slopes or 

valley floors. In areas with tephra, a layer near the surface that contains 

pumice fragments, volcanic glass, or amorphous weathering products of 

pyroclastic materials is common. These soils commonly have an ochric 

(typically thin and/or light-colored) or umbric (humus-rich with low base 

saturation) epipedon and a cambic (minimal soil development) subsoil 

horizon. Some have root-restricting bedrock or dense, compacted materials 

within a depth of 100 cm. Cryepts are moderately extensive in the United 

States. They occur in high-elevation mountains in the western States and in 

southern Alaska as well as in other mountainous areas of the world.

Cryepts

Humicryepts

Calcicryepts

Dystrocryepts

Haplocryepts
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-199

Key to Great Groups of Cryepts (Back to key to suborders)

Cryepts that have:

1. A thick, dark-colored, humus-rich, surface horizon ------- Humicryepts

These soils may have an umbric (humus-rich with low base saturation) or 

mollic (rich in humus and bases) epipedon.

2. A calcic (calcium carbonate accumulation) or petrocalcic 

(cemented by calcium carbonate) horizon within a depth of 

100 cm --------------------------------------------------------------------- Calcicryepts

Begin measuring depth below any O horizon.

3. Base saturation of < 50% in the upper 75 cm AND a lack of free 

carbonates within a depth of 200 cm -------------------------- Dystrocryepts

Base saturation is determined by NH4OAc lab method. Begin measuring

depth below any O horizon. See 12th edition of “Keys to Soil Taxonomy”

for detailed criteria regarding thickness and depth requirements. Free 

carbonates will effervesce with dilute HCl.

4. An ochric (typically thin and/or light-colored) epipedon and 

base saturation > 50% OR free carbonates within the 

profile --------------------------------------------------------------------- Haplocryepts

Base saturation is determined by NH4OAc lab method. Begin measuring

depth below any O horizon. Free carbonates will effervesce with dilute 

HCl.
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-200

Descriptions of Great Groups of Cryepts 

Humicryepts—These soils have a thick, dark umbric (humus-rich with low 

base saturation) or mollic (rich in humus and bases) epipedon. Humicryepts 

formed in deposits that are late Pleistocene or younger in age. They 

commonly have a dark brown surface layer and a brown cambic (minimal soil 

development) subsoil horizon. Some have root-restrictive bedrock or dense, 

compact material within a depth of 100 cm. The vegetation is mixed conifers, 

shrubs, or alpine meadow. These soils are of moderate extent in the highelevation mountains of western United States and Alaska. They also occur in 

other parts of the world, mostly in mountainous areas. (Back to Cryepts key)

Calcicryepts—These soils have a calcic (calcium carbonate accumulation) or 

petrocalcic (cemented by calcium carbonate) subsoil horizon within a depth of 

100 cm. They formed in drift, alluvium, colluvium, or residuum, mostly late 

Pleistocene or Holocene in age. They may have a high calcium carbonate 

content due to the influence from calcareous sedimentary rocks, such as 

limestone in the parent materials. These soils commonly have a thin dark 

brown or brown ochric (typically thin and/or light-colored) epipedon and a 

brown cambic (minimal soil development) subsoil horizon. Some have 

bedrock within a depth of 100 cm. The natural vegetation is mostly grass and 

shrubs or coniferous forest with widely spaced trees. These soils are mainly 

used for timber production, wildlife habitat, or grazing by domestic livestock. 

Calcicryepts are of small extent and occur in high-elevation, inland 

mountainous regions in the western United States. (Back to Cryepts key)

Dystrocryepts—These soils have low base saturation, and, as a result, 

relatively low natural fertility. They do not have free carbonates within a depth 

of 200 cm. They formed in drift, colluvium, alluvium, or loess (mostly late 

Pleistocene or Holocene in age) on mountain slopes and valley floors. Some 

have root-restrictive bedrock or dense, compact materials within a depth of 

100 cm. These soils commonly have a thin, dark brown ochric (typically thin 

and/or light-colored) epipedon and a brown cambic (minimal soil 

development) subsoil horizon. Natural vegetation is mostly coniferous forest. 

In the United States, Dystrocryepts are moderately extensive in the highelevation mountains of western States and in Alaska. They also occur in other 

mountainous areas of the world. (Back to Cryepts key)

Haplocryepts—These are the base-rich Cryepts that have free carbonates or 

base saturation of 50% or more. They formed in drift, colluvium, alluvium, or 

loess (late Pleistocene or Holocene in age). They commonly have a thin, dark 

brown ochric (typically thin and/or light-colored) epipedon and a brown cambic 

(minimal soil development) subsoil horizon. Some have root-restrictive 

bedrock within a depth of 100 cm. Natural vegetation is coniferous forest, 

mixed conifer and hardwood forest, or shrub steppe. These soils are used for 

wildlife habitat and livestock grazing in western States and are important 
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-201

agricultural soils in Alaska. They are are extensive in Alaska and of small 

extent in the high-elevation inland mountains of western States. They also 

occur in other parts of the world, mostly in mountainous areas. (Back to 

Cryepts key)
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Ustepts Great Groups

Ustepts are mainly the more or less freely drained Inceptisols of subhumid to 

semiarid regions. They have an ustic moisture regime (moisture is limited, but 

available, during portions of the growing season). They receive dominantly 

summer precipitation. They formed mostly in Pleistocene or Holocene 

deposits. Some of the soils with steep slopes formed in older deposits. Some 

Ustepts formed from Mollisols as a result of human-induced erosion of the 

former mollic epipedon, mostly through cultivation. Most Ustepts have an 

ochric (typically thin and/or light-colored) epipedon and a cambic (minimal soil 

development) subsoil horizon. Many are calcareous at a shallow depth and 

have an accumulation of calcium carbonate in the subsoil. A few have a 

duripan (layer cemented by silica) or an umbric (humus-rich with low base 

saturation) or mollic (rich in humus and bases) epipedon. The native 

vegetation commonly was grass, but some of the soils supported trees. Most 

of the soils are used as cropland or pasture. Ustepts are of moderate extent in 

the United States. They are most common on the Great Plains, mainly in 

Montana, Texas, and Oklahoma.

Ustepts

Durustepts

Calciustepts

Humustepts

Dystrustepts

Haplustepts
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Profile of an Ustept (specifically a Calciustept) in Texas. This 

soil has a cambic horizon (grayish brown) above a depth 

of 60 cm and a calcic horizon (white) below.

Return to Ustepts
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-204

Key to Great Groups of Ustepts (Back to key to suborders)

Ustepts that have:

1. A duripan (layer cemented by silica) within a depth of 

100 cm ----------------------------------------------------------------------- Durustepts

Begin measuring depth below any O horizon.

2. A calcic (calcium carbonate accumulation) horizon within a depth 

of 100 cm or a petrocalcic (cemented by calcium carbonate)

horizon within a depth of 150 cm AND free carbonates or sandy 

texture above ------------------------------------------------------------ Calciustepts

Begin measuring depth below any O horizon. Free carbonates will 

effervesce with dilute HCl.

3. A thick, dark-colored, humus-rich, surface horizon -------- Humustepts

These soils have an umbric (humus-rich with low base saturation) or mollic 

(rich in humus and bases) epipedon.

4. Base saturation of < 60% in the upper 75 cm AND a lack of free 

carbonates within a depth of 200 cm --------------------------- Dystrustepts

Base saturation is determined by NH4OAc lab method. Begin measuring

depth below any O horizon. Free carbonates will effervesce with dilute 

HCl.

5. Base saturation of > 60% in the upper 75 cm or free carbonates within 

a depth of 200 cm ------------------------------------------------------- Haplustepts

Base saturation is determined by NH4OAc lab method. Begin measuring

depth below any O horizon. Free carbonates will effervesce with dilute 

HCl.
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Descriptions of Great Groups of Usteps

Durustepts—These soils have a duripan (layer cemented by silica) within a 

depth of 100 cm. Commonly, they formed in the vicinity of volcanic cinders 

and ash falls. The parent materials are mainly volcanic tuffs, ash, cinders, and 

volcanic rocks. In the United States, the native vegetation is mostly grasses, 

shrubs, and trees. These soils are of small extent. Durustepts formed mainly 

in areas of Pleistocene deposits. They may be also on the leeward sides of 

some volcanic islands. (Back to Ustepts key)

Calciustepts—These soils have a calcic (calcium carbonate accumulation) or 

petrocalcic (cemented by calcium carbonate) horizon and are either 

calcareous or have a sandy texture in all overlying horizons. Precipitation has 

been insufficient to remove the carbonates from the upper horizons, or there 

is a continual external source of carbonates in dust or water. Calciustepts 

formed mostly in Pleistocene or older materials. On the Calciustepts in the 

United States, the vegetation was dominantly grass before the soils were 

cultivated. The soils are most extensive on the Great Plains in the United 

States but also occur in the intermountain valleys of the western States. (Back 

to Ustepts key)

Humustepts—These soils have a mollic (rich in humus and bases) or umbric 

(humus-rich with low base saturation) epipedon. Parent materials are mostly 

residuum, colluvium, or alluvium in mountain settings. Some have been 

influenced by volcanic ash. These soils commonly have a brownish cambic 

(minimal soil development) subsoil horizon. Some have root-limiting bedrock 

or a dense, compact layer in the profile. In the United States, Humustepts are 

found mostly in coastal California and in Hawaii. (Back to Ustepts key)

Dystrustepts—These are the acid Ustepts with low base saturation and 

relatively low natural fertility. They developed mostly in Pleistocene or 

Holocene deposits. Some of the soils with steep slopes formed in older 

deposits. Parent materials generally are acid, moderately or weakly 

consolidated sedimentary or metamorphic rocks or acid sediments. The 

vegetation was mostly forest. Most of these soils have warm or very warm 

temperatures. A common horizon sequence in Dystrustepts is an ochric 

(typically thin and/or light-colored) epipedon over a cambic (minimal soil 

development) subsoil horizon. Some of the steeper soils are shallow to rootlimiting bedrock or a dense, compact layer. In the United States, Dystrustepts 

are found mostly in coastal California and in Hawaii. A few are in the Rocky 

Mountains and on the Great Plains. (Back to Ustepts key)

Haplustepts—These are the more or less freely drained Ustepts that are 

calcareous at some depth or have high base saturation. They commonly have 

an ochric (typically thin and/or light-colored) epipedon over a cambic (minimal 

soil development) subsoil horizon. Some have an accumulation of calcium 
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-206

carbonate in the subsoil. The native vegetation commonly was grass, but 

some of the soils supported trees. Haplustepts in the United States are mostly 

on the Great Plains. (Back to Ustepts key)
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-207

Xerepts Great Groups

Xerepts are in regions that have Mediterranean-type climates, characterized 

by cool, moist winters and warm, dry summers. As their name implies, they 

have a xeric soil moisture regime. These soils are dry for extended periods in 

summer, but moisture moves through most of the soils in winter and is stored 

throughout the soil profile above the deep layers or above bedrock in most 

years. Xerepts formed mostly in Pleistocene or Holocene deposits. Some of 

the soils, mostly those with steep slopes, formed in older deposits. Most of the 

soils have an ochric (typically thin and/or light-colored) epipedon and a cambic 

(minimal soil development) subsoil horizon. Some have an umbric (humus-rich 

with low base saturation) or mollic (rich in humus and bases) epipedon or a 

duripan (layer cemented by silica) in the subsoil. A few have a fragipan (firm

and brittle but not cemented layer). Some are calcareous at a shallow depth 

and have an accumulation of calcium carbonate in the subsoil. The native 

vegetation commonly is coniferous forest on the colder sites and shrubs, 

grass, and widely spaced trees on the warmer sites. Xerepts are of moderate 

extent in the United States. They are most common in the States of California, 

Oregon, Washington, Idaho, and Utah.

Xerepts

Durixerepts

Fragixerepts

Humixerepts

Calcixerepts

Dystroxerepts

Haploxerepts
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-208

Profile of a Xerept (specifically a Haploxerept) in Tunisia. 

This soil has a reddish brown cambic horizon. Scale is in 

centimeters.

(Back to Xerepts)
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-209

Key to Great Groups of Xerepts (Back to key to suborders)

Xerepts that have:

1. A duripan (layer cemented by silica) within a depth of 

100 cm ----------------------------------------------------------------------- Durixerepts

Begin measuring depth below any O horizon.

2. A fragipan (firm and brittle but not cemented layer) within a depth of

100 cm --------------------------------------------------------------------- Fragixerepts

Begin measuring depth below any O horizon.

3. A thick, dark-colored, humus-rich, surface horizon ------- Humixerepts

These soils have an umbric (humus-rich with low base saturation) or mollic 

(rich in humus and bases) epipedon.

4. A calcic (calcium carbonate accumulation) horizon within a depth 

of 100 cm or a petrocalcic (cemented by calcium carbonate) horizon

within a depth of 150 cm AND free carbonates above ----- Calcixerepts

Begin measuring depth below any O horizon. Free carbonates will 

effervesce with dilute HCl.

5. Base saturation of < 60% in the upper 75 cm AND a lack of free 

carbonates within a depth of 200 cm -------------------------- Dystroxerepts

Base saturation is determined by NH4OAc lab method. Begin measuring

depth below any O horizon. Free carbonates will effervesce with dilute 

HCl.

6. Base saturation of > 60% in the upper 75 cm OR free carbonates 

within a depth of 200 cm -------------------------------------------- Haploxerepts

Base saturation is determined by NH4OAc lab method. Begin measuring

depth below any O horizon. Free carbonates will effervesce with dilute 

HCl.
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-210

Descriptions of Great Groups of Xerepts

Durixerepts—These soils have a duripan (layer cemented by silica) within a 

depth of 100 cm. They have a Mediterranean-type climate and receive a large 

part of their annual precipitation in winter. Commonly, water is perched above 

the duripan for part of winter or early spring. The vegetation was commonly a 

coniferous forest or a mixture of grasses and scattered trees. Durixerepts are 

not extensive in the United States as a whole but are extensive locally. (Back 

to Xerepts key)

Fragixerepts—These soils have a fragipan (firm and brittle but not cemented 

layer) within a depth of 100 cm. Commonly, they have a brownish cambic 

(minimal soil development) subsoil horizon that is underlain by a fragipan at a 

depth of about 70 cm. Most Fragixerepts have perched water above the pan 

in winter, and few roots penetrate the pan. Consequently, plants on these 

soils tend to have shallow root systems. Many Fragixerepts formed in latePleistocene deposits. They have mostly gentle or moderate slopes, but some 

are steep. Most of these soils are loamy and have been leached of any free 

carbonates. The Fragixerepts in the United States are mostly in Idaho and 

Oregon and are of small extent. (Back to Xerepts key)

Humixerepts—These soils commonly have an umbric (humus-rich with low 

base saturation) epipedon. A few may have a mollic (rich in humus and 

bases) epipedon. They have a cambic (minimal soil development) subsoil 

horizon, and some have bedrock within the profile. They formed mostly in 

residuum, colluvium, or mudflow deposits on mountain slopes, terraces, or 

outwash plains. Many also have an influence from volcanic ash. The 

vegetation includes trees, grasses, pasture, and cropland. Humixerepts are 

commonly found in the western United States, in areas ranging from California 

to Washington and Idaho. (Back to Xerepts key)

Calcixerepts—These soils have a calcic (calcium carbonate accumulation) or 

petrocalcic (cemented by calcium carbonate) horizon. They are calcareous in 

all overlying horizons. Either the parent materials had more carbonates than 

rainfall could remove from the upper horizons, or there is a continual external 

source of carbonates in dust or water. Calcixerepts formed mostly in 

Pleistocene sediments or younger materials on surfaces of comparable age. 

In the United States, the vegetation was dominantly grass and shrubs before 

the soils were cultivated. Calcixerepts are most extensive in California, Idaho, 

and Utah. (Back to Xerepts key)

Dystroxerepts—These are the acid Xerepts with relatively low base 

saturation and natural fertility. They developed mostly in Pleistocene or 

Holocene deposits. Some of the soils with steep slopes formed in older 

deposits. The typical horizon sequence in Dystroxerepts is an ochric (typically

thin and/or light-colored) epipedon over a cambic (minimal soil development) 
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-211

subsoil horizon. Some of the steep Dystroxerepts are shallow to rootrestricting bedrock or dense, compact materials. The parent materials 

generally are acid, moderately or weakly consolidated sedimentary or 

metamorphic rocks or acid sediments and commonly contain additions of 

volcanic ash near the surface. A few of the soils formed in saprolite derived 

from igneous rocks. The vegetation was mostly coniferous trees. Most 

Dystroxerepts are used as forest or pasture. Some, mostly the least sloping 

ones, are used as cropland. Dystroxerepts are extensive in the States of 

California, Washington, and Oregon. (Back to Xerepts key)

Haploxerepts—These soils are calcareous at some depth or have high base 

saturation. They are relatively high in natural fertility. Some of the soils have 

an accumulation of calcium carbonate in the subsoil. Haploxerepts are moist 

through winter and into spring but are thoroughly dry for much of the summer. 

Slopes are gentle to very steep, and the soils are shallow to very deep. The 

most common vegetation on Haploxerepts in the United States was 

coniferous forest on the cooler sites and grass and widely spaced trees on the 

warmer sites. Haploxerepts occur in the western United States and are

moderately extensive. (Back to Xerepts key)
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-212

Udepts Great Groups

Udepts are mainly the more or less freely drained Inceptisols of humid 

climates. They formed on nearly level to steep surfaces, mostly of latePleistocene or Holocene age. Some of the soils, in areas of very high 

precipitation, formed in older deposits. Most of the soils had or now have 

forest vegetation, but some support shrubs or grasses. A few formed from 

Mollisols by human-induced erosion of the mollic epipedon, mostly from 

cultivation. Most of the soils have an ochric (typically thin and/or light-colored) 

or umbric (humus-rich with low base saturation) epipedon and a cambic 

(minimal soil development) subsoil horizon. A few have a mollic (rich in humus 

and bases) epipedon. Some also have a sulfuric (highly acid due to oxidation 

and production of sulfuric acid) horizon, a fragipan (firm and brittle but not 

cemented layer), or a duripan (layer cemented by silica). The Udepts in the 

United States are most extensive in the Appalachian Mountains, on the 

Allegheny Plateau, and on the West Coast.

Udepts

Sulfudepts

Durudepts

Fragiudepts

Humudepts

Eutrudepts

Dystrudepts
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-213

Profile of a Udept (specifically a Dystrudept) in North 

Carolina. Bedrock is below a depth of about 18 inches.

Scale is in inches. (Photo courtesy of John Kelley)

(Back to Udepts)
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-214

Key to Great Groups of Udepts (Back to key to suborders)

Udepts that have:

1. A sulfuric (highly acid due to oxidation and production of sulfuric 

acid) horizon within a depth of 50 cm ----------------------------- Sulfudepts

Begin measuring depth below any O horizon.

2. A cemented horizon within a depth of 100 cm ------------------ Durudepts

Begin measuring depth below any O horizon.

3. A fragipan (firm and brittle but not cemented layer) within a depth of 

100 cm ---------------------------------------------------------------------- Fragiudepts

Begin measuring depth below any O horizon.

4. A thick, dark-colored, humus-rich, surface horizon --------- Humudepts

These soils have an umbric (humus-rich with low base saturation) or mollic 

(rich in humus and bases) epipedon.

5. Base saturation of > 60% in the upper 75 cm OR free carbonates 

throughout the soil ------------------------------------------------------ Eutrudepts

Base saturation is determined by NH4OAc lab method. Begin measuring

depth below any O horizon. Free carbonates will effervesce with dilute 

HCl.

6. Base saturation of < 60% in the upper 75 cm AND a lack of free 

carbonates within a depth of 200 cm ---------------------------- Dystrudepts

Base saturation is determined by NH4OAc lab method. Begin measuring

depth below any O horizon. Free carbonates will effervesce with dilute 

HCl.
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-215

Descriptions of Great Groups of Udepts

Sulfudepts—These are extremely acid soils that formed in sulfidic materials 

(acid-producing, oxidizable sulfur compounds) that have been exposed to 

oxygen, mainly as a result of surface mining, road construction, dredging, or 

other earthmoving operations. This exposure to oxygen resulted in the 

formation of a sulfuric (highly acid due to oxidation and production of sulfuric 

acid) horizon. These soils are of small extent in the United States. (Back to 

Udepts key)

Durudepts—These soils have a duripan (layer cemented by silica) that has its 

upper boundary within a depth of 100 cm. Most occur in areas that generally

have a Mediterranean-type climate, but they are in localized areas that have 

higher amounts of precipitation. Commonly, these soils have a perched water 

table above the duripan for part of winter or early spring. The vegetation 

commonly is coniferous forest. Durudepts are of very small extent in the 

United States. (Back to Udepts key)

Fragiudepts—These soils have a fragipan (firm and brittle but not cemented 

layer) within a depth of 100 cm. Commonly, they have a brownish cambic 

(minimal soil development) subsoil horizon that is underlain, at a depth of 

about 50 cm, by the fragipan. Most Fragiudepts have perched water above 

the pan at some time of the year, and few roots penetrate the pan. 

Consequently, plants on these soils tend to have shallow root systems. Many 

Fragiudepts formed in late-Pleistocene or Holocene deposits on gentle or 

moderate slopes. Some are strongly sloping. The parent materials of most 

Fragiudepts are loamy and either are acid or have only a small amount of free 

carbonates. A few of the materials are sandy and have an appreciable amount 

of fine sand and very fine sand. Most of the Fragiudepts in the United States 

are in the northeastern States and the States bordering the Mississippi and 

Ohio Rivers. A few are in the northern Lake States, and some are in the 

humid parts of the northwestern States. Fragiudepts are extensive. (Back to 

Udepts key)

Humudepts—These soils have a mollic (rich in humus and bases) or umbric 

(humus-rich with low base saturation) epipedon. They formed in a variety of 

parent materials, including residuum, colluvium, alluvium, and marine 

sediments. Some have an influence from volcanic ash. Most Humudepts have 

an ochric (typically thin and/or light-colored) epipedon and a cambic (minimal 

soil development) subsoil horizon. A few have root-restrictive bedrock within 

the profile. Most are under forest vegetation, but some support grasses or are 

used for pasture or cropland. In the United States, Humudepts are dominantly 

found near the California coast, in the Pacific Northwest, and at the higher 

elevations of the southern Appalachian Mountains in the East. (Back to 

Udepts key)
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-216

Eutrudepts—These are the base-rich, naturally fertile soils of humid regions. 

Many developed in Holocene or late-Pleistocene deposits. Some of the soils 

with steep slopes formed in older deposits. Parent materials commonly are 

calcareous sediments or basic sedimentary rocks. The vegetation was mostly 

deciduous hardwoods, but the gently sloping soils are now cultivated and 

many of the steeply sloping soils are used as pasture. Eutrudepts are not 

extensive in the United States. (Back to Udepts key)

Dystrudepts—These are the acid soils of humid regions and generally are 

low in natural fertility. They developed mostly in late-Pleistocene or Holocene 

deposits. Some developed on older, steeply sloping surfaces. The parent 

materials generally are acid, moderately or weakly consolidated sedimentary 

or metamorphic rocks or acid sediments. A few of the soils formed in saprolite 

derived from igneous rocks. The vegetation was mostly deciduous trees. Most 

of the Dystrudepts that formed in alluvium are now cultivated, and many of the 

other Dystrudepts are used as pasture. The typical horizon sequence in 

Dystrudepts is an ochric (typically thin and/or light-colored) epipedon over a 

cambic (minimal soil development) subsoil horizon. Some of the steeper 

Dystrudepts have a shallow, root-restrictive contact with bedrock or other 

compact, dense material. Dystrudepts are extensive in the United States and 

occur mostly in the eastern and southern States. (Back to Udepts key)
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-217

Mollisols Order

Mollisols are very dark-colored, naturally very fertile soils of grasslands.

General Characteristics

Mollisols are soils with a thick, friable, very dark-colored, organic-rich surface 

layer (mollic epipedon). In addition, they are naturally very fertile, having high 

base saturation throughout the profile. Depending on their environmental 

setting, Mollisols, especially those on older, relatively stable geomorphic 

surfaces, may have a variety of subsoil horizons. In relatively dry areas where 

leaching is not intense, calcic (calcium carbonate accumulation) horizons, 

petrocalcic (cemented by calcium carbonate) horizons, and duripans (layers

cemented by silica) are known to occur. In more humid environments, argillic 

(clay accumulation) and natric (high levels of illuvial clay and sodium) horizons 

commonly occur. A few Mollisols in wet areas have a light-colored, leached 

albic horizon.

Environment and Processes

Mollisols formed predominantly grasslands in temperate regions at midlatitudes in many parts of the world. To a lesser degree, they can be found in 

the Tropics or at high latitudes or high mountain elevations. Mollisols formed 

as the result of deep inputs of organic matter and nutrients from decaying 

roots, especially the short, mid, and tall grasses common to prairie and steppe 

areas (where there are high, below-ground biomass production and deep 

cycling of nutrients). Less commonly, Mollisols formed under forest vegetation, 

commonly from calcareous parent materials. Also contributing to the deep 

inputs and cycling of organic matter and nutrients is the activity of microbes, 

earthworms, ants, rodents, and other organisms.

In addition to accumulations of organic matter, Mollisols have high contents of 

base nutrients distributed throughout their profile, making them naturally 

highly fertile. This is generally because Mollisols formed from mostly non-acid 

parent materials, which supplied bases to the soil as they weathered, and in 

environments (subhumid to semiarid) where the soil was not subject to 

intense leaching of nutrients.

Location

Globally, Mollisols occupy about 7% of the ice-free land area. They are 

commonly located between the Aridisols of very dry areas and the Alfisols and 

Spodosols of more humid environments. In the United States and Canada,

they are common throughout the Great Plains and Prairie regions. Worldwide,

they are common in the subhumid to semiarid prairie and steppe regions of 

Europe, Asia, and South America.
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-218

Profile of a Mollisol (specifically a Calciustoll).

(Back to Key to Soil Orders)
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-219

Mollisols by suborder in the United States. Note: This map was 

produced before the introduction of the Gelolls suborder. Some 

areas shown as Cryolls in Alaska are now Gelolls.

Global distribution of Mollisols.
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-220

Mollisols Suborders

Classification of Mollisols

In the definitions of the suborders, emphasis is divided between the presence 

of a light-colored leached layer (albic horizon) with a fluctuating water table 

(Albolls), soil wetness (Aquolls), highly calcareous parent materials (Rendolls), 

cold soil temperature (Gelolls and Cryolls), and soil moisture regime (Xerolls, 

Ustolls, and Udolls). 

The great groups reflect a combination of important soil properties, including 

the presence of various diagnostic horizons in addition to the mollic epipedon, 

cold soil temperature, patterns of soil saturation, intense bioturbation from 

earthworms or other organisms, and a morphology that reflects strong soil 

development on stable landforms. 

The eight suborders are:

1. Albolls—Mollisols with a fluctuating water table and a light-colored zone of 

leaching (albic horizon)

2. Aquolls—wet Mollisols (aquic conditions in upper part)

3. Rendolls—Mollisols that formed in highly calcareous parent materials

4. Gelolls—very cold Mollisols that lack permafrost (gelic temperature 

regime)

5. Cryolls—cold Mollisols (cryic temperature regime)

6. Xerolls—moderately dry Mollisols (limited moisture that is supplied in 

winter and Mediterranean-type climate)

7. Ustolls—moderately dry Mollisols (limited moisture)

8. Udolls—Mollisols of humid regions with well-distributed rainfall
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-221

Mollisols

Albolls

Aquolls

Rendolls

Gelolls

Cryolls

Xerolls

Ustolls

Udolls
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-222

Keys to Suborders of Mollisols

Mollisols that have:

1. A seasonal high water table within a depth of 100 cm, a light-colored 

leached layer (albic horizon), and an argillic (clay accumulation) or 

natric (high levels of illuvial clay and sodium) horizon ----------- Albolls

See 12th edition of “Keys to Soil Taxonomy” for specific requirements for 

color, thickness, and location of the albic horizon. Artificially drained soils 

are included in Albolls. Redoximorphic features are evidence of a seasonal 

high water table. Albolls have a soil temperature regime warmer than cryic.

2. A seasonal high water table within a depth of 50 cm ------------- Aquolls

Redoximorphic features are evidence of a seasonal high water table. 

However, the intense accumulation of organic matter results in very dark 

colors that commonly mask the morphology of reddish and grayish colors

associated with redoximorphic features. Artificially drained soils are 

included in Aquolls. See 12th edition of “Keys to Soil Taxonomy” for 

specific requirements regarding color.

3. A calcium carbonate equivalent of > 40% --------------------------- Rendolls

These soils do not have an argillic or calcic subsoil horizon. They have a 

udic soil moisture regime or a cryic soil temperature regime. The mollic 

epipedon is < 50 cm thick.

4. Very cold average soil temperature -------------------------------------- Gelolls

These soils have a gelic soil temperature regime, but no permafrost.

5. Cold average soil temperature -------------------------------------------- Cryolls

These soils have a cryic soil temperature regime.

6. A Mediterranean-type climate ---------------------------------------------- Xerolls

These soils typically have a xeric soil moisture regime (cool and moist in 

winter and warm and dry in summer). Some have an aridic soil moisture 

regime that borders on xeric.

7. Somewhat limited soil moisture available for crop growth ------ Ustolls

These soils typically have an ustic soil moisture regime. Moisture is limited, 

but available, during portions of the growing season. Some have an aridic 

soil moisture regime that borders on ustic.

8. Seasonally well-distributed precipitation ------------------------------- Udolls

These soils have a udic soil moisture regime.
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-223

Albolls Great Groups

Albolls are the Mollisols that have an albic (light-colored and leached) horizon

and fluctuating ground water levels. Most of these soils are saturated with 

water at or near the soil surface at some time during winter or spring in most 

years. In summer, ground water commonly is not within a depth of 200 cm. 

Below the albic horizon, there is either an argillic (clay accumulation) or natric 

(high levels of illuvial clay and sodium) subsoil horizon. These soils developed 

mostly on broad, nearly level to sloping ridges, on back slopes, or in closed 

depressions. Most have episaturation (perched water table). Leaching is a 

significant process in Albolls due to the relatively large amounts of water 

moving through the profile. In the United States, most Albolls are in areas of 

late-Pleistocene deposits. Most Albolls developed under grass or under grass 

and shrub vegetation. Some Albolls are thought to have had forest vegetation 

that was replaced by grass in the early stages of soil development. Because 

they have gentle slopes, most of the Albolls in the United States are now 

cultivated.

Albolls

Natralbolls

Argialbolls
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-224

Profile of an Alboll (specifically an Argialboll) in China. The mollic 

epipedon, which extends from the surface to a depth of more 

than 3 feet, is separated by the intervening albic horizon 

(light gray). Scale is in feet.

Return to Albolls
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-225

Key to Great Groups of Albolls (Back to key to suborders)

Albolls that have:

1. A natric (high levels of illuvial clay and sodium) 

horizon ----------------------------------------------------------------------- Natralbolls

2. An argillic (clay accumulation) horizon --------------------------- Argialbolls
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-226

Descriptions of Great Groups of Albolls

Natralbolls—These soils have a natric (high levels of illuvial clay and sodium)

subsoil horizon. The natric horizon typically lies very close to the surface. 

These soils commonly have a thin dark surface layer overlying the lightcolored, leached albic horizon. The color of the upper part of the soil, after 

mixing to a depth of 18 cm, however, is dark enough for a mollic epipedon 

because both the epipedon and the natric horizon are dark colored. Ground 

water is shallow during part of the year, and capillary rise in many of the soils

has concentrated salts, including sodium salts, in the upper 50 cm of the 

profile. Natralbolls are known to occur only in subhumid and humid regions. 

They are in areas of late-Pleistocene till plains and lacustrine deposits or 

Holocene deposits. The vegetation consists of grasses and sedges. These

soils are used as cropland or rangeland or for grasses cut for hay. (Back to 

Albolls key)

Argialbolls—These soils have an argillic (clay accumulation) subsoil horizon. 

Most of the soils have very dark gray to black coatings of humus and clay on 

the peds in the upper part of the argillic horizon. In the United States, these 

soils are most extensive in the loess-covered areas of the midwestern States 

where the temperature regime is mesic. A very few of the soils have a frigid or 

thermic temperature regime. A distinct moisture deficiency in summer and a 

moisture surplus in winter and spring seem to be essential to the genesis of 

these soils. Argialbolls are associated on the landscape with all other 

suborders of Mollisols, except possibly Rendolls. Because they have gentle 

slopes, most of the Argialbolls in the United States are cultivated. (Back to 

Albolls key)
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-227

Aquolls Great Groups

Aquolls are the wet Mollisols. They have color dominated by low chroma, 

commonly in olive hues, and have high-contrast redoximorphic depletions in 

or below the epipedon. These soils commonly develop in low areas where 

water collects and stands, but some are on broad flats or on seepy hillsides. 

Most of the soils have had a vegetation of grasses, sedges, and forbs, but a 

few also have had forest vegetation. In the United States, Aquolls are most 

extensive in glaciated areas of the Midwestern States where the drift or loess 

was calcareous. Aquolls are saturated long enough to allow oxygen- depleted 

conditions to develop within the upper 50 cm (aquic conditions), or they are

artificially drained. 

Aquolls

Cryaquolls

Duraquolls

Natraquolls

Calciaquolls

Argiaquolls

Epiaquolls

Endoaquolls
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-228

Profile of an Aquoll (specifically an Endoaquoll) in Missouri. 

Scale is in feet.

Return to Aquolls
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-229

Key to Great Groups of Aquolls (Back to key to suborders)

Aquolls that have:

1. Cold average soil temperature --------------------------------------- Cryaquolls

These soils have a cryic soil temperature regime.

2. A duripan (layer cemented by silica) within a depth of 

100 cm ------------------------------------------------------------------------ Duraquolls

Begin measuring depth below any O horizon.

3. A natric (high levels of illuvial clay and sodium) 

horizon ---------------------------------------------------------------------- Natraquolls

4. A calcic (calcium carbonate accumulation) or gypsic (gypsum 

accumulation) horizon within a depth of 40 cm -------------- Calciaquolls

Begin measuring depth below any O horizon. These soils do not have an 

argillic horizon, except where it occurs in a buried soil.

5. An argillic (clay accumulation) horizon ------------------------- Argiaquolls

6. Episaturation (perched water table) -------------------------------- Epiaquolls

7. Endosaturation (saturated throughout the profile) --------- Endoaquolls
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-230

Descriptions of Great Groups of Aquolls

Cryaquolls—These are the cold Aquolls of high elevations or high latitudes. 

Because they are both wet and cold, most of these soils are not cultivated. 

Cryaquolls may have an argillic (clay accumulation) or cambic (minimal soil 

development) subsoil horizon or a shallow calcic (calcium carbonate 

accumulation) subsoil horizon. In the United States, these soils are mostly in 

high mountain valleys in the West, but they are not extensive. They are used 

mostly for grazing or for grasses cut for hay. (Back to Aquolls key)

Duraquolls—These soils have a duripan (layer cemented by silica). In the 

United States, these soils are restricted to areas that have a Mediterraneantype climate or to arid regions. They are wet in winter, but most are relatively

dry in summer. All Duraquolls are near enough to volcanoes to have received 

ash falls. The upper boundary of the duripan most commonly is about 50 to 75 

cm below the soil surface. These soils range from slightly acid to strongly 

alkaline. They are of small extent. (Back to Aquolls key)

Natraquolls—These soils have a natric (high levels of illuvial clay and 

sodium) subsoil horizon that typically lies very close to the surface. The thin 

overlying horizon commonly has a dry color that is too light for a mollic 

epipedon. The color of the upper part of the soil, after mixing to a depth of 18 

cm, however, is dark enough for a mollic epipedon because the natric horizon 

is nearly black. Ground water is shallow during most of the year, and capillary 

rise in many Natraquolls has concentrated salts, including sodium salts, in the 

upper 50 cm. Natraquolls are known to occur only in subhumid to arid regions. 

They are on flood plains and on the margins of lakes. They formed in latePleistocene or Holocene deposits. The vegetation consists of grasses, 

sedges, and shrubs. The soils are used as rangeland or for grasses cut for 

hay. They are rarely cultivated. Natraquolls are rare in the United States. 

(Back to Aquolls key)

Calciaquolls—These soils have a shallow calcic (calcium carbonate 

accumulation) or gypsic (gypsum accumulation) subsoil horizon. Capillary rise 

of calcium-rich water and the subsequent evapotranspiration were important 

processes in the genesis of these soils. In the humid parts of the United 

States, these soils commonly formed around the margins of shallow, 

intermittent ponds in closed depressions left by glaciers. In semiarid and arid 

areas, they are mainly on flood plains, low terraces, and the margins of 

Pleistocene lakes where ground water is shallow. A calcic horizon typically is 

at or very close to the soil surface. Carbonates in high concentrations act as a 

white pigment, and not all Calciaquolls have a surface horizon that is dark

colored when dry. Aquolls with a gypsic horizon are included in Calciaquolls, 

but no Calciaquolls with a gypsic horizon have been recognized in the United 

States. Calciaquolls are moderately extensive in the United States. Most are 

calcareous from the surface downward, and problems with plant chlorosis are 
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-231

common. In humid regions, potassium deficiency in plants can be severe. In 

arid regions, salinity may be a problem. (Back to Aquolls key)

Argiaquolls—These soils have an argillic (clay accumulation) subsoil horizon. 

The depth to ground water in these soils fluctuates appreciably and is 

commonly shallow in winter and spring but deep in summer. Most of the 

Argiaquolls in the United States have been drained and are used as cropland. 

They are extensive and widely distributed throughout the Midwest and the 

western parts of the country. (Back to Aquolls key)

Epiaquolls—These soils have episaturation. A perched water table is at or 

near the soil surface during wet periods (mostly in winter and early spring) but 

commonly does not occur during dry periods (in summer). The depth to 

ground water fluctuates appreciably. Most of the Epiaquolls in the United 

States have some artificial drainage, mostly surface drainage, and are used 

as cropland. The soils are extensive and widely distributed throughout the 

Midwest and the western parts of the country. (Back to Aquolls key)

Endoaquolls—These soils have endosaturation. They are saturated 

throughout the profile at some time. The depth to ground water fluctuates 

appreciably. Commonly, the ground water is at or near the soil surface in 

winter and spring but is deep in summer. Most of the Endoaquolls in the 

United States have been artificially drained and are used as cropland. They 

are extensive and widely distributed throughout the Midwest and the western 

parts of the country. (Back to Aquolls key)
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Rendolls Great Groups

Rendolls are the Mollisols of humid regions that formed in highly calcareous 

parent materials, such as limestone, chalk, drift composed mainly of 

limestone, or shell bars. These soils have a mollic (rich in humus and bases) 

epipedon that is less than 50 cm thick and that rests on the calcareous parent 

materials or on a cambic (minimal soil development) subsoil horizon that is 

rich in carbonates. A few of the soils are so rich in finely divided lime that the 

mollic epipedon is lighter in color than typical but is nevertheless rich in darkcolored humus and within the limits of a mollic epipedon. Rendolls are either 

cold (cryic soil temperature regime) or in areas with seasonally well-distributed 

rainfall (udic moisture regime), or both. These soils are not extensive in the 

United States, but they are extensive in some parts of the world, including 

parts of Europe and on the Mexican Yucatan peninsula. They formed under 

forest vegetation or under grass and shrubs.

Rendolls

Cryrendolls

Haprendolls
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-233

Profile of a Rendoll (specifically a Haprendoll). The mollic 

epipedon rests on chalk (white). Scale is in feet.

(Back to Rendolls)
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-234

Key to Great Groups of Rendolls (Back to key to suborders)

Rendolls that have:

1. Cold average soil temperature -------------------------------------- Cryrendolls

These soils have a cryic soil temperature regime.

2. Seasonally well-distributed rainfall ------------------------------- Haprendolls

These soils have a udic soil moisture regime.
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-235

Descriptions of Great Groups of Rendolls

Cryrendolls—These soils have a cryic soil temperature regime. The 

Cryrendolls in the United States are rare and occur only in the mountains of 

the western States. The vegetation is mostly grass and shrubs. Some of the 

soils have widely spaced coniferous trees. (Back to Rendolls key)

Haprendolls—These soils have a udic soil moisture regime. Some have a 

minimally developed subsoil (cambic horizon). Haprendolls are rare in the 

United States. (Back to Rendolls key)
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Gellols Great Groups

Gellols are the Mollisols of very cold regions. These soils have a gelic soil 

temperature regime but lack permafrost within the profile. They tend to be in 

landscape positions that are subject to wide variations in summer high 

temperatures and winter low temperatures, which result in mean annual soil 

temperatures of 0 degrees C or less, but without permafrost.

Gelolls Haplogelolls
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Key to Great Groups of Gelolls (Back to key to suborders)

Gelolls that have:

1. Only a mollic epipedon or a mollic epipedon and a cambic (minimal 

soil development) subsoil horizon ------------------------------- Haplogelolls
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Description of the Great Group of Gelolls

Haplogelolls—These soils are mostly well drained or excessively drained

soils at high latitudes. Most have high contents of rock fragments in the profile 

and may also be shallow to bedrock. They commonly have a relatively thin 

mollic epipedon and no other diagnostic horizons, but some may have a 

cambic (minimal soil development) subsoil horizon. The vegetation commonly 

is dwarf herbaceous vegetation, coniferous forest, or alpine tundra. 

Haplogelolls are not extensive. In the United States, these soils occur in 

Alaska. (Back to Gelolls key)
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Cryolls Great Groups

Cryolls are the cool or cold, more or less freely drained Mollisols. These soils

have various diagnostic subsoil horizons, including a duripan (layer cemented 

by silica), an argillic (clay accumulation) horizon, a natric (high levels of illuvial 

clay and sodium) horizon, a calcic (calcium carbonate accumulation) horizon,

or a cambic (minimal soil development) horizon. They are moderately 

extensive in the high mountains of the western United States. They also are 

extensive on the plains and mountains of Eastern Europe and in Asia. On the 

plains, they are mainly in areas of late-Pleistocene or Holocene deposits. In 

the mountains of the western United States, some of the soils may be on older 

surfaces, but the geomorphology of these areas has had little study.

Cryolls have a cryic temperature regime and a udic (seasonally welldistributed precipitation), ustic (moisture is limited, but available, during 

portions of the growing season), or xeric (cool and moist in winter and warm

and dry in summer) moisture regime.

The vegetation of the Cryolls was mostly grasses on the plains and either 

forest or grass in the mountains. In Alaska, spruce, birch, and aspen trees are 

common on these soils.

Cryolls

Duricryolls

Natricryolls

Palecryolls

Argicryolls

Calcicryolls

Haplocryolls
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Profile of a Cryoll (specifically a Natricryoll). The soil has a 

natric horizon below a depth of 14 inches. Scale is in 

inches.

(Back to Cryolls)
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Key to Great Groups of Cryolls (Back to key to suborders)

Cryolls that have:

1. A duripan (layer cemented by silica) within a depth of 

100 cm ------------------------------------------------------------------------ Duricryolls

Begin measuring depth below any O horizon.

2. A natric (high levels of illuvial clay and sodium) 

horizon ---------------------------------------------------------------------- Natricryolls

3. An argillic (clay accumulation) horizon beginning deeper than 60 cm 

AND loamy or clayey texture above ------------------------------- Palecryolls

Begin measuring depth below any O horizon.

4. An argillic (clay accumulation) horizon --------------------------- Argicryolls

5. A calcic (calcium carbonate accumulation) or petrocalcic (cemented 

by calcium carbonate) horizon within a depth of 100 cm AND free 

carbonates or sandy texture above ------------------------------- Calcicryolls

Begin measuring depth below any O horizon.

6. A cambic (minimal soil development) or calcic (calcium carbonate 

accumulation) horizon ----------------------------------------------- Haplocryolls
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Descriptions of Great Groups of Cryolls

Duricryolls—These soils have a duripan (layer cemented by silica) that has 

its upper boundary within a depth of 100 cm. They are of small extent in the 

mountain valleys of the western United States. Many had grasses and widely 

spaced conifer trees. Most of these soils are used as rangeland or forest. 

(Back to Cryolls key)

Natricryolls—These soils have a natric (high levels of illuvial clay and 

sodium) subsoil horizon. They are rare. (Back to Cryolls key)

Palecryolls—These soils have an argillic (clay accumulation) subsoil horizon

with an upper boundary more than 60 cm below the surface. Most of the 

Palecryolls in the United States are in the mountains of the western States. In 

other countries, at high latitudes, Palecryolls also developed on plains. 

Palecryolls have cool or short summers. Many of the soils had grasses and 

scattered conifer trees. In the United States, the vegetation at the time of 

settlement was either forest or grass. Most Palecryolls are used as rangeland 

or forest. Some are cultivated and used for small grain or hay. (Back to Cryolls 

key)

Argicryolls—These soils have an argillic (clay accumulation) subsoil horizon

close to the surface. Commonly, the upper part of the argillic horizon is in the 

mollic epipedon. Some of these soils have calcium carbonate accumulations 

below the argillic horizon. Argicryolls formed mainly in Pleistocene or 

Holocene deposits or are on surfaces of equivalent ages. They have cool or 

short summers. Most Argicryolls in the United States are in the mountains of 

the western States. A few are in Alaska. In other countries, at high latitudes, 

Argicryolls also developed on plains. Many of the soils had grasses and 

scattered conifer trees. In the United States, the vegetation at the time of 

settlement was either forest or grass. Most Agricryolls are used as rangeland 

or forest. Some are cultivated and used for small grain or hay. (Back to Cryolls 

key)

Calcicryolls—These soils have a calcic (calcium carbonate accumulation) or 

petrocalcic (cemented by calcium carbonate) subsoil horizon that has its 

upper boundary within a depth of 100 cm. They are calcareous, or would be 

calcareous if plowed, in all parts of the pedon above the calcic horizon, or they 

have a texture of loamy fine sand or coarser in all parts above the calcic or 

petrocalcic horizon. The mollic epipedon commonly includes part of the calcic 

horizon. These soils formed in materials that have a moderate to high 

percentage of carbonates. Many of the soils are in areas of deposits that 

originated from limestone. Calcicryolls occur in areas of cool or short 

summers. In the United States, the vegetation at the time of settlement was 

either grass or forest. The soils are of moderate extent in the mountains of the 

western United States. Most of the soils are used as rangeland, forest, or 
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-243

wildlife habitat. A few areas are cultivated and used for small grain or hay. 

(Back to Cryolls key)

Haplocryolls—Most Haplocryolls have a cambic subsoil horizon, and some 

have a calcic horizon. Haplocryolls formed mainly in Pleistocene or Holocene 

deposits or on surfaces of equivalent ages. They have cool or short summers. 

Many of the soils had grasses and scattered conifer trees. The vegetation at 

the time of settlement was either forest or grass. Most of the soils are used as 

rangeland or forest. Some are cultivated and used for small grain or hay. 

Haplocryolls are extensive in the United States. (Back to Cryolls key)
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Xerolls Great Groups

Xerolls are the Mollisols in regions that have Mediterranean-type climates,

characterized by cool, moist winters, and warm, dry summers. As their name 

implies, they generally have a xeric moisture regime. Some Xerolls that are 

marginal to Aridisols, however, have an aridic moisture regime. Xerolls are dry 

for extended periods in summer, but moisture moves through most of the soils 

in winter and is stored throughout the profile above the deep layers or above 

bedrock in most years. Soft, white winter wheat is the most common crop on 

the Xerolls in the United States where there is no irrigation and temperatures 

are moderate. In colder areas, these soils are used for spring wheat or for 

grazing. With irrigation, many crops are grown, especially in the warmer 

areas. The soils with steep slopes mainly support natural vegetation 

consisting of native annual or perennial grasses or of shrubs or trees. These 

soils are used mainly for grazing in spring or late fall. The native perennial 

grasses remain dormant during the dry summers, and the annual grasses 

grow mainly in spring and sometimes in fall, if rains occur before temperatures

become too low.

Characteristically, Xerolls have a relatively thick mollic epipedon, a cambic 

(minimal soil development) or argillic (clay accumulation) subsoil horizon, and 

an accumulation of carbonates in the lower part of the subsoil. Most horizons 

have neutral pH.

The Xerolls in the United States formed mainly in late-Pleistocene loess that 

varied in thickness and that overlay bedrock on nearly level to steep older 

surfaces. Tertiary lake sediments, older crystalline rocks, and alluvium are 

common parent materials in some areas. The vegetation at the time of 

settlement in the areas with moderate to cold temperatures was dominantly 

bunchgrass and shrubs; wheatgrass (Agropyron), fescue (Festuca), and 

bluegrass (Poa) species in association with sagebrush (Artemisia), bitterbrush 

(Purshia), and rabbitbrush (Chrysothamnus) species; and some scattered 

juniper (Juniperus) and pine (Pinus) species. In the Willamette Valley of 

Oregon, the vegetation was a savanna of perennial grasses, oaks, and 

Douglas fir. In the warmer areas of California, the vegetation was a savanna 

of annual grasses and oak species.

Xerolls are not extensive worldwide, but they are extensive in parts of Turkey 

and northern Africa near the Mediterranean Sea, in some of the southern 

republics of the former Soviet Union, and in Washington, Oregon, Idaho, Utah, 

Nevada, and California in the United States. 
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-245

Xerolls

Durixerolls

Natrixerolls

Palexerolls

Calcixerolls

Argixerolls

Haploxerolls
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-246

Profile of a Xeroll (specifically a Haploxeroll). 

The mollic epipedon is cobbly and rests 

directly on bedrock at a depth of about 

2 feet. Scale is in feet.

(Back to Xerolls)
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Key to Great Groups of Xerolls (Back to key to suborders)

Xerolls that have:

1. A duripan (layer cemented by silica) within a depth of 

100 cm ------------------------------------------------------------------------ Durixerolls

Begin measuring depth below any O horizon.

2. A natric (high levels of illuvial clay and sodium) 

horizon ---------------------------------------------------------------------- Natrixerolls

3. Either:

a) A petrocalcic (cemented by calcium carbonate) horizon within a 

depth of 150 cm, OR

b) An argillic (clay accumulation) horizon in which the clay content

does not decrease significantly within a depth of 150 cm and that 

has hue of 7.5YR or redder color (and there is no root-limiting 

layer within a depth of 150 cm), OR

c) An abrupt increase in clay content to an argillic (clay

accumulation) horizon with clayey texture (and no root-limiting 

layer within a depth of 50 cm) ----------------------------------- Palexerolls

Begin measuring depth below any O horizon. For item b, clay content must 

not decrease by > 20% of the maximum in the argillic horizon. For specific 

criteria regarding colors and regarding an abrupt clay increase (item c), 

see 12th edition of “Keys to Soil Taxonomy.”

4. A calcic (calcium carbonate accumulation) or gypsic (gypsum 

accumulation) horizon within a depth of 150 cm AND free carbonates 

or sandy texture above ------------------------------------------------ Calcixerolls

Begin measuring depth below any O horizon.

5. An argillic (clay accumulation) horizon --------------------------- Argixerolls

6. A cambic (minimal soil development) horizon or no diagnostic 

subsoil horizon --------------------------------------------------------- Haploxerolls
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-248

Descriptions of Great Groups of Xerolls

Durixerolls—These soils have a duripan (layer cemented by silica) that has 

its upper boundary within a depth of 100 cm. They formed mainly in 

Pleistocene sediments. In many of these soils, some of the materials near the 

surface are younger than the duripan. The parent materials commonly were 

derived, at least partially, from basic volcanic rocks, siliceous tuffs, pumice, or 

volcanic ash. They generally are gently sloping or moderately sloping. The 

natural vegetation on the Durixerolls in the United States was grass and 

shrubs. These soils occur in the western United States, but they are not 

extensive. (Back to Xerolls key)

Natrixerolls—These soils have a natric (high levels of illuvial clay and 

sodium) subsoil horizon. The natric horizon commonly has either prismatic or 

columnar structure, and there are accumulations of calcium carbonate in or 

below it. Some of the soils have a thin albic (light-colored and leached) 

horizon above the natric horizon. Natrixerolls formed mostly in mixed 

sediments of late-Pleistocene age, commonly in depressions, in basins, or on 

low terraces. Individual areas of these soils are mostly small. (Back to Xerolls 

key)

Palexerolls—These soils formed on old surfaces, commonly on basalt plains 

of Miocene or Pliocene age that later, during or after the late Pleistocene, 

received a thin covering of loess or of mixed loess and ash. Slopes range 

from nearly level to very steep. The soils commonly have a thick, dark brown 

or reddish brown, clayey argillic (clay accumulation) subsoil horizon that has 

an abrupt upper boundary, or they have a petrocalcic (cemented calcium 

carbonate) subsoil horizon. Calcium carbonate accumulations are common 

below or in the lower part of the argillic horizon. The native vegetation was 

mainly bunchgrasses and some shrubs. Some of the soils supported a 

coniferous forest with an understory of grasses and shrubs or supported an 

open forest or savanna. Palexerolls occur in parts of the western United 

States, but they are not extensive. (Back to Xerolls key)

Calcixerolls—These soils have a calcic (calcium carbonate accumulation) or 

gypsic (gypsum accumulation) subsoil horizon and are calcareous in all 

overlying horizons. Either the parent materials had more carbonates than

rainfall could remove from the upper horizons, or there is a continual external 

source of carbonates in dust or water. Calcixerolls formed mostly in latePleistocene sediments or in older materials on surfaces of comparable age. 

Slopes range from nearly level to very steep. In the United States, their native 

vegetation was mostly grass and shrubs. These soils are most extensive in 

the Great Basin of the western United States. (Back to Xerolls key)

Argixerolls—These soils have a relatively thin argillic (clay accumulation) 

subsoil horizon or one in which the percentage of clay decreases rapidly with 
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-249

increasing depth. Generally, the mollic epipedon is very dark brown and the 

argillic horizon is dark brown. Most of these soils have calcium carbonate 

accumulations below or in the lower part of the argillic horizon. Argixerolls 

formed mostly in mid-Pleistocene or earlier deposits or on surfaces of Tertiary 

age. Slopes range from nearly level to very steep. The natural vegetation is 

mostly grasses and shrubs, but some of the soils support coniferous forest 

vegetation with a grass and shrub understory and some have an open forest 

or savanna. (Back to Xerolls key)

Haploxerolls—These soils are characterized by little development in the 

subsoil. Commonly, they have a cambic (minimal soil development) subsoil 

horizon below the mollic epipedon. Some have only unaltered recent parent 

materials below the mollic epipedon. Many have horizons in which secondary 

carbonates have accumulated. A few have a calcic (calcium carbonate 

accumulation) subsoil horizon, but at least parts of the surface horizons are 

free of carbonates. Haploxerolls formed mainly in late-Pleistocene deposits or 

on surfaces of comparable age, but some formed on older surfaces. Parent 

materials of late-Wisconsinan loess mixed with some volcanic ash, glacial 

outwash or till, and alluvium from mixed sources are common. Slopes range 

from nearly level to very steep. The natural vegetation is mostly grasses and 

shrubs, but some of the soils support a coniferous forest with a grass and 

shrub understory and some support an open forest or savanna. (Back to 

Xerolls key)
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Ustolls Great Groups

Ustolls are the more or less freely drained Mollisols of subhumid to semiarid 

climates. Rainfall occurs mainly during a growing season, often in heavy 

showers, but is erratic. Drought is frequent and may be severe. During a 

drought, soil blowing becomes a problem. In nonirrigated areas, the low 

supply of moisture usually limits crop yields. Ustolls are extensive soils on the 

western Great Plains in the United States.

In addition to the mollic epipedon, most Ustolls have accumulations of calcium 

carbonate in the subsoil; a few of the soils that formed in noncalcareous 

materials do not. Ustolls may also have a cambic (minimal soil development), 

argillic (clay accumulation), petrocalcic (cemented by calcium carbonate), or 

natric (high levels of illuvial clay and sodium) subsoil horizon. If there is a 

natric horizon, there may be an albic (light-colored and leached) horizon 

overlying it, or if there is a cambic or argillic horizon, there may be a duripan 

(layer cemented by silica) below it. The presence or absence of these 

horizons is used, in part, as the basis for defining the great groups of Ustolls.

Most of the Ustolls on the Great Plains in the United States had grass 

vegetation when the country was settled. In drier areas they supported mostly 

short grasses, and in more humid areas they supported mixtures of short and 

tall grasses. Some of the Ustolls in the mountains of the western States 

supported forest vegetation. 

Ustolls formed in sediments and on surfaces of varying ages, ranging from 

Holocene to mid Pleistocene or earlier. Those outside of glaciated areas may 

have formed during two or more glacial and interglacial stages.

Ustolls

Durustolls

Natrustolls

Calciustolls

Paleustolls

Argiustolls

Vermustolls

Haplustolls
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Profile of an Ustoll (specifically a Calciustoll) in Texas. The mollic 

epipedon is underlain by a thick calcic horizon (white bodies 

are calcium carbonate accumulations). 

Back to Ustolls
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Key to Great Groups of Ustolls (Back to key to suborders)

Ustolls that have:

1. A duripan (layer cemented by silica) within a depth of 

100 cm ------------------------------------------------------------------------ Durustolls

Begin measuring depth below any O horizon.

2. A natric (high levels of illuvial clay and sodium) 

horizon ----------------------------------------------------------------------- Natrustolls

3. Either: 

a) A calcic (calcium carbonate accumulation) or gypsic (gypsum 

accumulation) horizon within a depth of 100 cm, OR

b) A petrocalcic horizon within a depth of 150 cm,

AND

c) Free carbonates or sandy texture above -------------------- Calciustolls

Begin measuring depth below any O horizon. Calciustolls do not have an 

argillic horizon above any calcic, gypsic, or petrocalcic horizon.

4. Either:

a) A petrocalcic (cemented by calcium carbonate) horizon within a 

depth of 150 cm, OR

b) An argillic (clay accumulation) horizon in which clay does not 

decrease significantly within a depth of 150 cm and that has hue of 

7.5YR or redder color (and there is no root-limiting layer within a 

depth of 150 cm), OR

c) An abrupt increase in clay content to an argillic (clay 

accumulation) horizon with clayey texture (and no root-limiting layer 

within a depth of 50 cm) ----------------------------------- Paleustolls

Begin measuring depth below any O horizon. For item b, clay content must 

not decrease by > 20% of the maximum in the argillic horizon. For specific 

criteria regarding colors and regarding an abrupt clay increase (item c), 

see 12th edition of “Keys to Soil Taxonomy.”

5. An argillic (clay accumulation) horizon --------------------------- Argiustolls
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6. Significant evidence of bioturbation ----------------------------- Vermustolls

Bioturbation must be evident in > 50%, by volume, of the surface layer and 

> 25% of a transition zone to the subsoil below.

7. A cambic (minimal soil development) horizon ----------------- Haplustolls
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Descriptions of Great Groups of Ustolls

Durustolls—These soils have a duripan (layer cemented by silica) with its 

upper boundary within a depth of 100 cm. They formed in the vicinity of 

cinders and ash falls. Their parent materials are mainly siliceous tuffs, 

volcanic ash, cinders, and basic volcanic rocks. In the United States, the 

natural vegetation is mostly grasses and shrubs. These soils are of small 

extent. They formed in late-Pleistocene deposits. They are on alluvial fans 

and terraces, on nearly level to rolling cinder fans and plains, and on hilly to 

very steep cinder cones. They may be common on the leeward sides of 

volcanic islands in the Lesser Antilles. (Back to Ustolls key)

Natrustolls—These soils have a natric (high levels of illuvial clay and sodium)

subsoil horizon. The most common horizon sequence is either a natric horizon 

in the lower part of the mollic epipedon or a thin albic (light-colored and 

leached) horizon over a natric horizon with columnar structure. Calcium 

carbonate or other salts may have accumulated below the natric horizon. Most 

of the areas of these soils are small and are nearly level or concave. 

Natrustolls formed mostly in late-Pleistocene sediments. (Back to Ustolls key)

Calciustolls—These soils have a gypsic (gypsum accumulation), calcic 

(calcium carbonate accumulation), or petrocalcic (cemented by calcium 

carbonate) subsoil horizon and are calcareous in all overlying horizons. Either 

the parent materials had more carbonates than the limited rainfall could 

remove from the upper horizons, or there is a continual external source of 

carbonates in dust or water. Calciustolls formed mostly in Pleistocene 

sediments or in older materials on surfaces of comparable age. In the United 

States, their vegetation was dominantly grass before the soils were cultivated. 

Calciustolls are most extensive on the Great Plains in the United States, but 

some occur in the intermountain valleys of the western States. (Back to 

Ustolls key)

Paleustolls—These are the Ustolls on old stable surfaces, as evidenced by 

the development of a thick, reddish argillic horizon, a clayey argillic horizon 

that has an abrupt upper boundary, or a petrocalcic (cemented by calcium 

carbonate) horizon. These soils commonly have been partly or completely 

calcified during the Holocene, and calcium carbonate has accumulated in the 

previously formed argillic horizon. The Paleustolls in the United States are 

mainly in the central and southern parts of the Great Plains. At the time of 

settlement, they had mostly grass vegetation. Their history during the 

Pleistocene has had little study. The petrocalcic horizon, where it occurs, may 

be complex, suggesting a number of alternating cycles of humidity and aridity 

and slow accretion of dust and sediment from the arid regions to the west. 

(Back to Ustolls key)
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-255

Argiustolls—These soils have an argillic (clay accumulation) subsoil horizon

in or below the mollic epipedon. Most of these soils have an argillic horizon 

that, with increasing depth, has a clay decrease of 20% or more (relative) from 

the maximum clay content within 150 cm of the mineral soil surface. Some of 

the soils have a root-limiting layer (densic, lithic, or paralithic contact) within a 

depth of 150 cm, and some have no hues of 7.5YR or redder or have chroma 

of 4 or less. Most of these soils have a zone of accumulation of calcium 

carbonate or other salts below the argillic horizon. Argiustolls formed mostly in 

late-Pleistocene deposits or on surfaces of comparable age. They occur in 

relatively stable positions. Slopes generally are moderate to nearly level, and 

most of the soils are cultivated. Argiustolls are extensive on the western Great 

Plains and also occur in the mountains and valleys of the western United 

States. (Back to Ustolls key)

Vermustolls—These soils have been intensively and repeatedly mixed by 

animals, mainly by earthworms and their predators. The most common 

horizon sequence is a mollic epipedon consisting mostly of wormcasts that

rests on parent materials that contain many worm channels filled with dark 

materials from the surface. There may be a horizon in which carbonates have 

precipitated, but mixing by animals may have destroyed such a horizon or 

prevented its formation. Vermustolls are not known to occur in North America, 

but they are extensive in Eastern Europe. (Back to Ustolls key)

Haplustolls—Most of these soils have a cambic (minimal soil development) 

subsoil horizon below the mollic epipedon, and most have a horizon in which 

carbonates or soluble salts have accumulated. A few that formed in 

noncalcareous sediments do not have a horizon of carbonate accumulation. 

Haplustolls formed mainly in late-Pleistocene or Holocene deposits or on 

surfaces of comparable age. Their vegetation has been dominantly grasses 

and forbs. These soils are extensive on the Great Plains of North America, in 

Eastern Europe, and on the Pampas in South America. Where slopes are 

suitable, most of the soils are used for grain and feed crops. (Back to Ustolls 

key)
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-256

Udolls Great Groups

Udolls are the more or less freely drained Mollisols of humid climates. In 

addition to the mollic epipedon, these soils may have a cambic (minimal soil 

development), calcic (calcium carbonate accumulation), natric (high levels of 

illuvial clay and sodium), or argillic (clay accumulation) subsoil horizon. They 

formed mainly in late-Pleistocene or Holocene deposits or on surfaces of 

comparable ages. In the United States, their vegetation at the time of 

settlement was dominantly a tall grass prairie, but some of the soils on 

Pleistocene surfaces appear to have supported at some time a boreal forest 

that was supplanted by grasses several thousand years ago.

Udolls formed in sediments and on surfaces of varying ages, ranging from

Holocene to mid Pleistocene or earlier. Those outside of glaciated areas may 

have formed during two or more glacial and interglacial stages.

Most of the Udolls are in the eastern part of the Great Plains or are east of the 

Great Plains. The soils are most extensive in Illinois, Iowa, and adjacent 

States. Where slopes are not too steep, nearly all of these soils are cultivated. 

Maize (corn) and soybeans are the major crops. Many Udolls have undergone 

significant erosion, which has resulted in mollic epipedons that are now 

noticeably thinner than they were in their native state.

Udolls

Natrudolls

Calciudolls

Paleudolls

Argiudolls

Vermudolls

Hapludolls
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-257

Profile of a Udoll (specifically an Argiudoll) in Iowa. The mollic 

epipedon has lost some of its original thickness due to water 

erosion. It is underlain by an argillic horizon. Scale is in cm.

Back to Udolls
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-258

Key to Great Groups of Udolls (Back to key to suborders)

Udolls that have:

1. A natric (high levels of illuvial clay and sodium) horizon --- Natrudolls

2. A calcic (calcium carbonate accumulation) or petrocalcic 

(cemented by calcium carbonate) horizon within a depth of 100 cm 

AND free carbonates above ------------------------------------------ Calciudolls

3. Either:

a) Cold average soil temperature (frigid soil temperature regime), an 

argillic (clay accumulation) horizon within a depth of 60 cm, and 

loamy or clayey textures above, OR

b) A clay content that does not decrease significantly within a depth 

of 150 cm AND an argillic (clay accumulation) horizon with hue of 

7.5YR or redder color in some part (and there is no root-limiting layer 

within a depth of 150 cm) ----------------------------------------------- Paleudolls

Begin measuring depth below any O horizon. For item c, clay must not 

decrease by > 20% of the maximum in the argillic horizon. See 12th edition 

of “Keys to Soil Taxonomy” for specific criteria regarding colors.

4. An argillic (clay accumulation) horizon ---------------------------- Argiudolls

5. Significant evidence of bioturbation ------------------------------ Vermudolls

Criterion requires > 50% by volume in the surface layer and > 25% in a 

transition zone to the subsoil below.

6. A cambic (minimal soil development) horizon ------------------ Hapludolls
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-259

Descriptions of Great Groups of Udolls

Natrudolls—These soils have a natric (high levels of illuvial clay and sodium)

subsoil horizon. Below the natric horizon, there is typically one horizon or 

more in which carbonates, sulfates, or other soluble salts have accumulated. 

Most Natrudolls are in small, nearly level or concave areas. These soils are 

most common in Argentina and on the northern Great Plains of the United 

States, where many of the parent materials contain salts. (Back to Udolls key)

Calciudolls—These soils have a calcic (calcium carbonate accumulation) or 

petrocalcic (cemented by calcium carbonate) subsoil horizon that has its 

upper boundary within a depth of 100 cm. They are calcareous in all 

subhorizons overlying the calcic horizon after the upper 18 cm has been 

mixed, unless the texture is coarser than loamy very fine sand or very fine 

sand. These soils are of small extent on the northern Great Plains of the 

United States. They are important in Argentina. (Back to Udolls key)

Paleudolls—These soils have a thick or deep argillic (clay accumulation) 

subsoil horizon. Most of these soils have a reddish hue and a clay content that 

decreases very gradually with increasing depth. Paleudolls are mainly on 

surfaces older than Wisconsinan and are thought to have formed during at 

least one glacial stage and one interglacial stage. Slopes are gentle to steep. 

These soils are mostly on the southern Great Plains and in the mountains of 

the western United States. They are extensive only locally. (Back to Udolls 

key)

Argiudolls—These soils have a relatively thin argillic (clay accumulation) 

subsoil horizon or one in which the percentage of clay decreases greatly with 

increasing depth. The mollic (rich in humus and bases) epipedon commonly is 

black to very dark brown, and the argillic horizon is mostly brownish. Many of 

these soils are noncalcareous to a considerable depth below the argillic 

horizon. Some Argiudolls have a zone of accumulation of calcium carbonate 

below the argillic horizon. Argiudolls formed mostly in late-Wisconsinan 

deposits or on surfaces of that age. Many or most of these soils supported 

boreal forests during the Pleistocene that were later replaced by tall grass 

prairies during the Holocene. Argiudolls are extensive in Iowa, Illinois, and 

adjacent States. (Back to Udolls key)

Vermudolls—These soils have been intensively mixed by earthworms or 

other burrowing animals and their predators. The most common horizon 

sequence is a mollic (rich in humus and bases) epipedon consisting of many 

wormcasts that overlies soil material containing many worm channels filled 

with dark material from the mollic epipedon. These soils are not known to 

occur to a significant extent of the United States, but they are important in 

other countries. (Back to Udolls key)
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-260

Hapludolls—These soils generally have a cambic (minimal soil development) 

subsoil horizon below a mollic epipedon. There may be a zone of calcium 

carbonate accumulation below the cambic horizon. Hapludolls formed mostly 

in Holocene or late-Pleistocene deposits or on surfaces of that age. Slopes 

generally are gentle, and most of the soils are cultivated. Hapludolls are 

extensive in Iowa, Minnesota, and adjacent States. (Back to Udolls key)
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-261

Oxisols Order

Oxisols are highly weathered tropical soils with low natural fertility.

General Characteristics

Oxisols are highly weathered soils that are low in natural fertility. Typically,

they have an ochric (typically thin and/or light-colored) epipedon underlain by 

an oxic (extremely weathered) subsoil horizon. The oxic horizon is 

characterized by having a low nutrient-holding ability (i.e., cation-exchange 

capacity) and few minerals capable of further weathering. Oxisols with a

clayey surface layer, but in which there is a noticeable further increase in clay 

content from the surface to the subsoil, have a kandic (very low cationexchange capacity) subsoil horizon which is otherwise very similar to an oxic 

horizon. A few Oxisols have a sombric subsoil horizon (dark layer in which 

organic matter has accumulated). The processes leading to the formation of 

the sombric horizon are poorly understood. The profiles of most Oxisols are 

distinctive in that their horizons are not readily apparent visually. While their 

surface horizons are typically somewhat darker than the subsoil, the transition 

of subsoil features is gradual. Subsoil horizons are generally relatively red, 

due to iron oxides coating the mineral grains. Oxisols commonly have a strong 

grade of granular structure. Although they may have a loamy feel, many are in 

fact quite clayey. The clay particles are strongly aggregated into fine granules

and, as a result, many Oxisols have a high degree of permeability.

Environment and Processes

Oxisols occur mostly in the humid tropics and subtropics. They are generally 

in climates that are warm year-round with little seasonal variation. Because 

these soils occur in environments that range from tropical rainforests to 

grassland savannas and even include some deserts, they can have any soil 

moisture regime, from aquic to aridic. Many Oxisols are on old landscapes 

and formed in parent materials that have undergone multiple cycles of 

weathering, erosion, redeposition, and renewed weathering. In these soils, 

nearly all of the original weatherable minerals that were present early in the 

cycle are now gone, leaving only highly resistant minerals such as quartz, 

kaolinite, gibbsite, and oxides of aluminum and iron. From a mineralogical 

perspective, these soils are relatively inert and have little potential for further 

development. Oxisols have very low reserves of soil nutrients. Most of the soil 

nutrients are concentrated in the living and dead plant tissues at the site and 

are quickly lost when the site is cleared and cultivated. Oxisols can be fairly 

high in organic matter, but the organic matter is generally very old and 

consists of inactive, stable forms that do not add much to the fertility of the 

soil. Bioturbation, especially by termites, is important in many Oxisols.
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-262

Location

Globally, Oxisols occupy about 8% of the ice-free land area. They are found 

primarily at latitudes between the Tropic of Cancer and the Tropic of Capricorn 

(at latitudes of about 23 degrees north and south, respectively). The most 

extensive areas of Oxisols are on the interior plateaus of South America, the 

lower portion of the Amazon Basin, portions of the Central African Basin, and 

parts of Asia, Australia, and several tropical and subtropical islands. Oxisols 

are of small extent in the United States. They occur only in Hawaii and Puerto 

Rico, and many of the taxa are not known to occur at those locations. 

Profile of an Oxisol.

(Back to Key to Soil Orders)
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-263

Oxisols by suborder in the United States.

Global distribution of Oxisols.
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-264

Oxisols Suborders

Classification of Oxisols

In the definitions of the suborders, emphasis is placed primarily on soil climate 

in the form of soil wetness (Aquox) and soil moisture regime (Torrox, Ustox, 

Perox, and Udox).

The great group level reflects a combination of important properties, including 

the presence of various diagnostic horizons, a very low effective cationexchange capacity (ECEC), relatively high base saturation, and the presence 

of plinthite (firm, iron oxide-rich concentration) as a continuous phase in a 

layer.

The five suborders are:

1. Aquox—wet Oxisols (aquic conditions in the upper part)

2. Torrox—Oxisols of arid areas

3. Ustox—moderately dry Oxisols with limited moisture

4. Perox—Oxisols of areas where precipitation exceeds evaporation in all 

months

5. Udox—Oxisols of humid regions with well-distributed rainfall

Oxisols

Aquox

Torrox

Ustox

Perox

Udox
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-265

Key to Suborders of Oxisols

Oxisols that have:

1. A seasonal high water table within a depth of 50 cm --------------- Aquox

A histic epipedon (wet, organic surface layer) and/or redoximorphic 

features are evidence of a seasonal high water table. See 12th edition of 

“Keys to Soil Taxonomy” for specific criteria regarding depth and color of 

redoximorphic features. Begin measuring depth below any O horizon. 

Artificially drained soils are included in Aquox.

2. Inadequate soil moisture for crop growth ------------------------------ Torrox

These soils have an aridic soil moisture regime.

3. Somewhat limited soil moisture available for crop growth -------- Ustox

These soils have either an ustic or xeric soil moisture regime.

4. More total precipitation than evaporation each month ------------- Perox

These soils have a perudic soil moisture regime.

5. Seasonally well-distributed precipitation -------------------------------- Udox

These soils have a udic soil moisture regime.
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-266

Aquox Great Groups

Aquox are the wet Oxisols in shallow depressions and in seepage areas at 

the base of slopes. They are saturated for long enough periods to allow the 

development of oxygen-depleted conditions within the upper 50 cm (aquic 

conditions), or they are artificially drained. Because the water table may 

fluctuate seasonally in the soils, iron tends to accumulate in the form of 

secondary nodules, concretions, and plinthite (firm, iron oxide-rich 

concentration). Most areas of these soils are small.

Aquox

Acraquox

Plinthaquox

Eutraquox

Haplaquox
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-267

Profile of an Aquox in Brazil. The gray and reddish colors of the 

subsoil are due to saturation with water. Scale is in 

decimeters.

Back to Aquox
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-268

Key to Great Groups of Aquox (Back to key to suborders)

Aquox that have:

1. An extremely low nutrient-holding capacity ---------------------- Acraquox

These soils have an effective cation-exchange capacity (ECEC) of < 1.50 

cmol (+) per kg clay and a pH value (1N KCl) of > 5.0 within a depth of 150

cm.

2. Plinthite (firm, iron oxide-rich concentration) forming a continuous 

phase within a depth of 125 cm ------------------------------------ Plinthaquox

Begin measuring depth below any O horizon. Plinthite is a firm, iron oxiderich concentration that irreversibly hardens after exposure to repeated wetdry cycles.

3. Moderate natural fertility ------------------------------------------------ Eutraquox

These soils have base saturation (by NH4OAc) of > 35% in all horizons 

within a depth of 125 cm.

4. Low natural fertility ------------------------------------------------------ Haplaquox

These soils have base saturation (by NH4OAc) of < 35% in some horizon 

within a depth of 125 cm and an effective cation-exchange capacity 

(ECEC) of > 1.50 cmol (+) per kg clay within a depth of 150 cm. Begin 

measuring depth below any O horizon.
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-269

Descriptions of Great Groups of Aquox

Acraquox—These soils have an extremely low cation-exchange capacity 

(CEC), and some have a net anion-exchange capacity. As a result, these soils 

have very little capacity to retain base cations (positively charged plant 

nutrients). Because of a low buffering capacity, small but frequent applications 

of fertilizer and lime may be needed. However, because the low CEC also 

facilitates the downward movement of the cations into the soil, careful nutrient 

management can lead to the enhancement of a favorable rooting depth for

crops, especially where drainage is applied. (Back to Aquox key)

Plinthaquox—These soils have continuous plinthite (firm, iron oxide-rich 

concentration) within a depth of 125 cm. (Back to Aquox key)

Eutraquox—These soils have base saturation (measured by NH4OAc at pH 

7) of more than 35% in all horizons to a depth of 125 cm. Most formed in 

mafic (base-rich) parent materials. Although a significant proportion of the 

exchange sites are occupied by base cations (positively charged plant 

nutrients), the total CEC of these soils is still low. As a result, the initial supply 

of naturally occurring plant nutrients is rapidly depleted by cropping. (Back to 

Aquox key)

Haplaquox—These soils have a low base saturation in one or more 

subhorizons within a depth of 125 cm. (Back to Aquox key)
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-270

Torrox Great Groups

Torrox are the Oxisols of arid regions. They have an aridic moisture regime, 

and, due to limited leaching, many of them have a higher base saturation than 

other Oxisols. Unless they are irrigated, Torrox are unsuitable for growing 

common agronomic crops. However, they are well suited to a variety of crops 

if irrigation water and fertilizer are applied. Torrox are known to occur only in 

Hawaii in the United States and perhaps in some areas of Australia.

Torrox

Acrotorrox

Eutrotorrox

Haplotorrox
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-271

Profile of a Torrox (specifically a Haplotorrox) in Hawaii. This soil 

formed in colluvium containing large basalt boulders. Scale is in 

centimeters.

Back to Torrox
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-272

Key to Great Groups of Torrox (Back to key to suborders)

Torrox that have:

1. An extremely low nutrient-holding capacity --------------------- Acrotorrox

These soils have an effective cation-exchange capacity (ECEC) of < 1.50 

cmol (+) per kg clay and a pH value (1N KCl) of > 5.0 within a depth of 150

cm.

2. Moderate natural fertility ---------------------------------------------- Eutrotorrox

These soils have base saturation (by NH4OAc) of > 35% in all horizons 

within a depth of 125 cm.

3. Low natural fertility ----------------------------------------------------- Haplotorrox

These soils have base saturation (by NH4OAc) of < 35% in some horizon 

within a depth of 125 cm and an effective cation-exchange capacity 

(ECEC) of > 1.50 cmol (+) per kg clay within a depth of 150 cm. Begin 

measuring depth below any O horizon.
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-273

Descriptions of Great Groups of Torrox

Acrotorrox—These soils have an extremely low cation-exchange capacity

(CEC), and some have a net anion-exchange capacity. As a result, these soils 

have very little capacity to retain base cations (positively charged plant 

nutrients) such as Ca2+, Mg2+, and K+. Because of a low buffering capacity, 

small but frequent applications of fertilizer and lime may be needed. However, 

because the low CEC also facilitates the downward movement of the cations 

into the soil, careful nutrient management can lead to the enhancement of a 

favorable rooting depth for crops. (Back to Torrox key)

Eutrotorrox—These soils have base saturation (measured by NH4OAc at pH 

7) of more than 35% in all horizons to a depth of 125 cm. Most formed in 

mafic (base-rich) parent material. Although a significant proportion of the 

exchange sites are occupied by base cations (positively charged plant 

nutrients), the total CEC of these soils is still low. As a result, the initial supply 

of naturally occurring plant nutrients is rapidly depleted by cropping.

Eutrotorrox occur mainly in Hawaii, where they are used for irrigated crops. 

(Back to Torrox key)

Haplotorrox—These soils have a low base saturation in one or more 

subhorizons within a depth of 125 cm. (Back to Torrox key)
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-274

Ustox Great Groups

Ustox are the Oxisols of subhumid to semiarid climates. They have either an 

ustic (moisture is limited, but available, during portions of the growing season) 

or xeric (most moisture is supplied in winter) moisture regime. Due to the 

minimal difference between summer and winter temperatures, there is little 

practical distinction between the ustic and xeric regimes for these soils. 

Because of natural rainfall, these soils are moist in most years for at least 90 

days (a period that usually is long enough for one rain-fed crop). Crops are 

not grown continuously because there is inadequate moisture for at least 90 

days in most years. The range of natural rainfall for Ustox allows two crops to 

be grown in some areas but only one crop in other areas, unless 

supplemental irrigation is available. Ustox may be the most extensive 

suborder, occurring over a large portion of the interior of South America and in 

extensive areas of Africa. They are also found in Hawaii, Puerto Rico, and the 

Pacific Basin. A few Ustox are in areas with a xeric soil moisture regime, such 

as in Australia. 

Ustox

Sombriustox

Acrustox

Eutrustox

Kandiustox

Haplustox
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-275

Profile of an Ustox (specifically a Eutrustox) in Rwanda.

This soil is found at cool, high elevations and has a 

thick, dark umbric epipedon over the red oxic 

horizon. Scale is in decimeters.

Back to Ustox
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-276

Key to Great Groups of Ustox (Back to key to suborders)

Ustox that have:

1. A sombric horizon (dark layer in which organic matter has 

accumulated) within a depth of 150 cm ------------------------ Sombriustox

Begin measuring depth below any O horizon.

2. An extremely low nutrient-holding capacity ----------------------- Acrustox

These soils have an effective cation-exchange capacity (ECEC) of < 1.50 

cmol (+) per kg clay and a pH value (1N KCl) of > 5.0 within a depth of 150 

cm. Begin measuring depth below any O horizon.

3. Moderate natural fertility ------------------------------------------------ Eutrustox

These soils have base saturation (by NH4OAc) of > 35% in all horizons 

within a depth of 125 cm. Begin measuring depth below any O horizon.

4. A kandic (very low cation-exchange capacity) horizon within a depth 

of 150 cm ------------------------------------------------------------------- Kandiustox

Begin measuring depth below any O horizon. These soils have < 10% 

weatherable minerals in the fine and very fine sand fraction of the kandic 

horizon, and there is a noticeable increase in clay from the clayey surface

layer to the subsoil.

5. Low natural fertility ------------------------------------------------------- Haplustox

These soils have base saturation (by NH4OAc) of < 35% in some horizon 

within a 125 cm and an effective cation-exchange capacity (ECEC) of 

> 1.50 cmol (+) per kg clay within a depth of 150 cm. Begin measuring 

depth below any O horizon.
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-277

Descriptions of Great Groups of Ustox

Sombriustox—These soils have a sombric subsoil horizon (dark layer in 

which organic matter has accumulated). They have an increase in content of 

organic carbon in the subsoil. The subsoil is easily mistaken for a buried 

surface layer. Commonly, this layer has andic soil properties or some 

properties of spodic materials. The only known pedons of Sombriustox are 

near the Rift Valley in Africa. (Back to Ustox key)

Acrustox—These soils have an extremely low cation-exchange capacity

(CEC), and some have a net anion-exchange capacity. As a result, these soils 

have very little capacity to retain base cations (positively charged plant 

nutrients) such as Ca2+, Mg2+, and K+. Because of a low buffering capacity, 

small but frequent applications of fertilizer and lime may be needed. However, 

because the low CEC also facilitates the downward movement of the cations 

into the soil, careful nutrient management can lead to the enhancement of a 

favorable rooting depth for crops. Supplemental irrigation is needed for 

continuous cropping. (Back to Ustox key)

Eutrustox—These soils have high base saturation throughout the profile. 

They are well known by local farmers because of their relatively high natural 

fertility. Although a significant proportion of the exchange sites are occupied 

by base cations (positively charged plant nutrients), the total CEC of these 

soils is still low. As a result, the initial supply of naturally occurring plant 

nutrients is rapidly depleted by cropping. Supplemental irrigation is needed for 

continuous cropping. Commonly, these soils supported natural forests while 

the surrounding areas that had simliar rainfall but low base saturation in the 

soil supported savannas. Currently, forest vegetation is rare because the 

forests have been completely cut by native farmers. The reason Ustox have 

high base saturation throughout their profile is not known, but these soils tend 

to occur over or near mafic (basic) rocks, such as limestone and basalt. (Back 

to Ustox key)

Kandiustox—These soils have a clayey surface layer and a kandic (very low 

cation-exchange capacity) subsoil horizon that has its upper boundary within a 

depth of 150 cm. There is a noticeable increase in clay content from the 

surface to the subsoil. They have, in all subhorizons of the kandic horizon 

within 150 cm of the mineral soil surface, an apparent ECEC of 1.50 or more 

cmol(+) per kg clay or a pH value (1N KCl) of less than 5.0. They also have 

base saturation (by NH4OAc) of less than 35% in some horizon within a depth 

of 125 cm. The subsoil has a moderate grade of blocky structure in most 

pedons. The epipedons are either dark- or light-colored. The subsoil shows 

evidence of translocated clay only in a few areas. In some pedons, it tends to 

have a weak or moderate grade of blocky structure. In most of the soils, 

however, there is a strong secondary structure that is fine granular. 
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-278

Supplemental irrigation is needed for continuous cropping. (Back to Ustox 

key)

Haplustox—These soils have little difference in clay content between the 

surface and subsoil horizons. They have, in all subhorizons of an oxic horizon 

within a depth of 150 cm, an apparent ECEC of 1.50 or more cmol(+) per kg 

clay or a pH value (1N KCl) of less than 5.0. They have base saturation (by 

NH4OAc) of less than 35% in some horizon within a depth of 125 cm. The 

subsoil has granular structure, and the epipedons may be either dark- or lightcolored. Haplustox are dark red to yellow and have all intervening colors in the 

subsoil. They occur in vast areas of central South America and Africa. 

Supplemental irrigation is needed for continuous cropping. (Back to Ustox 

key)
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-279

Perox Great Groups

Perox are the well drained Oxisols of climates where precipitation exceeds 

evaporation in most months. They have a perudic soil moisture regime. 

Clearing and burning are difficult in areas of these soils because of 

atmospheric wetness. Curing many seed crops and storing produce are also 

difficult. There are no large areas with a perudic soil moisture regime, but 

areas of this regime appear distinctive enough to show and identify on some 

small-scale soil maps.

Perox

Sombriperox

Acroperox

Eutroperox

Kandiperox

Haploperox
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-280

Perox soils are found on the highest mountains (in the background) of this 

Puerto Rican landscape. They receive much more precipitation than the 

soils in the foreground (which consist of Ustox).

Return to Perox
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-281

Key to Great Groups of Perox (Back to key to suborders)

Perox that have:

1. A sombric horizon (dark layer in which organic matter has 

accumulated) within a depth of 150 cm ------------------------ Sombriperox

Begin measuring depth below any O horizon.

2. An extremely low nutrient-holding capacity --------------------- Acroperox

These soils have an effective cation-exchange capacity (ECEC) of 

< 1.50 cmol (+) per kg clay and a pH value (1N KCl) of > 5.0 within a depth 

of 150 cm. Begin measuring depth below any O horizon.

3. Moderate natural fertility ---------------------------------------------- Eutroperox

These soils have base saturation (by NH4OAc) of > 35% in all horizons 

within a depth of 125 cm. Begin measuring depth below any O horizon.

4. A kandic (very low cation-exchange capacity) horizon within a depth 

of 150 cm ------------------------------------------------------------------- Kandiperox

Begin measuring depth below any O horizon. These soils have < 10% 

weatherable minerals in the fine and very fine sand fraction of the kandic 

horizon, and there is a noticeable increase in clay from the clayey surface 

layer to the subsoil.

5. Low natural fertility ----------------------------------------------------- Haploperox

These soils have base saturation (by NH4OAc) of < 35% in some horizon 

within a depth of 125 cm and an effective cation-exchange capacity 

(ECEC) of > 1.50 cmol (+) per kg clay within a depth of 150 cm. Begin 

measuring depth below any O horizon.
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-282

Descriptions of Great Groups of Perox

Sombriperox—These soils have a sombric subsoil horizon (dark layer in 

which organic matter has accumulated). They are not known to occur in the 

United States but are expected to be found in other parts of the world. (Back 

to Perox key)

Acroperox—These soils have an extremely low cation-exchange capacity

(CEC), and some have a net anion-exchange capacity. As a result, these soils 

have very little capacity to retain base cations (positively charged plant 

nutrients) such as Ca2+, Mg2+, and K+. (Back to Perox key)

Eutroperox—These soils have high base saturation throughout the profile. 

Although a significant proportion of the exchange sites are occupied by base 

cations (positively charged plant nutrients), the total CEC of these soils is still 

low. As a result, the initial supply of naturally occurring plant nutrients is 

rapidly depleted by cropping. (Back to Perox key)

Kandiperox—These soils have a clayey surface layer and a kandic (very low 

cation-exchange capacity) horizon that has its upper boundary within a depth 

of 150 cm. There is a noticeable increase in clay content from the surface to 

the subsoil. These soils have, in all subhorizons within a depth of 150 cm, an 

apparent ECEC of 1.50 or more cmol(+) per kg clay or a pH value (1N KCl) of 

less than 5.0. They also have base saturation (by NH4OAc) of less than 35% 

in some horizon within a depth of 125 cm. The subsoil has granular structure, 

and the epipedons may be either dark- or light-colored. (Back to Perox key)

Haploperox—These soils have little difference in clay content between the 

surface and subsoil layers. They have, in all subhorizons of an oxic horizon 

within a depth of 150 cm, an apparent ECEC of 1.50 or more cmol(+) per kg 

clay or a pH value (1N KCl) of less than 5.0. They also have base saturation 

(by NH4OAc) of less than 35% in some horizon within a depth of 125 cm of the 

mineral soil surface. The subsoil has granular structure, and the epipedons 

may be either dark- or light-colored. (Back to Perox key)
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-283

Udox Great Groups

Udox are the well drained Oxisols of areas with seasonally well-distributed 

precipitation. They have a udic soil moisture regime. They are moist because 

of natural rainfall in most years and are dry in some parts for less than 90 

days, a period that is short enough for rain-fed crops to be grown continuously 

in most years. There are fewer than 90 days during which crops are not 

planted. In local terms, there are 1 to 3 months that are considered dry in 

most years. Udox are an extensive suborder, occurring mostly in South 

America and in parts of Africa and Asia.

Udox

Sombriudox

Acrudox

Eutrudox

Kandiudox

Hapludox
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-284

Profile of a Udox in Puerto Rico. This soil is an Acrudox and 

therefore has a very low nutrient-holding capacity.

Return to Udox
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-285

Key to Great Groups of Udox (Back to key to suborders)

Udox that have:

1. A sombric horizon (dark layer in which organic matter has 

accumulated) within a depth of 150 cm ------------------------- Sombriudox

Begin measuring depth below any O horizon.

2. An extremely low nutrient-holding capacity ------------------------ Acrudox

These soils have an effective cation-exchange capacity (ECEC) of < 1.50 

cmol (+) per kg clay and a pH value (1N KCl) of > 5.0 within a depth of 150

cm. Begin measuring depth below any O horizon.

3. Moderate natural fertility ------------------------------------------------- Eutrudox

These soils have base saturation (by NH4OAc) of > 35% in all horizons to 

a depth of 125 cm. Begin measuring depth below any O horizon.

4. A kandic (very low cation-exchange capacity) horizon within a depth 

of 150 cm -------------------------------------------------------------------- Kandiudox

These soils have < 10% weatherable minerals in the fine and very fine 

sand fraction of the kandic horizon, and there is a noticeable increase in 

clay from the clayey surface layer to the subsoil. Begin measuring depth 

below any O horizon.

5. Low natural fertility -------------------------------------------------------- Hapludox

These soils have base saturation (by NH4OAc) of < 35% in some horizon 

within a depth of 125 cm and an effective cation-exchange capacity 

(ECEC) of > 1.50 cmol (+) per kg clay within a depth of 150 cm. Begin 

measuring depth below any O horizon.
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-286

Descriptions of Great Groups of Udox

Sombriudox—These soils have a sombric subsoil horizon (dark layer in 

which organic matter has accumulated). They are poorly understood. They 

have an increase in content of organic carbon in the subsoil. The only known 

pedons are near the Rift Valley in Africa. (Back to Udox key)

Acrudox—These soils have an extremely low cation-exchange capacity, and 

some have a net anion-exchange capacity. As a result, these soils have very 

little capacity to retain base cations (positively charged plant nutrients) such 

as Ca2+, Mg2+, and K+. Frequent but small applications of fertilizer and lime 

are required for agriculture. Because the low CEC also facilitates the 

downward movement of these cations into the soil, careful nutrient 

management can lead to the enhancement of a favorable rooting depth for 

crops. Also, because CEC is low, the amount of exchangeable aluminum in 

the subsoil is low. This deficiency can be corrected by leaching basic cations 

from lime and fertilizer. (Back to Udox key)

Eutrudox—These soils have high base saturation throughout the profile. 

Although a significant portion of the exchange sites are occupied by base 

cations (positively charged plant nutrients), the total CEC of these soils is still 

low. As a result, the initial supply of naturally occurring plant nutrients is

rapidly depleted by cropping. Eutrudox are highly valued by shifting cultivators 

and are most common in areas near mafic (basic) rock. (Back to Udox key)

Kandiudox—These soils have a clayey surface layer and a kandic (very low 

cation-exchange capacity) subsoil horizon that has its upper boundary within a 

depth of 150 cm. There is a noticeable increase in clay content from the 

surface layer to the subsoil. These soils have, in all subhorizons of a kandic 

horizon within a depth of 150 cm, an apparent ECEC of 1.50 or more cmol(+) 

per kg clay or a pH value (1N KCl) of less than 5.0. They also have base 

saturation (by NH4OAc) of less than 35% in some horizon within a depth of 

125 cm. The subsoil has a moderate grade of blocky structure in most 

pedons. The epipedons are either dark- or light-colored. (Back to Udox key)

Hapludox—These soils have little difference in clay content between the 

surface and subsoil layers. They have, in all subhorizons of an oxic horizon 

within a depth of 150 cm, an apparent ECEC of 1.50 or more cmol(+) per kg 

clay or a pH value (1N KCl) of less than 5.0. They also have base saturation 

(by NH4OAc) of less than 35% in some horizon within 125 cm of the mineral 

soil surface. The subsoil has granular structure, and the epipedons may be 

either dark- or light-colored. These soils commonly are acid, and the subsoil 

ranges from dark red to pale yellow. Hapludox are common in the uplands of 

Africa, the central part of Indonesia, and many other areas. (Back to Udox 

key)
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-287

Spodosols Order

Spodosols are acid soils with low fertility and accumulations of organic 

matter and iron and aluminum oxides in the subsoil.

General Characteristics

The dominant profile characteristic of Spodosols is a reddish or brownish 

spodic (accumulation of translocated organic matter in complex with aluminum

and also commonly iron) subsoil horizon. Commonly, there is an albic (lightcolored and leached) horizon above the spodic horizon. Spodosols tend to be 

quite acid throughout the profile. Some Spodosols have additional kinds of 

horizons below the spodic horizon, such as a root-restrictive fragipan (firm and 

brittle but not cemented layer) or a clay-enriched argillic horizon. A few 

Spodosols have an iron-cemented placic horizon. Most Spodosols are sandy, 

and a few are loamy.

Environment and Processes

Spodosols are most extensive in areas of cool, moist climates in the humid 

boreal zones. Most are well drained to somewhat poorly drained. A few 

Spodosols, however, are in warm tropical or sub-tropical regions, mostly in 

areas of quartz-rich sands that have a fluctuating level of ground water. 

Spodosols formed mostly in sandy or loamy parent materials under coniferous 

forest vegetation. Some are in hardwood forests, in tropical rainforests, or 

under palms. The critical processes leading to the formation of these soils 

include the production of organic acids from decaying leaf litter. These acids 

form organo-metallic complexes with iron and aluminum, which then move 

downward in the profile and accumulate in the subsoil. Spodosols are very

acid and low in native fertility. 

Location

Globally, Spodosols occupy about 3% of the ice-free land area. In North 

America, Spodosols are common in the Great Lakes region, New England, 

and eastern Canada. They are also found in the Pacific Northwest and 

southern Alaska as well as on the coastal plain of the southeastern United 

States. Spodosols are common in northern Europe and Russia and are also 

found in New Zealand and southern Australia.
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-288

Profile of a Spodosol.

(Back to Key to Soil Orders)
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-289

Spodosols by suborder in the United States. Note: This map was 

produced before the introduction of the Gelods suborder. Some 

areas shown as Cryods in Alaska are now Gelods.

Global distribution of Spodosols.
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-290

Spodosols Suborders

Classification of Spodosols

In the definitions of the suborders, emphasis is divided between soil wetness 

(Aquods), cold soil temperatures (Gelods and Cryods), spodic horizons 

dominated by illuvial accumulation of organic matter (Humods), and varying 

amounts of illuvial carbon, aluminum, and iron in the spodic horizons 

(Orthods). 

The great group level reflects a combination of important properties, including 

the presence of diagnostic horizons other than a spodic horizon, the presence 

of cemented layers, prevalence of either aluminum or carbon as illuvial 

constituents, cold soil temperature, and kinds of soil saturation. 

The five suborders are:

1. Aquods—wet Spodosols (aquic conditions in the upper part)

2. Gelods—very cold Spodosols that lack permafrost (gelic temperature 

regime)

3. Cryods—cold Spodosols (cryic temperature regime)

4. Humods—Spodosols with a large accumulation of carbon in the spodic 

horizon

5. Orthods—simple Spodosols with varying amounts of illuvial carbon, 

aluminum, and iron in the spodic horizon

Spodosols

Aquods

Gelods

Cryods

Humods

Orthods
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-291

Key to Suborders of Spodosols

Spodosols that have:

1. A seasonal high water table within a depth of 50 cm ------------- Aquods

Evidence of the water table may be a histic epipedon (wet, organic surface 

layer) and/or redoximorphic features (gray and red color patterns). See 

12th edition of “Keys to Soil Taxonomy” for specific criteria regarding 

location of redoximorphic features. Begin measuring depth below any O 

horizon. Artificially drained soils are included in Aquods.

2. Very cold average soil temperature ------------------------------------- Gelods

These soils have a gelic soil temperature regime.

3. Cold average soil temperature -------------------------------------------- Cryods

These soils have a cryic soil temperature regime.

4. High amounts of organic carbon in the subsoil ------------------- Humods

These soils have > 6.0% organic carbon in a layer > 10 cm thick within the 

spodic horizon.

5. A simple spodic horizon --------------------------------------------------- Orthods

These soils have varying amounts of illuvial carbon, aluminum, and iron in 

the spodic horizon but have < 6.0% organic carbon in all layers > 10 cm 

thick within the spodic horizon.
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-292

Aquods Great Groups

Aquods are the wet Spodosols. They are saturated close to the surface for a 

long enough period during the year to become devoid of oxygen (aquic 

conditions). Most of the soils have a nearly white albic (light-colored and 

leached) horizon thick enough to persist under cultivation or have, where they 

are the wettest, a black surface horizon resting on a dark reddish brown 

spodic (accumulation of translocated organic matter in complex with aluminum

and also commonly iron) subsoil horizon that is virtually free of iron. Other 

Aquods have a placic (cemented by iron and organic matter) horizon or a 

duripan (layer cemented by silica) or are cemented by an amorphous mixture 

of sesquioxides and organic matter. The Aquods that do not have a placic 

horizon typically have a transitional horizon between the albic horizon and the 

spodic horizon.

Aquods formed mainly in sandy materials of Pleistocene age. They may have 

any soil temperature regime. Water-loving plants of a very wide variety, 

ranging from sphagnum in cold areas to palms in the tropics, grow on these 

soils. In the United States, except in New Jersey and Florida, relatively few

Aquods are cultivated.

Aquods

Cryaquods

Alaquods

Fragiaquods

Placaquods

Duraquods

Epiaquods

Endoaquods
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-293

Profile of an Aquod (specifically an Endoaquod) in Florida. The spodic 

horizon (black) begins at a depth of about 62 cm.

Back to Aquods
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-294

Key to Great Groups of Aquods (Back to key to suborders)

Aquods that have: 

1. Cold average soil temperature --------------------------------------- Cryaquods

These soils have a cryic soil temperature regime.

2. A very low content of iron in the spodic (accumulation of 

translocated organic matter in complex with aluminum and also 

commonly iron) horizon -------------------------------------------------- Alaquods

These soils have < 0.10% iron (by ammonium oxalate) in > 75% of the 

spodic horizon. 

3. A fragipan (firm and brittle but not cemented layer) within a depth of 

100 cm --------------------------------------------------------------------- Fragiaquods

Begin measuring depth below any O horizon.

4. A placic (cemented by iron and organic matter) horizon within a 

depth of 100 cm ---------------------------------------------------------- Placaquods

Begin measuring depth below any O horizon. The placic horizon is present 

in > 50% of each pedon. By definition, a placic horizon in Spodosols is 

< 2.5 cm thick.

5. A cemented soil layer within a depth of 100 cm --------------- Duraquods

Begin measuring depth below any O horizon. The cemented layer is 

present in > 90% of each pedon. 

6. Episaturation (perched water table) -------------------------------- Epiaquods

7. Endosaturation (saturated throughout the profile) --------- Endoaquods
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-295

Descriptions of Great Groups of Aquods

Cryaquods—These soils occur in cold climates, mainly at high latitudes. 

Because they are both cold and wet, Cryaquods are generally precluded from 

cropping. Most are covered with coniferous forest or tundra vegetation. Aquic 

conditions (saturated and depleted of oxygen) persist during much of the 

growing season. In the United States, Cryaquods occur in Alaska. (Back to 

Aquods key)

Alaquods—These soils have a water table that fluctuates seasonally. During 

wet periods, iron is chemically reduced (thus making it mobile) and moved out 

of the soil profile. The spodic horizon consists mostly of an accumulation of 

organic matter and aluminum and commonly has few or no redoximorphic 

concentrations (reddish to black accumulations of iron-manganese oxides). 

The albic (light-colored and leached) horizon in the drier Alaquods is typically 

thick. The wettest Alaquods have no albic horizon but generally have 

uncoated sand grains above the spodic horizon. Alaquods typically have 

sandy texture. Because of their high humus content, however, some of them 

feel and behave like loamy soils. Alaquods occur primarily in the southeastern 

United States. (Back to Aquods key)

Fragiaquods—These soils have a fragipan (firm and brittle but not cemented 

layer) below the spodic (accumulation of translocated organic matter in 

complex with aluminum and also commonly iron) horizon, typically at a depth 

of 40 to 75 cm. Plant roots are shallow, and the soil clinging to the roots of 

tree-throws has in many areas formed a microrelief of 20 to 50 cm. Most of 

these soils are in fairly cold regions. Although these soils are nearly level, few 

of them are cultivated, except for those used for the production of hay. Most 

are forested. Fragiaquods are not extensive in the United States. (Back to 

Aquods key)

Placaquods—These soils have a thin (< 2.5 cm) placic (cemented by iron 

and organic matter) horizon. The placic horizon commonly has a very irregular 

(involute) boundary. The horizons above the placic horizon have low-contrast 

mottles that are probably caused by organic matter. Placaquods are known to 

occur only in areas of perhumid oceanic climates. Many have, or used to 

have, sphagnum and heath vegetation. (Back to Aquods key)

Duraquods—These soils have a cemented soil layer that in many areas is a 

combination of iron and/or aluminum and organic matter. In some of these 

soils, silica also is a cementing agent. Duraquods are seasonally saturated 

above the cemented soil layer but not necessarily below it. In the United 

States, these soils occur mostly in the Pacific Northwest and in the Lakes 

States region. They also are known to occur in New Zealand, in areas of kauri 

trees. The cemented soil layer in Duraquods severely restricts plant rooting. 

Currently, few of these soils are recognized worldwide. (Back to Aquods key)
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-296

Epiaquods—These soils have a perched water table above one or more 

relatively impermeable layers. They typically have a considerable 

accumulation of iron in addition to aluminum and organic matter. The relatively 

impermeable layers commonly are argillic (clay accumulation) subsoil 

horizons or dense, compacted glacial till. (Back to Aquods key)

Endoaquods—These soils are saturated throughout the soil profile. They 

have a considerable accumulation of iron, in addition to aluminum and organic 

carbon, in the spodic horizon. The spodic horizon has few to many 

redoximorphic features (gray and red color patterns) and commonly is brown 

to dark reddish brown. Most Endoaquods have an albic (light-colored and

leached) horizon. Some of the wettest do not have an albic horizon but 

commonly have uncoated sand grains above the spodic horizon. (Back to 

Aquods key)
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-297

Gelods Great Groups

Gelods are the Spodosols of very cold, high-latitude regions. They have a 

gelic temperature regime but lack permafrost within the profile. These soils 

tend to be in landscape positions that are subject to wide variations in summer 

high temperatures and winter low temperatures, which result in mean annual 

soil temperatures of 0 degrees C or less, but without permafrost.

Gelods

Humigelods

Haplogelods
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-298

Key to Great Groups of Gelods (Back to key to suborders)

Gelods that have:

1. High amounts of organic carbon in the subsoil -------------- Humigelods

These soils have > 6.0% organic carbon in a layer > 10 cm thick within the 

spodic horizon.

2. A simple spodic horizon --------------------------------------------- Haplogelods
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-299

Descriptions of Great Groups of Gelods

Humigelods—These soils commonly have an O horizon over a very thin or 

intermittent albic (light-colored and leached) horizon, which overlies a darkcolored spodic (accumulation of translocated organic matter in complex with 

aluminum and also commonly iron) subsoil horizon. They have a high content 

of organic carbon in the upper part of the spodic horizon. The vegetation 

commonly is coniferous forest or alpine tundra. Humigelods are not extensive. 

In the United States, they occur in Alaska. (Back to Gelods key)

Haplogelods—These soils are mostly well drained or excessively drained. 

Most have high contents of rock fragments in the profile. They commonly have

a thin ochric (typically thin and/or light-colored) epipedon, and possibly an 

albic (light-colored and leached) horizon, over a spodic (accumulation of 

translocated organic matter in complex with aluminum and also commonly 

iron) subsoil horizon. The vegetation commonly is coniferous forest or alpine 

tundra. Haplogelods are not extensive. In the United States, they occur in 

Alaska. (Back to Gelods key)
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-300

Cryods Great Groups

Cryods are Spodosols of high latitudes and/or high elevations. In the United 

States, they occur mostly in southeast Alaska and in the mountains of 

Washington, Oregon, and northern Idaho. Some are in the mountains of New 

York and northern New England. Many Cryods formed in volcanic ash or 

glacial drift, and some formed in residuum or colluvium on mountain slopes. 

Cryods commonly have an O horizon over a very thin or intermittent albic

(light-colored and leached) horizon, which overlies a well developed spodic 

(accumulation of translocated organic matter in complex with aluminum and 

also commonly iron) subsoil horizon. Some of the soils have a thin placic 

(cemented by iron and organic matter) horizon or another cemented soil layer 

within a depth of 100 cm. In many Cryods, the content of organic carbon in the 

upper part of the spodic horizon is relatively high. The vegetation is mostly 

coniferous forest or alpine tundra.

Cryods

Placocryods

Duricryods

Humicryods

Haplocryods
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-301

Profile of a Cryod (specifically a Haplocryod) in Alaska. 

This soil formed in glacial till that is only about 200 

years old. The thin albic and spodic horizons are within 

the upper 1 foot of the profile. Scale is in feet.

Back to Cryods
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-302

Key to Great Groups of Cryods (Back to key to suborders)

Cryods that have:

1. A placic (cemented by iron and organic matter) horizon within a 

depth of 100 cm --------------------------------------------------------- Placocryods

Begin measuring depth below any O horizon. The placic horizon is present 

in > 50% of each pedon. By definition, a placic horizon in Spodosols is 

< 2.5 cm thick.

2. A cemented soil layer within a depth of 100 cm --------------- Duricryods

Begin measuring depth below any O horizon. The cemented layer is 

present in > 90% of each pedon. 

3. High amounts of organic carbon in the subsoil -------------- Humicryods

These soils have > 6.0% organic carbon in a layer > 10 cm thick within the 

spodic horizon.

4. A simple spodic horizon --------------------------------------------- Haplocryods
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-303

Descriptions of Great Groups of Cryods

Placocryods—These soils have a thin (< 2.5 cm) placic (cemented by iron 

and organic matter) horizon, typically within the spodic (accumulation of 

translocated organic matter in complex with aluminum and also commonly 

iron) subsoil horizon. In the United States, they are known to occur only in 

Alaska. They are thought to be rare elsewhere in the world. (Back to Cryods 

key)

Duricryods—These soils have a cemented horizon either within or below the 

spodic (accumulation of translocated organic matter in complex with aluminum

and also commonly iron) subsoil horizon. The cemented horizon is considered 

to be ortstein if it is more than 2.5 cm thick and part of the spodic horizon. In 

the United States, these soils are known to occur in the State of Washington. 

They probably also occur in other cold regions of the world. (Back to Cryods 

key)

Humicryods—These soils commonly have a thick O horizon over a very thin 

or intermittent albic (light-colored and leached) horizon, which overlies a darkcolored spodic (accumulation of translocated organic matter in complex with 

aluminum and also commonly iron) subsoil horizon. They have a high content 

of organic carbon in the upper part of the spodic horizon. The vegetation 

commonly is coniferous forest or alpine tundra. In the United States, 

Humicryods occur mostly in southeast Alaska and in the mountains of 

Washington and Oregon. Some are in the higher mountains of the Northeast. 

(Back to Cryods key)

Haplocryods—These soils have horizons that are mostly thin but may be 

strongly contrasting. The base of the spodic (accumulation of translocated 

organic matter in complex with aluminum and also commonly iron) horizon is 

generally less than 50 cm below the mineral soil surface. Some Haplocryods 

have permafrost at varying depths below the spodic horizon, between 100 and 

200 cm. Others have, below the spodic horizon, another sequum with an 

argillic (clay accumulation) or kandic (very low cation-exchange capacity) 

subsoil horizon. In the United States, Haplocryods occur mostly in Alaska and 

in the higher mountains of the West and Northeast. (Back to Cryods key)
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-304

Humods Great Groups

Humods are the relatively freely drained Spodosols that have a large 

accumulation of organic carbon in the spodic (accumulation of translocated 

organic matter in complex with aluminum and also commonly iron) subsoil 

horizon. Undisturbed Humods may have either a thin, intermittent or a distinct, 

continuous albic (light-colored and leached) horizon over a spodic horizon, 

which in its upper part is nearly black and has reddish hue. The hue typically 

becomes yellower with increasing depth.

Humods formed predominantly in Pleistocene or Holocene deposits. In the 

United States, they developed mainly under a coniferous forest. In Western 

Europe, they are common in areas of sandy materials where heather (Calluna 

vulgaris) is, or used to be, a dominant plant. In tropical regions, most Humods 

supported rainforests.

Humods are not extensive in the United States. They are known to occur in 

northern New England and in the Pacific Northwest, mostly in small areas.

Humods

Placohumods

Durihumods

Fragihumods

Haplohumods
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-305

Profile of a Humod (specifically a Haplohumod). 

This soil has a thick albic horizon (white) 

underlain at a depth of about 70 cm by a 

spodic horizon with a high content of organic 

carbon. Scale is in cm.

Back to Humods
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-306

Key to Great Groups of Humods (Back to key to suborders)

Humods that have:

1. A placic (cemented by iron and organic matter) horizon within a 

depth of 100 cm ------------------------------------------------------- Placohumods

Begin measuring depth below any O horizon. The placic horizon is present 

in > 50% of each pedon. By definition, a placic horizon in Spodosols is

< 2.5 cm thick.

2. A cemented soil layer within a depth of 100 cm ------------- Durihumods

Begin measuring depth below any O horizon. The cemented layer is 

present in > 90% of each pedon. 

3. A fragipan (firm and brittle but not cemented layer) within a depth of 

100 cm -------------------------------------------------------------------- Fragihumods

Begin measuring depth below any O horizon.

4. A favorable rooting depth of > 100 cm ----------------------- Haplohumods
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Descriptions of Great Groups of Humods

Placohumods—These soils have a thin (< 2.5 cm) placic (cemented by iron 

and organic matter) horizon within the spodic (accumulation of translocated 

organic matter in complex with aluminum and also commonly iron) subsoil 

horizon. They are probably rare in the world but are known to occur in eastern 

Canada and in parts of Western Europe, where their vegetation is, or used to 

be, heather. (Back to Humods key)

Durihumods—These soils have a layer that is cemented, commonly with 

sesquioxides and organic matter. The cemented horizon is considered to be 

ortstein if it is more than 2.5 cm thick and part of the spodic horizon. Some 

Durihumods have a duripan (layer cemented by silica). Durihumods are not 

common in the United States but occur in some areas in the Pacific Northwest 

and the Southeast. (Back to Humods key)

Fragihumods—These soils have a fragipan (firm and brittle but not cemented 

layer) below the spodic (accumulation of translocated organic matter in 

complex with aluminum and also commonly iron) horizon. They are not known 

to occur in the United States. (Back to Humods key)

Haplohumods—These soils have a favorable rooting depth (no rootrestrictive layers) to > 100 cm. They are commonly in cool to warm, moist 

coastal regions, although some are in inland areas at higher elevations. In the 

United States, these soils occur in the Northeast and the Pacific Northwest. 

They are also in the southeastern United States, particularly in areas of sandy 

deposits. In Western Europe, most of these soils at some time supported 

heather (Calluna vulgaris). If undisturbed, these soils generally have an 

umbric (humus-rich with low base saturation) epipedon and an albic (lightcolored and leached) horizon overlying a spodic (accumulation of translocated 

organic matter in complex with aluminum and also commonly iron) subsoil 

horizon with reddish hue and a black upper subhorizon rich in organic carbon. 

Some of these soils, however, have an ochric (typically thin and/or lightcolored) epipedon. (Back to Humods key)
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Orthods Great Groups

Orthods are the relatively freely drained Spodosols that have a spodic 

(accumulation of translocated organic matter in complex with aluminum and 

also commonly iron) subsoil horizon. These are the most common Spodosols 

in the northern parts of Europe and in the United States. They formed 

predominantly in coarse, acid Pleistocene or Holocene deposits under mostly 

coniferous forest vegetation. If undisturbed, Orthods typically have an O 

horizon, an albic (light-colored and leached) horizon, and a spodic subsoil 

horizon and may have a fragipan (firm and brittle but not cemented layer). 

Some of these soils, however, have been mixed by the roots of fallen trees or 

by animals and have a very thin albic horizon or no albic horizon. In cultivated 

areas, the albic horizon is commonly mixed with part of the spodic horizon. In 

the United Sates, Orthods are extensive in both the southeastern and 

northeastern parts, in the Great Lakes States, and in the mountains of the 

West.

Orthods

Placorthods

Durorthods

Fragiorthods

Alorthods

Haplorthods
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Profile of an Orthod (specifically a Haplorthod). The 

albic horizon (white) extends from a depth of about

1 to 5 inches and is underlain by a reddish spodic 

horizon that extends to a depth of about 11 inches. 

Scale is in inches.

Back to Orthods
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Key to Great Groups of Orthods (Back to key to suborders)

Orthods that have:

1. A placic (cemented by iron and organic matter) horizon within a 

depth of 100 cm --------------------------------------------------------- Placorthods

Begin measuring depth below any O horizon. The placic horizon is present

in > 50% of each pedon. By definition, a placic horizon in Spodosols is 

< 2.5 cm thick.

2. A cemented soil layer within a depth of 100 cm --------------- Durorthods

Begin measuring depth below any O horizon. The cemented layer is 

present in > 90% of each pedon. 

3. A fragipan (firm and brittle but not cemented layer) within a depth of 

100 cm --------------------------------------------------------------------- Fragiorthods

Begin measuring depth below any O horizon.

4. A very low content of iron in the spodic (accumulation of 

translocated organic matter in complex with aluminum and also 

commonly iron) horizon ------------------------------------------------- Alorthods

These soils have < 0.10% iron (by ammonium oxalate) in > 75% of the 

spodic horizon. 

5. A simple spodic horizon and a favorable rooting 

depth ------------------------------------------------------------------------ Haplorthods
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Descriptions of Great Groups of Orthods

Placorthods—These soils have a thin (< 2.5 cm) placic (cemented by iron 

and organic matter) horizon within the spodic (accumulation of translocated 

organic matter in complex with aluminum and also commonly iron) subsoil 

horizon. They are not known to occur in the continental United States and are 

thought to be rare elsewhere in the world. (Back to Orthods key)

Durorthods—These soils have a layer that is cemented, commonly with 

combinations of iron, aluminum, and organic matter. The cemented horizon is 

considered to be ortstein if it is more than 2.5 cm thick and part of the spodic 

horizon. Some Durorthods may have a duripan (layer cemented by silica). 

(Back to Orthods key)

Fragiorthods—These soils have a fragipan (firm and brittle but not cemented 

layer) below the spodic (accumulation of translocated organic matter in 

complex with aluminum and also commonly iron) subsoil horizon. They formed 

mainly in loamy deposits, but some formed in sandy deposits containing a 

considerable amount of fine and very fine sand. The fragipan may be thick, or 

it may have formed in the eluvial (E ́) horizon of a lower sequum that overlies 

an argillic (clay accumulation) subsoil horizon. Generally, a lighter-colored 

eluvial horizon has developed above the fragipan, presumably because of the 

lateral movement of ground water. Trees growing on Fragiorthods are typically 

shallow-rooted and are sometimes blown down. When the trees are blown 

down, there is a mixing of soil horizons above the fragipan. As a 

consequence, many of these soils have a weakly expressed albic (lightcolored and leached) horizon or no albic horizon or have a thin, intermittent 

albic horizon below part of the spodic horizon. A distinct microrelief is a normal 

feature, unless the soils have been smoothed by cultivation. In the United 

States, Fragiorthods are mostly in cold climates in the northern part of the 

country. (Back to Orthods key)

Alorthods—These soils have accumulations of aluminum that occur in

relatively high amounts compared to accumulations of iron. They formed 

predominantly in sandy deposits. They have low amounts of iron 

accumulations either because of intensive leaching or because of parent 

materials that had a low iron content. Alorthods typically have a thick albic 

(light-colored and leached) horizon and an ochric (typically thin and/or lightcolored) epipedon. They are more common in areas of warm climates than in 

cool environments. In the United States, they occur mainly in the Southeast. 

(Back to Orthods key)

Haplorthods—These are the relatively freely drained Orthods that either have 

an albic (light-colored and leached) horizon and a spodic (accumulation of 

translocated organic matter in complex with aluminum and also commonly 

iron) subsoil horizon or, under cultivation, commonly have only a spodic 
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horizon below an Ap horizon. The spodic horizon may rest on a lower sequum 

with an argillic (clay accumulation) subsoil horizon or kandic (very low cationexchange capacity) subsoil horizon over relatively unaltered unconsolidated 

materials or on rock. Most Haplorthods have, or used to have, forest 

vegetation, mainly conifers but also hardwoods in some areas. A majority of 

these soils formed in sandy deposits or in materials weathered from 

sandstone or quartzite. (Back to Orthods key)
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Ultisols Order

Ultisols are soils with low base-status and a clay-enriched subsoil. 

General Characteristics

Typically, Ultisols have a surface layer consisting of an ochric (typically thin 

and/or light-colored) epipedon underlain by a clay-enriched subsoil. The 

subsoil is either an argillic (clay accumulation) horizon or, where the cationexchange capacity is very low, a kandic horizon. The subsoil is commonly 

reddish in color due to coatings of iron oxides on mineral grains. Between the 

surface horizon and subsoil there is commonly a lighter-colored zone of 

leaching. All Ultisols have low base saturation, which makes them low in 

native fertility. A few wet Ultisols have an umbric (humus-rich with low base 

saturation) epipedon.

Environment and Processes

Ultisols mostly formed in warm, humid climates under forest vegetation. 

Commonly, they are in areas where there is a moisture deficit part of the year 

but where overall annual precipitation exceeds evaporation, conditions that 

result in a net leaching environment. Ultisols form in a wide range of parent 

materials, but generally the materials are acidic and infertile and do not 

provide a large amount of bases as they weather. Ultisols commonly are on 

relatively old landscapes that have undergone significant weathering so that 

the clay fraction in the subsoil tends to be dominated by low-activity minerals,

such as kaolinite and iron and aluminum oxides. The fertility level of Ultisols is 

low, and plant nutrients are typically concentrated in the upper part of the 

profile (due to biocycling) and decrease in amount with increasing depth. The 

number of bases released by weathering is typically equal to or less than the 

number removed by leaching, and most of the bases are held in the 

vegetation and the upper soil horizons. 

Location

Globally, Ultisols occupy about 8% of the ice-free land area. In the United 

States, Ultisols are mostly located in the Southeast, in the Piedmont, 

Appalachian Plateau, Ridge and Valley, and Coastal Plain physiographic 

regions, but also occur in the Pacific Northwest and in central California. 

Ultisols are also found in Southeast Asia, the Upper Amazon Basin in South 

America, and the Congo Basin in central Africa.
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Profile of an Ultisol.

(Back to Key to Soil Orders)
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Ultisols by suborder in the United States.

Global distribution of Ultisols.
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Ultisols Suborders

Classification of Ultisols

In the definitions of the suborders, emphasis is placed mostly on soil climate 

in the form of soil wetness (Aquults) and soil moisture regime (Udults, Ustults, 

and Xerults). One suborder (Humults) emphasizes the accumulation of 

organic carbon in the soil. 

The great group level reflects a combination of important properties, including 

the presence of diagnostic horizons other than an argillic (clay accumulation) 

subsoil horizon and ochric (typically thin and/or light-colored) epipedon, an 

abrupt texture change to a slowly permeable layer, dark red colors, significant 

levels of plinthite (firm, iron oxide-rich concentration) in the subsoil, patterns of 

soil saturation, and a morphology that reflects advanced soil development on 

stable landforms.

The five suborders are:

1. Aquults—wet Ultisols (aquic conditions in the upper part)

2. Humults—Ultisols with a high amount of organic carbon accumulations in 

the subsoil

3. Udults—Ultisols of humid regions with well-distributed rainfall

4. Ustults—moderately dry Ultisols (limited moisture)

5. Xerults—moderately dry Ultisols (limited moisture that is supplied in winter

and a Mediterranean-type climate)

Ultisols

Aquults

Humults

Udults

Ustults

Xerults
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Key to Suborders of Ultisols

Ultisols that have:

1. A seasonal high water table within a depth of 50 cm ------------- Aquults

Evidence of a water table includes redoximorphic features (gray and red 

color patterns). See 12th edition of “Keys to Soil Taxonomy” for criteria 

regarding amount, color, and location of redoximorphic features. Begin 

measuring depth below any O horizon. Artificially drained sites are 

included in Aquults.

2. High amounts of organic carbon in the subsoil ------------------- Humults

These soils have > 0.9% organic carbon in the upper 15 cm of the subsoil,

or they have > 12 kg/m2in the upper 100 cm of the profile.

3. Seasonally well-distributed precipitation ------------------------------- Udults

These soils have a udic soil moisture regime.

4. Somewhat limited soil moisture available for plant growth ----- Ustults

These soils have an ustic soil moisture regime. Moisture is limited, but 

available, during portions of the growing season.

5. A Mediterranean-type climate --------------------------------------------- Xerults

These soils have a xeric soil moisture regime (cool and moist in winter and 

warm and dry in summer).
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Aquults Great Groups

Aquults are the Ultisols in wet areas where ground water is very close to the 

surface during part of each year, usually in winter and spring at middle 

latitudes, and is frequently deep at other times. Saturation persists long 

enough during the year for the soil to become devoid of oxygen in the upper 

part (aquic conditions). These soils are mostly grayish or olive in the subsoil 

and formed mainly in alluvium and marine deposits that are Pleistocene in age 

or older. Aquults are extensive on the coastal plains in the United States, 

particularly along the Atlantic Ocean and the Gulf of Mexico. Their slopes are 

gentle. Most of these soils had, and many still have, forest vegetation. Aquults 

have an ochric (typically thin and/or light-colored) or umbric (humus-rich with 

low base saturation) epipedon and an argillic (clay accumulation) or kandic 

(very low cation-exchange capacity) subsoil horizon. Some have a fragipan 

(firm and brittle but not cemented layer), and others have plinthite (firm, iron 

oxide-rich concentration) in or below the argillic or kandic horizon.

Aquults

Plinthaquults

Fragiaquults

Albaquults

Kandiaquults

Kanhaplaquults

Paleaquults

Umbraquults

Epiaquults

Endoaquults
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Profile of an Aquult (specifically a Plinthaquult) 

in Venezuela. Some of the red bodies in the 

subsoil are plinthite. Scale is in centimeters.

Back to Aquults
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Key to Great Groups of Aquults (Back to key to suborders)

Aquults that have:

1. Plinthite making up > 50% of a layer within a depth of 

150 cm -------------------------------------------------------------------- Plinthaquults

Begin measuring depth below any O horizon. Plinthite is a firm, iron oxiderich concentration that irreversibly hardens after exposure to repeated wetdry cycles.

2. A fragipan (firm and brittle but not cemented layer) within a depth of 

100 cm --------------------------------------------------------------------- Fragiaquults

Begin measuring depth below any O horizon.

3. An abrupt increase in clay content to the subsoil 

horizon ----------------------------------------------------------------------- Albaquults

See “abrupt texture change” definition for specific criteria. Also, the Ksat

class must be moderately low or lower in the subsoil (most likely to be met 

if the texture is sandy clay, silty clay, or clay). Albaquults may have either 

an argillic or kandic subsoil horizon.

4. A kandic (very low cation-exchange capacity) horizon and a clay

content that does not decrease significantly within a depth of 

150 cm -------------------------------------------------------------------- Kandiaquults

Clay content must not decrease by > 20% of the maximum in the kandic

horizon, unless clay depletions (gray areas where clay and iron have been 

lost) are present in the layer and the clay content increases again by > 3% 

below. Kandiaquults cannot have a root-limiting layer within a depth of 150 

cm. Begin measuring depth below any O horizon.

5. A kandic (very low cation-exchange capacity) horizon and a

significant decrease in clay content within a depth of 

150 cm ----------------------------------------------------------------- Kanhaplaquults

Begin measuring depth below any O horizon.

6. A clay content that does not decrease significantly within a depth of 

150 cm ---------------------------------------------------------------------- Paleaquults

Clay content must not decrease by > 20% of the maximum in the argillic

horizon, unless clay depletions (gray areas where clay and iron have been 

lost) are present in the layer and the clay content increases again by > 3% 

below. Paleaquults cannot have a root-limiting layer within a depth of 150 

cm. Begin measuring depth below any O horizon.

7. A thick, dark-colored, humus-rich, surface horizon ------- Umbraquults

These soils have either a mollic (rich in humus and bases) or umbric 

(humus-rich with low base saturation) epipedon.
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8. Episaturation (perched water table) -------------------------------- Epiaquults

9. Endosaturation (saturated throughout the profile) --------- Endoaquults
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Descriptions of Great Groups of Aquults

Plinthaquults—These soils have plinthite (firm, iron oxide-rich concentration) 

that either forms a continuous phase or constitutes more than half the matrix 

of some subhorizon within a depth of 150 cm. These soils are mostly in 

intertropical areas and constitute a small area of Puerto Rico. (Back to Aquults 

key)

Fragiaquults—These soils have a fragipan (firm and brittle but not cemented 

layer) with an upper boundary within a depth of 100 cm. Normally, the 

fragipan lies below the argillic (clay accumulation) or kandic (very low cationexchange capacity) horizon. In a few of the soils, however, it may be in the 

lower part of the argillic or kandic subsoil horizon. (Back to Aquults key)

Albaquults—These soils have a marked increase in percentage of clay at an 

abrupt boundary at the top of the argillic (clay accumulation) or kandic (very 

low cation-exchange capacity) subsoil horizon. The subsoil horizon generally 

is clayey and has moderately low or lower hydraulic conductivity. In most 

years water is perched for short duration above the argillic or kandic horizon 

prior to complete soil saturation. Slopes are nearly level, and draining the soils 

is difficult. In the southeastern United States, these soils formed mostly in 

acid, late-Pleistocene sediments and are on low marine or stream terraces. 

Most have a season when the upper horizons are dry. Some of the Albaquults 

in the United States have been cleared for grazing or cropping, but many are 

forested. (Back to Aquults key)

Kandiaquults—These soils have a kandic (very low cation-exchange 

capacity) horizon and a clay distribution in which the percentage of clay does 

not decrease from its maximum amount by as much as 20 percent within a 

depth of 150 cm. Kandiaquults are not known to occur in the United States. 

The great group is provided for use in other parts of the world. (Back to 

Aquults key)

Kanhaplaquults—These soils have a kandic (very low cation-exchange 

capacity) horizon in which clay content decreases significantly within a depth 

of 150 cm. They have a thin or moderately thick zone of maximum clay 

content. Kanhaplaquults are not known to occur in the United States. The 

great group is provided for use in other parts of the world. (Back to Aquults 

key)

Paleaquults—These soils have an argillic (clay accumulation) subsoil horizon

and a percentage of clay that does not decrease from its maximum amount by 

as much as 20 percent within a depth of 150 cm. Paleaquults formed mostly 

on mid-Pleistocene or older land surfaces, on high marine or river terraces, or

on old deltas. They are of large extent in the southeastern United States. The 
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natural vegetation consisted of forest plants, mostly water-tolerant conifers or 

hardwood trees. (Back to Aquults key)

Umbraquults—These soils commonly have an umbric (humus-rich with low 

base saturation) epipedon, but a few with high base-status in the upper part of 

the soil may have a mollic (rich in humus and bases) epipedon. Umbraquults 

are among the wettest of the Aquults, and their argillic (clay accumulation) 

subsoil horizon is not as strongly developed as that of other Aquults. 

Umbraquults have a thin or moderately thick zone of maximum clay content. 

The natural vegetation consisted mostly of water-tolerant trees and herbs 

(hydrophytic vegetation). (Back to Aquults key)

Epiaquults—These soils have water perched on a less permeable layer, 

commonly an argillic subsoil horizon. Slopes generally are nearly level. Before 

cultivation, most Epiaquults supported either deciduous broadleaf or 

coniferous forest. (Back to Aquults key)

Endoaquults—These soils have a ground-water level that fluctuates from the 

soil surface to depths that may exceed 2 meters. Generally, they are nearly 

level. Before cultivation, most Endoaquults supported either deciduous 

broadleaf or coniferous forest plants. (Back to Aquults key)
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Humults Great Groups

Humults are the freely drained, humus-rich Ultisols of mid or low latitudes. At 

mid latitudes they are mostly dark colored, but at low latitudes their content of 

humus is not necessarily reflected by color. These soils are mainly in 

mountainous areas that have high amounts of rainfall but also have a 

moisture deficiency during some season. Most of the Humults in the United 

States formed in basic rock on surfaces that are late Pleistocene in age or 

older. Slopes commonly are strong. If the soils are cultivated, the subsoil 

horizon may be exposed at the surface due to erosion. To keep the eroded 

and uneroded soils in the same classification, the suborder criteria were

written in terms of the carbon content of the whole soil or the argillic (clay 

accumulation) or kandic (very low cation-exchange capacity) subsoil horizon. 

The natural vegetation was mostly coniferous forest plants at mid latitudes 

and rainforests at low latitudes.

Humults

Sombrihumults

Plinthohumults

Kandihumults

Kanhaplohumults

Palehumults

Haplohumults
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Profile of a Humult (specifically a Palehumult) in Brazil. This soil has a 

thick argillic subsoil horizon with a relatively high content of organic 

carbon.

Back to Humults
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Key to Great Groups of Humults (Back to key to suborders)

Humults that have:

1. A sombric subsoil horizon (dark layer in which organic matter has 

accumulated) within a depth of 100 cm -------------------- Sombrihumults

Begin measuring depth below any O horizon.

2. Plinthite making up > 50% of a layer within a depth of 

150 cm ----------------------------------------------------------------- Plinthohumults

Begin measuring depth below any O horizon. Plinthite is a firm, iron oxiderich concentration that irreversibly hardens after exposure to repeated wetdry cycles.

3. A kandic (very low cation-exchange capacity) horizon and a clay 

content that does not decrease significantly within a depth of 

150 cm ------------------------------------------------------------------- Kandihumults

Clay content must not decrease by > 20% of the maximum in the kandic

horizon, unless skeletans (ped surfaces stripped of clay) are present in the 

layer and the clay content increases again by > 3% below. Kandihumults 

cannot have a root-limiting layer within a depth of 150 cm. Begin 

measuring depth below any O horizon.

4. A kandic (very low cation-exchange capacity) horizon and a 

significant decrease in clay content within a depth of 

150 cm -------------------------------------------------------------- Kanhaplohumults

Begin measuring depth below any O horizon.

5. An argillic (clay accumulation) subsoil horizon and a clay 

content that does not decrease significantly within a depth of 

150 cm --------------------------------------------------------------------- Palehumults

Clay content must not decrease by > 20% of the maximum in the argillic

horizon, unless skeletans (ped surfaces stripped of clay) are present in the 

layer and the clay content increases again by > 3% below. Palehumults 

cannot have a root-limiting layer within a depth of 150 cm. Begin 

measuring depth below any O horizon.

6. An argillic (clay accumulation) horizon and a significant decrease in 

clay content within a depth of 150 cm ------------------------ Haplohumults

Begin measuring depth below any O horizon.
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Descriptions of Great Groups of Humults

Sombrihumults—These soils have a sombric subsoil horizon (dark layer in 

which organic matter has accumulated) that has its upper boundary within a 

depth of 100 cm. They are not known to occur in the United States. The great 

group is provided for use in other parts of the world. (Back to Humults key)

Plinthohumults—These soils have plinthite (firm, iron oxide-rich 

concentration) that forms a continuous phase in, or constitutes more than half 

the volume of, one or more horizons within a depth of 150 cm. They are not 

known to occur in the United States. The great group has been provided for 

use in other parts of the world. (Back to Humults key)

Kandihumults—These soils have a kandic (very low cation-exchange

capacity) subsoil horizon and a clay distribution in which the percentage of 

clay does not decrease from its maximum amount by as much as 20% within 

a depth of 150 cm. In the United States, a small extent of Kandihumults 

occurs in California and Hawaii. (Back to Humults key)

Kanhaplohumults—These soils have a kandic (very low cation-exchange

capacity) subsoil horizon and have a clay distribution in which the percentage 

of clay decreases significantly within a depth of 150 cm. They have a thin to 

moderately thick zone of maximum clay content. Kanhaplohumults are of very 

small extent worldwide. In the United States, they are known to occur only in 

California and Hawaii. (Back to Humults key)

Palehumults—These soils are on old stable surfaces. In these soils, the

percentage of clay does not decrease from its maximum amount by as much 

as 20% within a depth of 150 cm or the layer in which the clay percentage 

decreases has at least 5% of the volume consisting of skeletans (ped 

surfaces stripped of clay) and there is at least a 3% increase in clay content 

below this layer. These soils have an argillic (clay accumulation) subsoil 

horizon. They are moderately extensive and occur mostly in California, 

western Oregon and Washington, and Hawaii. Like other Humults in the 

United States, they formed mostly in material weathered from basic rocks or in 

alluvium derived from basic rocks. Most Palehumults have had forest 

vegetation, and many still have forest vegetation. (Back to Humults key)

Haplohumults—These soils have an argillic (clay accumulation) subsoil 

horizon with a significant decrease in clay content within a depth of 150 cm.

They have a thin to moderately thick zone of maximum clay content. The 

Haplohumults in the United States are mainly in mountains close to the Pacific 

Ocean and have gentle to very steep slopes. Most Haplohumults have 

coniferous forest vegetation. Their content of organic carbon commonly is 

high relative to that of most other kinds of soil. Some of these soils have an 

umbric (humus-rich with low base saturation) epipedon, but many have a color 
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value and chroma higher than the value and chroma of umbric epipedons. 

(Back to Humults key)
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Udults Great Groups

Udults are the more or less freely drained, humus-poor Ultisols in humid 

areas with seasonally well-distributed precipitation (udic moisture regime). 

Most have light-colored upper horizons and commonly a grayish horizon that 

rests on a yellowish brown to reddish argillic (clay accumulation) or kandic 

(very low cation-exchange capacity) subsoil horizon. A few that developed 

from basic rocks have a dark brown or reddish brown surface horizon that 

rests on a dark red or dusky red argillic or kandic horizon. Some have a 

fragipan (firm and brittle but not cemented layer) or plinthite (firm, iron oxiderich concentration), or both, in or below the argillic or kandic horizon. Udults 

developed in sediments and on surfaces that range in age from late 

Pleistocene to Pliocene or possibly older. Many are cultivated, either with the 

use of soil amendments or in a system in which they are cropped for a very 

few years and then returned to forest to allow nutrient recycling. Most of these 

soils have or had forest vegetation, but some support a savanna that probably 

resulted from human-caused fire activity.

Udults

Plinthudults

Fragiudults

Kandiudults

Kanhapludults

Paleudults

Rhodudults

Hapludults
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Profile of a Udult (specifically a Kandiudult) in North 

Carolina. A reddish yellow kandic horizon begins below 

a depth of about 12 inches. Scale is in inches.

Back to Udults
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Key to Great Groups of Udults (Back to key to suborders)

Udults that have:

1. Plinthite making up > 50% of a layer within a depth of 

150 cm ---------------------------------------------------------------------- Plinthudults

Begin measuring depth below any O horizon. Plinthite is a firm, iron oxiderich concentration that irreversibly hardens after exposure to repeated wetdry cycles.

2. A fragipan (firm and brittle but not cemented layer) within a depth of 

100 cm ----------------------------------------------------------------------- Fragiudults

3. A kandic (very low cation-exchange capacity) horizon and a clay 

content that does not decrease significantly within a depth of 

150 cm ---------------------------------------------------------------------- Kandiudults

Clay content must not decrease by > 20% of the maximum in the kandic

horizon, unless skeletans (ped surfaces stripped of clay) are present in the 

layer and the clay content increases again by > 3% below. Kandiudults 

cannot have a root-limiting layer within a depth of 150 cm. Begin 

measuring depth below any O horizon.

4. A kandic (very low cation-exchange capacity) horizon and 

a significant decrease in clay content within a depth of 

150 cm ------------------------------------------------------------------ Kanhapludults

Begin measuring depth below any O horizon.

5. An argillic (clay accumulation) subsoil horizon and a clay 

content that does not decrease significantly within a depth of 

150 cm ------------------------------------------------------------------------ Paleudults

Clay content must not decrease by > 20% of the maximum in the argillic

horizon, unless skeletans (ped surfaces stripped of clay) are present in the 

layer and the clay content increases again by > 3% below. Paleudults 

cannot have a root-limiting layer within a depth of 150 cm. Begin 

measuring depth below any O horizon.

6. Both a dark surface layer and a dark, reddish argillic (clay 

accumulation) subsoil horizon ------------------------------------- Rhodudults

The surface layer and subsoil have moist value of < 3. The hue of the 

argillic horizon is mostly 2.5YR or redder and its value increases no more 

than 1 unit when dry. The argillic color requirement applies to a depth of 

100 cm, or to the entire argillic horizon if thinner. Begin measuring depth 

below any O horizon.

7. An argillic (clay accumulation) horizon and a significant decrease in

clay content within a depth of 150 cm ----------------------------- Hapludults

Begin measuring depth below any O horizon.
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-332

Descriptions of Great Groups of Udults

Plinthudults—These soils are the more or less freely drained Udults that 

have a large amount of plinthite (firm, iron oxide-rich concentration) in the 

subsoil. They are mainly in intertropical regions and, in some areas, are 

extensive. They are not known to occur in the United States. (Back to Udults 

key)

Fragiudults—These soils have a fragipan (firm and brittle but not cemented 

layer) within a depth of 100 cm. They formed mainly in loamy alluvium or in 

residuum. The fragipan commonly has an upper boundary between 50 and 

100 cm below the mineral soil surface. Ground water is perched above the 

pan at some period during the year, and many of the soils have thick clay 

depletions (gray areas where clay and iron have been lost) near the top of the 

fragipan and in vertical seams between structural units. Fragiudults are 

principally found on gentle slopes throughout the southeastern United States. 

The vegetation was a forest of either mixed conifers or broadleaf deciduous 

trees. (Back to Udults key)

Kandiudults—These soils have a kandic (very low cation-exchange capacity) 

subsoil horizon and a clay distribution in which the percentage of clay does 

not decrease from its maximum amount by as much as 20% within a depth of 

150 cm, or the layer in which the clay percentage decreases has at least 5%

of the volume consisting of skeletans (ped surfaces stripped of clay) and there 

is at least a 3% (absolute) increase in clay content below this layer.

Kandiudults are mostly found on older geomorphic surfaces of the 

southeastern Coastal Plain of the United States. (Back to Udults key)

Kanhapludults—These soils have a kandic (very low cation-exchange

capacity) subsoil horizon. They are less than 150 cm deep, or the kandic 

horizon has a clay distribution in which the clay content decreases 

significantly within a depth of 150 cm. These soils have a thin to moderately 

thick zone of maximum clay content. The natural vegetation consisted of 

forest plants. Many of the soils have been cleared and are used as cropland 

or pasture. Kanhapludults are commonly found in the Southeastern Piedmont 

of the United States. (Back to Udults key)

Paleudults—These soils are on very old, stable land surfaces. They have a 

thick argillic (clay accumulation) subsoil horizon. They have a clay distribution 

in which the percentage of clay does not decrease from its maximum amount 

by as much as 20% within a depth of 150 cm, or the layer in which the clay 

percentage decreases has at least 5% of the volume consisting of skeletans 

(ped surfaces stripped of clay) and there is at least a 3% (absolute) increase 

in clay content below this layer. Slopes generally are gently sloping or nearly 

level. The natural vegetation consisted of forest plants, mostly hardwoods or 

mixed conifers and hardwoods. Many of these soils have been cleared and 
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-333

are used as cropland or pasture. Paleudults are extensive in the southeastern 

and middle Atlantic areas of the United States. (Back to Udults key)

Rhodudults—These soils have a dark-colored epipedon and an argillic (clay 

accumulation) subsoil horizon with dark reddish colors throughout the upper 

part. They have a thin or moderately thick zone of maximum clay content. 

Rhodudults formed under forest vegetation and commonly from basic rocks or 

sediments. They tend to have more total phosphorus than most other Udults. 

They also tend to be less erosive. Rhodudults range from nearly level to very 

steep. Most of these soils formed in sediments or on surfaces dating to the

latter half of the Pleistocene (Illinoian or Wisconsinan). Where slopes are 

suitable, most of the soils are used as cropland. In many of the soils, the plow 

layer includes material that was part of the argillic horizon. (Back to Udults 

key)

Hapludults—These soils have an argillic (clay accumulation) subsoil horizon

and a clay distribution that decreases significantly within a depth of 150 cm. 

They have a thin or moderately thick zone of maximum clay content. Most of 

the soils formed in areas of acid rocks or sediments on surfaces that are at 

least Pleistocene in age. Where the soils are not cultivated, the vegetation 

consists of hardwood trees or conifers. Hapludults are extensive in the 

southeastern United States, in the mid Atlantic States, and on the Coastal 

Plain along the Gulf of Mexico in the southern States east of the Mississippi 

River. Slopes generally are gently sloping to steep, but a few of the soils on 

the lowest part of the Coastal Plain are nearly level. (Back to Udults key)
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-334

Ustults Great Groups

Ustults are the more or less freely drained Ultisols of subhumid to semiarid 

regions where amouns of rainfall are moderately low to high but 

evapotranspiration exceeds precipitation most of the year. Some Ustults have 

a single dry season each year, such as those in a monsoon climate, and 

others have alternating moist and dry periods throughout the growing season. 

Ustults have an ustic soil moisture regime and a relatively low content of 

organic carbon. 

Most Ustults have an ochric (typically thin and/or light-colored) epipedon that 

overlies an argillic (clay accumulation) or kandic (very low cation-exchange

capacity) subsoil horizon, which may or may not contain plinthite (firm, iron 

oxide-rich concentration). A petroferric (cemented by iron with little or no 

organic matter) contact is common in some parts of the world. 

The vegetation commonly consists of forest or savanna plants. Ustults are of 

small extent in the United States.

Ustults

Plinthustults

Kandiustults

Kanhaplustults

Paleustults

Rhodustults

Haplustults
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-335

Profile of an Ustult (specifically a Kandiustult) in Zambia. This soil 

has a reddish yellow kandic horizon beginning at a depth of 

about 20 centimeters. Scale is in decimeters.

Back to Ustults
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-336

Key to Great Groups of Ustults (Back to key to suborders)

Ustults that have:

1. Plinthite making up > 50% of a layer within a depth of

150 cm --------------------------------------------------------------------- Plinthustults

Begin measuring depth below any O horizon. Plinthite is a firm, iron oxiderich concentration that irreversibly hardens after exposure to repeated wetdry cycles.

2. A kandic (very low cation-exchange capacity) horizon and a 

clay content does not decrease significantly within a depth of 

150 cm --------------------------------------------------------------------- Kandiustults

Clay content must not decrease by > 20% of the maximum in the kandic

horizon, unless skeletans (ped surfaces stripped of clay) are present in the 

layer and the clay content increases again by > 3% below. Kandiustults 

cannot have a root-limiting layer within a depth of 150 cm. Begin 

measuring depth below any O horizon.

3. A kandic (very low cation-exchange capacity) horizon and 

a significant decrease in clay content within a depth of 

150 cm ------------------------------------------------------------------ Kanhaplustults

Begin measuring depth below any O horizon.

4. An argillic (clay accumulation) subsoil horizon and a clay 

content that does not decrease significantly within a depth of 

150 cm ----------------------------------------------------------------------- Paleustults

Clay content must not decrease by > 20% of the maximum in the argillic

horizon, unless skeletans (ped surfaces stripped of clay) are present in the 

layer and the clay content increases again by > 3% below. Paleustults 

cannot have a root-limiting layer within a depth of 150 cm. Begin 

measuring depth below any O horizon.

5. Both a dark surface layer and a dark, reddish argillic (clay 

accumulation) subsoil horizon ------------------------------------- Rhodustults

The surface and subsoil have moist value of < 3. The hue of the argillic 

horizon is mostly 2.5YR or redder and its value increases no more than 1 

unit when dry. Argillic color requirement applies to a depth of 100 cm, or to 

the entire argillic horizon if thinner. Begin measuring depth below any O 

horizon.

6. An argillic (clay accumulation) horizon and a significant decrease in 

clay content within a depth of 150 cm ---------------------------- Haplustults

Begin measuring depth below any O horizon.
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-337

Descriptions of Great Groups of Ustults

Plinthustults—These soils have a large amount of plinthite (firm, iron oxiderich concentration) in the subsoil. Slopes are mostly gentle or moderate. 

These soils are not known to occur in the United States or in Puerto Rico. The 

great group is provided for use in other parts of the world. (Back to Ustults 

key)

Kandiustults—These soils have a kandic (very low cation-exchange 

capacity) subsoil horizon. They have a clay distribution in which the 

percentage of clay does not decrease from its maximum amount by as much 

as 20% within a depth of 150 cm. The natural vegetation consisted of forest. 

Many of the soils have been cleared and are used as cropland or pasture. 

(Back to Ustults key)

Kanhaplustults—These soils have a kandic (very low cation-exchange 

capacity) subsoil horizon. They have a clay distribution in which the 

percentage of clay decreases significantly within a depth of 150 cm. These 

soils have a thin to moderately thick zone of maximum clay content. Slopes 

range from gentle to very steep. Many of the soils are in tropical climates and 

are farmed by means of shifting cultivation. Kanhaplustults are not known to 

occur in the United States. They are defined for use in other parts of the 

world. (Back to Ustults key)

Paleustults—These soils have an argillic (clay accumulation) subsoil horizon. 

They have a clay distribution in which the percentage of clay does not 

decrease from its maximum amount by as much as 20% within 150 cm from 

the mineral soil surface. Many of the soils have a thick argillic horizon. 

Commonly, there are small or moderate amounts of plinthite (firm, iron oxiderich concentration) at some depth in the soils. Paleustults are on old stable 

surfaces that have gentle slopes. They are very rare in the United States and 

are known to occur only in California. (Back to Ustults key)

Rhodustults—These soils have a dark-colored epipedon and a dark red or 

dusky red argillic (clay accumulation) subsoil horizon. They are moderately 

deep or have a clay distribution in which the percentage of clay decreases 

significantly within a depth of 150 cm. These soils have a thin to moderately 

thick zone of maximum clay content. They formed mainly in material 

weathered from basic rocks. They tend to have more total phosphorus than 

most other Ustults. Rhodustults have gentle to steep slopes. Most of these

soils formed in alluvium or on slopes dating from the latter half of the 

Pleistocene, either Illinoian or Wisconsinan. Rhodustults are rare in the United 

States and are known to occur only on the Pacific Trust Islands. (Back to 

Ustults key)
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-338

Haplustults—These soils have a thin or moderately thick zone of maximum 

clay content in the argillic (clay accumulation) subsoil horizon. Slopes range 

from gentle to very steep. Many of these soils are in tropical climates and are 

farmed by means of shifting cultivation. Haplustults are of small extent in the 

United States and occur mainly in Texas, California, and Puerto Rico. They 

are extensive in some parts of the world. (Back to Ustults key)
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-339

Xerults Great Groups

Xerults are the more or less freely drained Ultisols of Mediterranean-type

climates. These soils have a xeric moisture regime (cool and moist in winter

and warm and dry in summer) and a moderate or small amount of organic 

matter. They generally have an ochric (typically thin and/or light-colored) or 

umbric (humus-rich with low base saturation) epipedon that rests on a 

brownish to reddish argillic (clay accumulation) or kandic (very low cationexchange capacity) subsoil horizon. They are not extensive in the United 

States, except in California and Oregon, mostly in the mountains. They are 

gently sloping to very steep. The natural vegetation consisted mostly of 

coniferous forest plants.

Xerults

Palexerults

Haploxerults
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-340

Key to Great Groups of Xerults (Back to key to suborders)

Xerults that have:

1. An argillic (clay accumulation) or kandic (very low cation-exchange

capacity) subsoil horizon and a clay content that does not decrease 

significantly within a depth of 150 cm ---------------------------- Palexerults

Clay content must not decrease by > 20% of the maximum in the subsoil, 

unless skeletans (ped surfaces stripped of clay) are present in the layer 

and the clay content increases again by > 3% below. Palexerults cannot 

have a root-limiting layer within a depth of 150 cm. Begin measuring depth 

below any O horizon.

2. An argillic (clay accumulation) or kandic (very low cation-exchange

capacity) subsoil horizon and a significant decrease in clay content 

within a depth of 150 cm -------------------------------------------- Haploxerults

Begin measuring depth below any O horizon.
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-341

Descriptions of Great Groups of Xerults

Palexerults—These soils have a clay distribution in which the percentage of 

clay does not decrease from its maximum amount by as much as 20% within 

a depth of 150 cm, or the layer in which the clay percentage decreases has at 

least 5% of the volume consisting of skeletans (ped surfaces stripped of clay) 

and there is at least a 3% increase in clay content below this layer. These 

soils are mainly on older surfaces in the Cascades and coastal mountains of 

Oregon and California. Slopes are gentle to very steep. Palexerults are 

probably rare in the world, but they may be extensive locally in areas of humid 

Mediterranean-type climates. (Back to Xerults key)

Haploxerults—These soils have a clay distribution in which the percentage of 

clay decreases significantly within a depth of 150 cm. These soils have a thin 

or moderately thick zone of maximum clay content. Typically, they have an 

ochric (typically thin and/or light-colored) epipedon that rests on a brownish or 

reddish argillic (clay accumulation) or kandic (very low cation-exchange

capacity) subsoil horizon. In the United States, they occur mostly in the Sierra 

Nevada and Cascade Mountains and support coniferous forest plants. 

Haploxerults are moderately extensive in those mountains but are rare 

elsewhere in the United States. (Back to Xerults key)
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-342

Vertisols Order

Vertisols are very clayey soils that shrink and crack when dry and 

expand when wet.

General Characteristics

Vertisols are very clayey soils that have deep, wide cracks during dry 

seasons. All Vertisols are dominated by clay minerals (smectites) that 

dramatically shrink when dry and swell when moistened. These soils tend to 

be very sticky and plastic when wet and very firm and hard when dry. They 

are commonly very dark in color due to the deep mixing resulting from the 

shrink-swell cycles which churn the soil. Vertisols commonly have slickensides 

(shiny, striated structural surfaces) that are produced by the shrink-swell 

process. As a result of the churning, distinct soil horizons are often difficult to 

discern. Many Vertisols have a mollic (rich in humus and bases) epipedon and 

a cambic (minimal soil development) subsoil horizon. A few have an argillic 

(clay accumulation), natric (high levels of illuvial clay and sodium), or calcic 

(calcium carbonate accumulation) subsoil horizon.

Environment and Processes

Vertisols generally have gentle slopes, but a few are strongly sloping. The 

natural vegetation is predominantly grass or savanna, but a few are in forest 

or desert shrub. Vertisols are known to occur in a wide range of climatic 

environments, but all Vertisols require a climate (whether cool or hot) in which 

seasonal drying occurs. The ideal climate seems to be that with a wet-dry 

monsoonal pattern that accentuates the shrink-swell cycles. 

Vertisols form over a variety of parent materials, most of which are neutral or 

calcareous. The materials include calcareous sandstone, shale, chalk, 

limestone, ash deposits, base-rich igneous rocks, and clayey alluvium derived 

from these or similar sources. Conditions that allow relatively low leaching are 

important so that the clays do not weather to mineral species with less 

potential for shrinking and swelling. 

Some Vertisols, mostly those in more humid areas, developed a unique 

pattern of microrelief known as gilgai. Gilgai is an intricate pattern of microhighs and -lows that repeat over scales of about 3 to 10 meters. These areas 

have complex patterns of hydrology and vegetation.

Location

Globally, Vertisols occupy about 2% of the ice-free land area. These soils

occur mostly between latitudes of about 50 degrees north and 45 degrees 

south. They are common in parts of Sudan, Ethiopia, China, India, 
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-343

Bangladesh, Australia, and Uruguay. In the United States, Vertisols are 

common in the southern parts of the Great Plains and Mississippi River Valley

and in the Alabama and Mississippi Blackland Prairie, the Red River Valley, 

western South Dakota, central California, and a few areas in the Pacific 

Northwest.

Profile of a Vertisol.

(Back to Key to Soil Orders)
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-344

Vertisols by suborder in the United States.

Global distribution of Vertisols.
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-345

Vertisols Suborders

Classification of Vertisols

In the definitions of the suborders, emphasis is given to soil climate. Five 

suborders are based on moisture regime (Aquerts, Xererts, Usterts, Torrerts, 

and Uderts) and one is based on temperature regime (Cryerts). The defined 

moisture regimes, however, have little significance because water from rainfall 

commonly runs into cracks so that the soils are remoistened from both above 

and below. Because of the difficulty in defining soil moisture regimes in 

Vertisols, the duration that cracks are open or closed is used to differentiate 

the suborders. 

The great group level reflects a combination of important properties, including 

the presence of various diagnostic horizons, the presence of cemented layers, 

electrical conductivity and pH, significant carbon accumulation in the upper 

part of the soil, and patterns of soil saturation. 

The six suborders are:

1. Aquerts—wet Vertisols (aquic conditions in the upper part) 

2. Cryerts—cold Vertisols (cryic temperature regime)

3. Xererts—Vertisols with seasonal cracking pattern indicative of a xeric 

moisture regime (cool and moist in winter and warm and dry in summer)

4. Torrerts—Vertisols with seasonal cracking pattern indicative of a torric 

(arid) moisture regime

5. Usterts—Vertisols with seasonal cracking pattern indicative of an ustic 

moisture regime (moisture is limited, but available, during portions of the 

growing season)

6. Uderts—Vertisols with seasonal cracking pattern indicative of a udic 

moisture regime (seasonally well-distributed rainfall)
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-346

Vertisols

Aquerts

Cryerts

Xererts

Torrerts

Usterts

Uderts
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-347

Key to Suborders of Vertisols

Vertisols that have:

1. A seasonal high water table within a depth of 50 cm ------------- Aquerts

Evidence of a water table includes redoximorphic features (gray and red 

color patterns). See 12th edition of “Keys to Soil Taxonomy” for criteria 

regarding amount, color, and location of redoximorphic features. Begin 

measuring depth below any O horizon. Artificially drained sites are 

included in Aquerts.

2. Cold average soil temperature -------------------------------------------- Cryerts

These soils have a cryic soil temperature regime.

3. Cracks in the upper 50 cm that are open > 90 consecutive days in 

summer and closed > 90 consecutive days in winter ------------- Xererts

Seasonal cracking pattern pertains to nonirrigated soils. Cracks are > 5

mm wide and extend through > 25 cm of the upper 50 cm. Xererts have a 

frigid, mesic, or thermic soil temperature regime. 

4. Cracks in the upper 50 cm that are closed < 60 consecutive days 

when the soil temperature is > 8 oC ------------------------------------ Torrerts

Seasonal cracking pattern pertains to nonirrigated soils. Soil temperature 

is recorded at a depth of 50 cm. The temperature of 8

oC is used to 

approximate the growing season.

5. Cracks in the upper 50 cm that are open > 90 cumulative days each 

year --------------------------------------------------------------------------------- Usterts

Seasonal cracking pattern pertains to nonirrigated soils. Cracks are > 5

mm wide and extend through > 25 cm of the upper 50 cm. 

6. Cracks in the upper 50 cm that are open < 90 cumulative days each 

year ---------------------------------------------------------------------------------- Uderts

Seasonal cracking pattern pertains to nonirrigated soils. Cracks are > 5

mm wide and extend through > 25 cm of the upper 50 cm. 
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-348

Aquerts Great Groups

Aquerts are the wet Vertisols. They are saturated to the surface by ground

water for a long enough period during the year to become devoid of oxygen 

(aquic conditions). However, they also are dry for periods long enough during 

the year for cracks to open. These soils are typically in low areas, such as 

glacial lake plains, flood plains, stream terraces, and depressions.

Aquerts

Sulfaquerts

Salaquerts

Duraquerts

Natraquerts

Calciaquerts

Dystraquerts

Epiaquerts

Endoaquerts
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-349

Profile of an Aquert (specifically an Endoaquert) in Thailand. 

Redoximorphic features are throughout the profile. A 

lithologic discontinuity to underlying loamy material is at 

a depth of about 110 centimeters. Scale is in centimeters.

Return to Aquerts
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-350

Key to Great Groups of Aquerts (Back to key to suborders)

Aquerts that have:

1. A sulfuric (highly acid due to oxidation and production of sulfuric 

acid) horizon OR sulfidic materials (acid-producing, oxidizable sulfur 

compounds) within a depth of 100 cm --------------------------- Sulfaquerts

Begin measuring depth below any O horizon.

2. A salic (high content of salts) horizon within a depth of 

100 cm ------------------------------------------------------------------------ Salaquerts

Begin measuring depth below any O horizon.

3. A duripan (layer cemented by silica) within a depth of 

100 cm ----------------------------------------------------------------------- Duraquerts

Begin measuring depth below any O horizon.

4. A natric (high levels of illuvial clay and sodium) horizon within a 

depth of 100 cm ---------------------------------------------------------- Natraquerts

Begin measuring depth below any O horizon.

5. A calcic (calcium carbonate accumulation) horizon within a depth of 

100 cm --------------------------------------------------------------------- Calciaquerts

Begin measuring depth below any O horizon.

6. Very strongly acid pH (and low salinity) in the upper 

50 cm ---------------------------------------------------------------------- Dystraquerts

The pH is < 4.5 in 0.01 M CaCl2 (5.0 or less in 1:1 water). Electrical 

conductivity in the saturation extract is < 4.0 dS/m. The pH and salinity 

criteria must be met in at least 25 cm of the upper 50 cm.

7. Episaturation (perched water table) -------------------------------- Epiaquerts

8. Endosaturation (saturated throughout the profile) --------- Endoaquerts
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-351

Descriptions of Great Groups of Aquerts

Sulfaquerts—These soils have an appreciable amount of sulfides close to the 

soil surface as either sulfidic materials (acid-producing, oxidizable sulfur 

compounds) or a sulfuric (highly acid due to oxidation and production of 

sulfuric acid) horizon. The sulfides are close enough to the surface for the 

soils to become extremely acid and virtually barren if drained. These soils are 

not known to occur in the United States. They have been recognized in 

coastal areas, including river deltas in South Asia. (Back to Aquerts key)

Salaquerts—These soils have a salic (high content of salts) horizon. They are 

not known to occur in the United States. (Back to Aquerts key)

Duraquerts—These soils have a duripan (layer cemented by silica). They 

commonly are derived from volcanic materials. In the United States, all of 

these soils have aquic conditions (saturated and depleted of oxygen) for part 

of the year and either have a xeric moisture regime (cool and moist in winter 

and warm and dry in summer) or border a xeric moisture regime. (Back to 

Aquerts key)

Natraquerts—These soils have a natric (high levels of illuvial clay and 

sodium) subsoil horizon. In many areas, they occur on flood plains or glacial 

lake plains. The high sodium content limits their use as cropland. Most of the 

soils are used as rangeland, pasture, or hayland. Most Natraquerts in the 

United States formed in alluvium or lacustrine deposits derived dominantly 

from sedimentary rocks. Natraquerts occur in North Dakota, South Dakota, 

Montana, Iowa, Nebraska, and Texas. (Back to Aquerts key)

Calciaquerts—These soils have a calcic (calcium carbonate accumulation)

subsoil horizon. In addition, they commonly have a mollic (rich in humus and 

bases) epipedon. In the United States, these soils occur on the northern Great 

Plains. They are used mostly for crops. (Back to Aquerts key)

Dystraquerts—These soils have dominantly low pH values and low electrical 

conductivity in the upper 50 cm. They commonly occur in low areas, such as 

flood plains and terraces, but a few occur on uplands. These soils are found in 

portions of the southern United States. Many of them are cropped or are in 

pasture, but some are forested. (Back to Aquerts key)

Epiaquerts—These soils have one or more soil layers that perch water. 

These layers are commonly close to the surface. Epiaquerts occur on a 

variety of landforms, including flood plains, glacial lake plains, and 

depressions. In the United States, they occur in several western States, on 

the northern Great Plains, and in the South. They also occur in Puerto Rico. 

(Back to Aquerts key)
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Part 4—Keys to the Orders, Suborders, and Great Groups 4-352

Endoaquerts—These soils are saturated throughout the profile. They occur in 

the far western States, on the northern Great Plains, and in the southern 

States. (Back to Aquerts key)
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Cryerts Great Groups

Cryerts are the Vertisols of cold regions. They have a cryic soil temperature 

regime. Because these soils are fine textured and in cold temperature 

regimes, they periodically shrink and swell, the process which forms the 

diagnostic characteristics of Vertisols. Cracks commonly open once a year, 

late in the summer. Cryerts occur on the cold prairies of Canada, where they 

commonly are derived from lacustrine deposits. They also occur in the Rocky 

Mountains of the United States.

Cryerts

Humicryerts

Haplocryerts
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-354

Key to Great Groups of Cryerts (Back to key to suborders)

Cryerts that have:

1. High amounts of organic carbon in the soil ------------------- Humicryerts

These soils have > 10 kg/m2organic carbon in the upper 50 cm of the 

profile. Begin measuring depth below any O horizon.

2. Moderate to low amounts of organic carbon ----------------- Haplocryerts

These soils have < 10 kg/m2organic carbon in the upper 50 cm of the 

profile. Begin measuring depth below any O horizon.
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-355

Descriptions of Great Groups of Cryerts

Humicryerts—These soils have relatively high amounts of organic carbon in 

the upper part of the profile. They formed under a variety of vegetative types. 

They occur in the Rocky Mountains of the United States. (Back to Cryerts key)

Haplocryerts—These soils have low to moderate accumulations of organic 

carbon. They occur in the Rocky Mountains of the United States. They are of 

limited extent. (Back to Cryerts key)
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-356

Xererts Great Groups

Xererts are the Vertisols of Mediterranean-type climates, which are typified by 

cool, wet winters and warm, dry summers. These soils have cracks that 

regularly close and open each year. Because the soils become dry every 

summer and are remoistened in winter, damage to structures and roads from 

shrink-swell processes is very significant. If not irrigated, these soils are used 

for small grain or grazing. In the United States, most Xererts supported 

grasses before they were cultivated.

Xererts

Durixererts

Calcixererts

Haploxererts
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-357

Profile of a Xerert (specifically a Haploxerert) in Jordan. Scale is 

in decimeters.

Back to Xererts
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-358

Key to Great Groups of Xererts (Back to key to suborders)

Xererts that have:

1. A duripan (layer cemented by silica) within a depth of 

100 cm ----------------------------------------------------------------------- Durixererts

Begin measuring depth below any O horizon.

2. A calcic (calcium carbonate accumulation) or petrocalcic 

(cemented by calcium carbonate) horizon within a depth of 

100 cm ---------------------------------------------------------------------- Calcixererts

Begin measuring depth below any O horizon.

3. Little or no calcium carbonate accumulation and no 

cementation ------------------------------------------------------------- Haploxererts
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-359

Descriptions of Great Groups of Xererts

Durixererts—These soils have a duripan (layer cemented by silica). Most are 

derived from basic igneous rocks, but some formed in mixed alluvium. The 

Durixererts in the United States occur in California and Idaho and are used for 

crop production or rangeland. (Back to Xererts key)

Calcixererts—These soils have a calcic (calcium carbonate accumulation) or 

petrocalcic (cemented by calcium carbonate) subsoil horizon. Many of these 

soils formed in fine textured sedimentary rocks. In the United States, some 

Calcixererts are used for irrigated crops. The rest are used as rangeland, 

pasture, or nonirrigated cropland. (Back to Xererts key)

Haploxererts—These are the most common Xererts. These soils formed in a 

variety of parent materials, including volcanic and sedimentary rocks, 

lacustrine deposits, and alluvium. In many areas, they are used for grazing by 

livestock. In some areas, they are used for citrus, small grain, truck crops, or 

rice. (Back to Xererts key)
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-360

Torrerts Great Groups

Torrerts are the Vertisols of arid climates. Due to limited amounts of 

precipitation, these soils have cracks that commonly stay open for most of the 

year but may close for at least a few days in most years during brief periods of 

precipitation. The cracks are closed for less than 60 days during the growing 

season. Many of these soils dot the landscape in closed depressions that may 

be ponded from time to time by runoff from higher areas. Torrerts that occupy 

larger, more continuous areas of the landscape are commonly underlain by 

parent materials, such as basalt, that tend to weather to smectite. As a result 

of the arid climate, some (but not all) Torrerts have accumulations of salts in 

the profile.

The Torrerts in the United States occur mostly in the Southwest. A few are in 

Hawaii. Most commonly, they are used as rangeland.

Torrerts

Salitorrerts

Gypsitorrerts

Calcitorrerts

Haplotorrerts
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-361

Profile of a Torrert (specifically a Gypsitorrert) in Sudan. 

The white streaks in the subsoil (near knife) are 

accumulations of gypsum.

Back to Torrerts
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-362

Key to Great Groups of Torrerts (Back to key to suborders)

Torrerts that have:

1. A salic (high content of salts) horizon within a depth of 

100 cm ----------------------------------------------------------------------- Salitorrerts

2. A gypsic (gypsum accumulation) horizon within a depth of

100 cm -------------------------------------------------------------------- Gypsitorrerts

3. A calcic (calcium carbonate accumulation) or petrocalcic 

(cemented by calcium carbonate) horizon within a depth of

100 cm --------------------------------------------------------------------- Calcitorrerts

4. Little or no accumulation of salts in the profile ------------- Haplotorrerts
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-363

Descriptions of Great Groups of Torrerts

Salitorrerts—These soils have a salic (high content of salts) horizon. In 

some, salts accumulate because of a periodic water table. In others, scant 

precipitation and the impermeable nature of the soil facilitate the accumulation 

of salts. Salitorrerts are rare in the United States. (Back to Torrerts key)

Gypsitorrerts—These soils have a gypsic (gypsum accumulation) subsoil 

horizon that has its upper boundary within a depth of 100 cm. They occur in 

arid areas of the world where the parent materials have a high content of

gypsum. A high shrink-swell potential and the gypsum content limit many 

construction activities (urban development) unless measures that counteract 

the shrinking and swelling and the dissolution of gypsum are applied. These 

soils occur in the southwestern United States. (Back to Torrerts key)

Calcitorrerts—These soils have a calcic (calcium carbonate accumulation) or 

petrocalcic (cemented by calcium carbonate) subsoil horizon. They are used 

mostly for grazing by livestock. Some are used for irrigated alfalfa. 

Calcitorrerts occur in Arizona and other areas of the southwestern United 

States. (Back to Torrerts key)

Haplotorrerts—These soils have little or no salt accumulation in the profile. 

This is the most extensive great group of Torrerts. In the United States, these 

soils occur in many of the western States and in Texas and Hawaii. They are 

used as urban land, cropland, or rangeland. (Back to Torrerts key)
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-364

Usterts Great Groups

Usterts are the Vertisols in temperate climates that do not receive high 

amounts of rainfall during the summer, in areas with monsoonal climate, and 

in tropical and subtropical areas that have two rainy and two dry seasons. 

Cracks in the soil generally open and close once or twice each year. Usterts 

are extensive in Texas, on the Great Plains, in Australia, in Africa south of the 

Sahara, and in India. Many of these soils formed in gently sloping areas of 

fine textured marine deposits or alluvium. Some are derived from basic 

igneous rocks. Usterts, if irrigated are intensively cultivated, but large areas 

are used for grazing because of a lack of the machinery needed for tillage.

Usterts

Dystrusterts

Salusterts

Gypsiusterts

Calciusterts

Haplusterts
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-365

Profile of an Ustert (specifically a Hapustert) in India. 

Slickensides are throughout profile. Note the large 

slickenside exposed in the lower part of the profile. 

Scale colors are decimeters.

Back to Usterts
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-366

Key to Great Groups of Usterts (Back to key to suborders)

Usterts that have:

1. Very strongly acid pH (and low salinity) in the upper 

50 cm ----------------------------------------------------------------------- Dystrusterts

The pH is < 4.5 in 0.01 M CaCl2 (5.0 or less in 1:1 water). Electrical 

conductivity in the saturation extract is < 4.0 dS/m. The pH and salinity 

criteria must be met in at least 25 cm of the upper 50 cm. Begin measuring 

depth below any O horizon.

2. A salic (high content of salts) horizon within a depth of 

100 cm ------------------------------------------------------------------------- Salusterts

Begin measuring depth below any O horizon.

3. A gypsic (gypsum accumulation) horizon within a depth of 

100 cm --------------------------------------------------------------------- Gypsiusterts

Begin measuring depth below any O horizon.

4. A calcic (calcium carbonate accumulation) or petrocalcic 

(cemented by calcium carbonate) horizon within a depth of 

100 cm ---------------------------------------------------------------------- Calciusterts

Begin measuring depth below any O horizon.

5. Little or no accumulation of salts in the profile --------------- Haplusterts
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-367

Descriptions of Great Groups of Usterts

Dystrusterts—These soils have a dominant pH value of 5.0 or less (in 1:1 

water) and an electrical conductivity of less than 4.0 dS/m within a depth of 50 

cm. They commonly form in acid marine clays and are underlain by more 

alkaline parent materials. They are not known to occur in the United States. 

(Back to Usterts key)

Salusterts—These soils have a salic (high content of salts) horizon. Many are 

derived from salty marine deposits. Salts are difficult to leach in Salusterts due 

to the very slow permeability associated with these soils. As a result, few 

areas are used as cropland. These soils are not known to occur in the United 

States. (Back to Usterts key)

Gypsiusterts—These soils have a gypsic (gypsum accumulation) horizon. 

They are derived from parent materials rich in gypsum. Even in an ustic 

moisture regime, the gypsum is not leached because of the very slow 

permeability associated with these soils. Some of the soils occur on the edges 

of depressions where gypsum accumulates at the capillary fringe. 

Gypsiusterts are rare in the United States but do occur in Texas, where they 

are used as rangeland or pasture. (Back to Usterts key)

Calciusterts—These soils have a calcic (calcium carbonate accumulation) or 

petrocalcic (cemented by calcium carbonate) horizon. These soils commonly 

are derived from parent materials rich in carbonates, such as marine deposits 

or eolian material. Some Calciusterts have a mollic (rich in humus and bases) 

epipedon. Although Calciusterts have limited acreage in the United States, 

they are significant in other parts of the world. (Back to Usterts key)

Haplusterts—These are the most common Usterts. They have little or no 

accumulation of salts and a pH value of > 5.0 (in 1:1 water). These soils are 

derived from a variety of parent materials, including sedimentary rocks, 

alluvium, marl, and basic igneous rocks. Slopes range from nearly level to 

strongly sloping. Haplusterts occur in many western and southwestern States, 

on the northern Great Plains, and in Puerto Rico and the Virgin Islands. They 

are used as rangeland, cropland, or pasture. (Back to Usterts key)
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-368

Uderts Great Groups

Uderts are the Vertisols of humid areas with seasonally well-distributed 

rainfall. These soils have cracks that open and close, depending upon the 

amount of precipitation. In some years the cracks may not open completely.

The Uderts in the United States occur on gentle slopes and are derived 

dominantly from marine shales, chalks, marls, and alluvium. At one time,

many of these soils supported native grasses while some supported 

hardwood or pine forest. Currently, these soils are used for pasture, row 

crops, or woodland. 

Uderts

Dystruderts

Hapluderts
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-369

Profile of an Udert (specifically a Hapludert) in Texas. Soil churning due to 

shrinking and swelling has thrust the underlying gray marl upward to the 

surface (center of photo). The separation of the gray and dark parts of the 

profile is lined with large slickensides.

Back to Uderts
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-370

Key to Great Groups of Uderts (Back to key to suborders)

Uderts that have:

1. Very strongly acid pH (and low salinity) in the upper 

50 cm ------------------------------------------------------------------------ Dystruderts

The pH is < 4.5 in 0.01 M CaCl2 (5.0 or less in 1:1 water). Electrical 

conductivity in the saturation extract is < 4.0 dS/m. The pH and salinity 

criteria must be met in at least 25 cm of the upper 50 cm. Begin measuring 

depth below any O horizon.

2. pH > 5.0 (in 1:1 water) in the upper 50 cm ----------------------- Hapluderts

Begin measuring depth below any O horizon.
Illustrated Guide to Soil Taxonomy

Part 4—Keys to the Orders, Suborders, and Great Groups 4-371

Descriptions of Great Groups of Uderts

Dystruderts—These are the acid Uderts. They are derived dominantly from 

acid, fine textured materials and occur on alluvial plains, deltas, interfluves, 

and side slopes. Commonly, these soils are underlain by sediments high in 

bases. Some of these soils have diagnostic horizons, including argillic (clay 

accumulation), calcic (calcium carbonate accumulation), and gypsic (gypsum 

accumulation) subsoil horizons. In the United States, these soils occur in 

Texas and in the southeastern States, including the Alabama and Mississippi 

Blackland Prairie. They are used as cropland, pasture, or woodland. (Back to 

Uderts key)

Hapluderts—These soils have pH values that are dominantly above 5.0 (1:1 

water) in the upper 50 cm. They typically have high base saturation, and some 

have diagnostic horizons, including argillic (clay accumulation) subsoil 

horizons. Hapluderts occur on uplands and in lower areas. They formed in a 

variety of fine textured parent material, including alluvium. In the United 

States, these soils occur in the Southeast, the northern Great Plains, and the 

Pacific Northwest. (Back to Uderts key)

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