Chapter

3

Revised by Soil Science Division Staff.

Examination and 

Description of Soil Profiles

Introduction

Adescription of the soils is essential in any soil survey. This

chapter provides standards and guidelines for describing the 

 soil. It contains standard technical terms and their definitions

for most soil properties and features and provides information for 

describing the necessary related facts. For some soils, standard terms 

are not adequate and must be supplemented by a narrative. Some soil 

properties change through time. Many properties must be observed over 

time and summarized if one is to fully understand the soil being described 

and its response to short-term environmental changes. Examples are the 

length of time that cracks remain open, the patterns of soil temperature 

and moisture, and the variations in size, shape, and hardness of clods in 

the surface layer of tilled soils.

This chapter does not discuss every possible soil property. For some 

soils, other properties need to be described. Good judgment is needed 

to decide what properties merit detailed attention for any given pedon 

(sampling unit). Observations must not be limited by preconceived ideas 

about what is important.

Although the format of the description and the order in which 

individual properties are described are less important than the content 

of the description, a standard format has distinct advantages. The reader 

can find information more rapidly, and the writer is less likely to omit

important features. Furthermore, a standard format makes data entry 

into a computer database more efficient. Any standardized forms need to

allow enough space for all possible information.

Each investigation of the internal properties of a soil is made on 

a soil body with certain dimensions. The body may be larger than a 

pedon (e.g., a backhoe pit) or represent only a portion of a pedon (e.g., 
84 Chapter 

a sample from a hand auger). During field operations, many soils are

investigated by examining the soil material removed by a sampling tube 

or auger. For rapid investigations of thin soils, a small pit can be dug 

and a section of soil removed with a spade. All of these are samples of 

pedons. Knowledge of the internal properties of a soil is derived mainly 

from studies of such samples. Samples can be studied more rapidly than 

entire pedons; consequently, a much larger number can be studied and 

for several more places. For many soils, the information obtained from 

a small sample amply describes the pedon from which it is taken. For 

other soils, however, important properties of a pedon are not observable 

in a smaller sample and detailed studies of the entire pedon are needed. 

Complete study of an entire pedon requires the exposure of a vertical 

section and the removal of horizontal sections layer by layer. Horizons 

are studied in both horizontal and vertical dimensions. The kind of 

exposure (e.g., bucket auger, push tube, small hand-dug pit, backhoe pit, 

road cut, etc.) should be identified in the soil description.

The information in this chapter, which focuses on the standards and 

guidelines for describing a soil profile in the field, is complemented by

that provided in chapters 2, 6, 10, and 11. Chapter 2 provides information 

related to describing the site surrounding the soil profile. Chapter 6

discusses the use of proximal sensors to measure some soil properties 

quickly and efficiently at field and larger scales by using field-based

electronic technology. Chapter 10 provides information specific to

describing subaqueous soils. Chapter 11 discusses soils heavily impacted 

by human activity. 

General Terms Used to Describe Soils

This section describes several of the general terms for internal 

elements of the soil. Other more specific terms are described or defined

in the following sections. 

Pedon

A pedon is a three-dimensional body of soil that has sufficient area

(roughly 1 to 10 m2) and depth (up to 200 cm) to be used in describing 

the internal arrangement of horizons and in collecting representative 

samples for laboratory analysis (see chapter 4). The pedon is the 

individual classified with Soil Taxonomy. Multiple pedons that have the

same classification and occur together in landscapes are used in defining

soil series. Conceptually, these contiguous pedons are called polypedons 

(see chapter 4). 
Soil Survey Manual 

Soil Profile

A soil profile is smaller than a pedon. It is exposed by a twodimensional vertical cut through the soil. It is commonly conceived as a 

plane at right angles to the soil surface. In practice, a description of a soil 

profile includes soil properties that can be determined only by inspecting

volumes of soil. However, the volume of soil described from a profile

is almost always less than the volume of soil defined by a full pedon

because observations of the soil profile are generally made to only a few

decimeters behind the face of the exposed profile. A pedon description

is commonly based on examination of a profile, and the properties of

the pedon are inferred from the properties of the profile. The width of a

profile ranges from a few decimeters to several meters or more. The size

of the profile should be sufficient to include the largest structural units.

Soil Horizon

A soil horizon is a layer, approximately parallel to the surface of 

the soil, that is distinguishable from adjacent layers by a distinctive set 

of properties produced by the soil-forming processes (i.e., pedogenesis). 

The term “layer” is used instead of “horizon” if the properties are 

inherited from the parent material, such as sedimentary strata. Horizons, 

in contrast, display the effects of pedogenesis, such as the obliteration of 

sedimentary strata and accumulation of illuvial clay.

Solum

The solum (plural, sola) of a soil consists of a set of horizons that are 

related through the same period of pedogenesis. It includes all horizons 

now forming. It may also include a bisequum (discussed below). It 

does not include a buried soil or layer unless it has acquired some of its 

properties by currently active soil-forming processes. The solum of a soil 

is not necessarily confined to the zone of major biological activity. Its

genetic horizons may be expressed faintly to prominently. A solum does 

not have a maximum or minimum thickness.

Solum and soils are not synonymous. Some soils include layers that 

are not affected by soil formation. These layers are not part of the solum. 

The number of genetic horizons ranges from one to many. An A horizon 

that is 10 cm thick overlying bedrock is by itself the solum. A soil that 

consists only of recently deposited new soil material or recently exposed 

soft sediment generally does not have a solum.

In terms of soil horizons as described in this chapter, a solum consists 

of O, V, A, E, and B horizons and their transitional horizons. Included 
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are horizons with an accumulation of carbonates or more soluble salts if 

these horizons are either within, or contiguous to, other genetic horizons 

and are judged to be at least partly produced during the same period of 

soil formation.

The lower limit of the solum, in a general sense, in many soils should 

be related to the depth of rooting for perennial plants, assuming that water 

state and chemistry are not limiting. In some soils, the lower limit can 

be set only arbitrarily and is defined in relation to the particular soil. For

example, horizons of carbonate accumulation are easily visualized as part 

of the solum in many soils in arid and semiarid environments. However, 

to conceive of cemented horizons of carbonates that may extend for 5 

meters or more below the surface as part of the modern solum is more 

difficult. Such massive carbonate horizons represent pedogenesis over

hundreds of thousands of years and are referred to as relict paleosols. 

Gleyed soil material begins in some soils a few centimeters below the 

surface and continues practically unchanged to a depth of many meters. 

Gleying immediately below the A horizon is likely to be related to the 

processes of soil formation in the modern soil. At great depth, gleying 

is likely to be relict or related to processes that are more geological than 

pedological. The same kind of problem exists for some deeply weathered 

soils—the deepest material penetrated by roots is very similar to the 

weathered material at much greater depth.

For some soils, digging deep enough to reveal all of the relationships 

between soils and plants is not practical. Plant roots, for example, may 

derive much of their moisture from fractured bedrock. Descriptions 

should indicate the nature of the soil-rock contact and determinations 

about the upper part of the underlying rock.

Not everyone will agree about the exact extent of the solum in 

some soils. For example, a certain level of subjectivity is involved in 

differentiating transitional BC or CB horizons from C horizons or in 

determining which properties observed in the soil are the product of 

active pedogenic processes. The concept of the solum remains useful for 

discussions about the nature of soils and soil profiles but is generally not

used as a part of any technical definitions.

Sequum

A sequum (plural, sequa) consists of a B horizon and any overlying 

eluvial horizons. A single sequum is considered to be the product of a 

specific combination of soil-forming processes.

Most soils have only one sequum, but some have two or more. For 

example, a new sequence of horizons that meet the criteria for a Spodosol 
Soil Survey Manual 

can form in the upper part of a previously existing Alfisol, producing an

eluviated zone and a spodic horizon underlain by another eluviated zone 

overlying an argillic horizon. Such a soil has two sequa. Soils in which 

two sequa have formed, one above the other in the same deposit, are said 

to be bisequal.

If two sequa formed in different deposits at different times, the soil 

is not bisequal. For example, a soil having an A-E-B horizon sequence 

may form in material that was deposited over another soil that already 

had an A-E-B horizon sequence. Each set of A-E-B horizons is a sequum, 

but the combination is not a bisequum; the lower set is a buried soil. If 

the horizons of the upper sequum extend into the underlying sequum, the 

affected layer is considered part of the upper sequum. For example, the 

A horizon of the lower soil may retain some of its original characteristics 

and also have some characteristics of the overlying soil. In this case, the 

soils are also not considered bisequal; the upper part of the lower soil 

is the parent material of the lower part of the currently forming soil. In 

many soils the distinction cannot be made with certainty. If some of the 

C material of the upper sequum remains, the distinction is clear.

Studying Pedons

Site Selection

Pedons representative of an extensive mappable area are generally 

more useful than pedons that represent a transitional area to another 

soil. For detailed study of a soil, a pedon is tentatively selected and then 

examined preliminarily to determine whether or not it represents the 

desired segment of the soil’s range. This is a critical step. Typically, only 

a few pedons can be studied in detail due to the time and expense involved 

in exposing, describing, photographing, and sampling soil profiles and

performing necessary laboratory analysis. It is very important that the 

site selected for study is a representative sample of the overall soil body 

in the landscape because data from the site will be used to classify the 

soil pedon and correlate it with other similar pedons.

Information Recorded

For a soil description to be of greatest value, detailed information 

about its setting should be recorded (see chapter 2). Important items 

include location (identified by latitude and longitude, including datum, or

another acceptable geographic location system), the part of the landscape 
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that the pedon represents (i.e., landform, position on landform, any 

applicable microfeature), elevation, aspect, parent material, vegetation, 

land use, and erosion or other disturbance affecting the soil profile.

The level of detail will depend on the objectives. A complete setting 

description should include information about the pedon and other soils 

conterminous with the pedon. It also may include information on any 

features that differ from the central concept of the soil series for which 

the described pedon is named (if a series has been defined).

The description of a body of soil in the field, whether an entire pedon

or a soil profile within it, should record the kinds of horizons or layers,

their depth and thickness, and the properties of each. Generally, external 

features, such as slope, surface stoniness, erosion, and vegetation, are 

observed for the area around the pedon, which is considered to be part of 

the same soil body. Internal features, such as color, texture, and structure, 

are observed from the study of the pedon. 

Observing Pedons

In order to observe a pedon fully, including soil structure (size and 

kind), horizon boundary topography, and short-range variability in 

horizon thickness, a pit exposing a vertical face approximately 1 meter 

across to an appropriate depth (fig. 3-1) is adequate for most soils.

Excavations associated with roads, railways, gravel pits, and other soil 

disturbances provide easy access for studying soils. Old exposures, 

however, must be used cautiously. In these areas, the soils can dry out 

or freeze and thaw from both the surface and the sides. In addition, the 

soil structure may be more pronounced than is typical, salts may have 

accumulated near the edges of exposures or been removed by seepage, 

plinthite may have irreversibly hardened to ironstone, or other changes 

may have taken place.

For hand- or backhoe-dug pits, care must be taken to ensure that the 

pit conforms to safety regulations. Loose sandy soils and wet soils are 

particularly susceptible to cave-ins.

After the sides of the pit are cleaned of all loose material disturbed by 

digging, the exposed vertical faces are examined, typically starting at the 

top and working downward, to identify significant changes in properties.

Boundaries between layers are marked on the face of the pit, and the 

layers are identified and described.

Photographs should be taken after the layers have been identified but

before the vertical section is disturbed in the description-writing process. 

An estimation of the volume of stones or other features also is done 

before the layers are disturbed.
Soil Survey Manual 

Figure 3-1

A shallow soil pit with a face that has been cleaned and prepared for describing the soil 

profile. This soil (a Fibristel in Alaska) has been dug to the depth of permafrost (about 

40 cm).

If bulk samples are to be collected for laboratory analysis, it generally 

is best to begin with the bottom layer and work upward. This prevents 

material from the upper layers falling onto the face of lower layers before 

they have been sampled.

A horizontal view of each horizon is useful. This exposes structural 

units that otherwise may not be readily observable from the vertical pit 

face. Patterns of color within structural units, variations of particle size 

from the outside to the inside of structural units, and the pattern in which 

roots penetrate structural units are commonly seen more clearly in a 

horizontal section (fig. 3-2).

Measuring Depth to and Thickness of Horizons and 

Layers

Soil Surface

When describing soil profiles, depth is measured from the soilsurface.

Generally, the soil surface is the top of the mineral soil. For soils with 

an O horizon (Oi, Oe, or Oa), it is the top of the O horizon. Fresh leaf or 

needle fall that has not undergone observable decomposition is excluded 
90 Chapter 

Figure 3-2

A horizontal view (looking down) of a fragipan from a soil (a Fragiudalf) in Tennessee. 

The horizon has prismatic structure with gray seams between prisms and reddish 

redoximorphic features, mostly within the prisms. This view allows the structure and 

color patterns of the horizon to be easily observed. The exposed area is approximately 

30 by 40 cm.

from the concept of an O horizon and may be described separately as 

a surface feature. Profile measurements begin below any fresh leaf or

needle fall.

For soils that have a cover of 80 percent or more rock or pararock 

fragments (as in some areas of rubbly colluvial materials), the top of the 

soil is considered the mean height of the tops of the rock or pararock 

fragments. Depth measurements are taken from this height.

It is important to note that, when measuring depth and thickness for 

taxonomic purposes, the “mineral soil surface” is commonly specified

as the datum to use in measurements. This essentially excludes any 

overlying O horizon and is therefore not synonymous with the soil surface 

as defined here for making soil descriptions. See Keys to Soil Taxonomy

(Soil Survey Staff, 2014b or later version) for further information.
Soil Survey Manual 

Depth Measurements

The depth to a horizon or layer boundary commonly differs within 

short distances, even within a pedon. The part of the pedon that is typical 

or most common is described. In the soil description, the horizon or layer 

designation is listed, followed by the values that represent the depths 

from the soil surface to the upper and lower boundaries (e.g., Bt1 - 8 to 

20 cm). The depth to the lower boundary of a horizon or layer is the depth 

to the upper boundary of the horizon or layer beneath it. Variation in the 

depths of the boundaries is recorded in the description of the horizon or 

layer. The depth limits of the deepest horizon or layer described include 

only that part actually seen. 

In some soils, the variations in depths to boundaries are so complex 

that the usual terms used to describe the boundary topography are 

inadequate. These variations are described separately, e.g., “depth to the 

lower boundary is mainly 30 to 40 cm, but tongues extend to depths of

60 to 80 cm.” The lower boundary of a horizon or layer and the upper 

boundary of the horizon or layer below share a common irregularity.

Thickness Measurements

The thickness of each horizon or layer is the vertical distance 

between the upper and lower boundaries. Overall thickness may vary 

within a pedon, and this variation should be noted in the description. 

A range in thickness may be given, e.g., “thickness ranges from 20 to 

30 cm.” This range is not calculated from the range of upper and lower

boundary depths. Instead, the range is calculated from evaluations across 

the exposure at different lateral points. For example, the upper boundary 

of a horizon may range in depth from 25 to 45 cm and the lower boundary 

from 50 to 75 cm. Taking the extremes of these two ranges, it is incorrect 

to conclude that the horizon thickness ranges from as little as 5 cm to as 

much as 50 cm when in fact it may be 20 to 30 cm in the field.

Designations for Horizons and Layers

Soils vary widely in the degree to which horizons are expressed. 

Relatively fresh parent materials, such as recent deposits of alluvium, 

eolian sands, or mantles of volcanic ash, may have no recognizable 

genetic horizons but may have distinct layers that reflect different modes

of deposition. As soil formation proceeds, horizons in their early stages 

may be detected only by very careful examination. As horizons increase 

in age, they generally are more easily identified in the field. However,

only one or two different horizons may be readily apparent in some very 
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old, deeply weathered soils in tropical areas where annual precipitation is 

high. This section provides the standard nomenclature and definitions for

a system used to assign symbols to soil horizons and layers.

Background and Concepts for Use of Designations

Different kinds of layers are identified by different symbols. Designations are provided for layers that have been changed by soil formation 

and for those that have not. Each horizon designation indicates either that 

the original material has been changed in certain ways or that there has 

been little or no change. The designation is assigned after comparison 

of the observed properties of the layer with properties inferred for the 

material before it was affected by soil formation. The processes that have 

caused the change need not be known; properties of soils relative to those 

of an estimated parent material are the criteria for judgment. The parent 

material inferred for the horizon in question, not the material below the 

solum, is used as the basis of comparison. The inferred parent material 

commonly is very similar to, or the same as, the soil material below the 

solum.

Designations show the describer’s interpretations of genetic relationships among the layers within a soil. Layers do not need to be identified

by symbols in order to make a good description, but the usefulness of soil 

descriptions is greatly enhanced by the proper use of designations. The 

designations provide a sort of shorthand nomenclature conveying the important properties observed by the person describing the soil as well as 

the genetic inferences made by that person regarding the formation of the 

soil. The definitions of the symbols provided below are generally more

qualitative than quantitative. There is a small degree of subjectivity that 

allows some freedom for the describer to convey their theory of how the 

soil formed. There may be a certain level of inconsistency in the way 

different describers label the horizons of the same profile. For example,

one describer may label a horizon “C” while another may label it “CB” 

or one may record a subtle lithologic discontinuity that another person 

does not observe. 

Designations are not substitutes for descriptions. If both designations 

and adequate descriptions of a soil are provided, the reader has the 

interpretation made by the person who described the soil and also the 

evidence on which the interpretation was based.

Genetic horizons are not equivalent to the diagnostic horizons of 

Soil Taxonomy. Designations of genetic horizons express a qualitative 

judgment about the kind of changes that are believed to have taken 

place. Diagnostic horizons are quantitatively defined features used to
Soil Survey Manual 

differentiate taxa. Changes implied by genetic horizon designations 

may not be large enough to justify recognition of diagnostic criteria. 

For example, the designation “Bt” does not always indicate an argillic 

horizon. Furthermore, the diagnostic horizons may not be coextensive 

with genetic horizons.

Basic System of Horizon and Layer Designations

Four kinds of symbols are used in various combinations to designate 

horizons and layers:

Capital letters.—Used to designate the master horizons and layers. 

Lowercase letters.—Used assuffixes to indicate specific characteristics

of master horizons and layers.

Numbers.—Used both as suffixes to indicate vertical subdivisions

within a horizon orlayer and as prefixesto indicate discontinuities.

Special symbols.—Used to indicate layers formed in human-transported 

material or sequences of horizons having otherwise identical 

designations. 

Master Horizons and Layers

The capital letters O, L, V, A, E, B, C, R, M, and W represent the 

master horizons and layers of soils. These letters are the base symbols 

to which other characters are added to complete the designations. Most 

horizons and layers have a designation using one capital letter symbol; 

some have two.

O Horizons or Layers

O horizons or layers are dominated by organic soil materials. Some 

are saturated with water for long periods; some were once saturated but 

are now artificially drained; and others have never been saturated.

Some O horizons or layers consist of slightly decomposed to highly 

decomposed litter (such as leaves, needles, twigs, moss, and lichens) 

that was deposited on the surface of either mineral or organic soils. 

Others consist of organic materials that were deposited under saturated 

conditions and have decomposed to varying stages. The mineral fraction 

of such material constitutes only a small percentage of the volume of 

the material and generally much less than half of its weight. Some soils 

consist entirely of materials designated as O horizons or layers. 

An O horizon or layer may be at the surface of a mineral soil or, if 

buried, at any depth below the surface. A horizon formed by illuviation 

of organic material into a mineral subsoil is not an O horizon, although 

some horizons that formed in this manner contain a large amount of 
94 Chapter 

organic matter. Horizons or layers composed of limnic materials are not 

designated as O horizons. 

L Horizons or Layers

L horizons or layers include both organic and mineral limnic materials 

that were either: 

1. Deposited in water by precipitation or through the actions of 

aquatic organisms, such as algae and diatoms; or 

2. Derived from underwater and floating aquatic plants and subsequently modified by aquatic animals.

L horizons or layers include coprogenous earth (sedimentary peat), 

diatomaceous earth, and marl. They are described only for Histosols 

(decomposed plant material) and not for mineral soils. They have only 

the following suffixes: co, di, or ma (described below). They do not have 

the subordinate distinctions of the other master horizons and layers.

V Horizons

V horizons are mineral horizons that formed at the soil surface 

or below a layer of rock fragments (e.g., desert pavement), a physical 

or biological crust, or recently deposited eolian material. They are 

characterized by the predominance of vesicular pores and have platy, 

prismatic, or columnar structure.

Porosity in a V horizon may include vughs and collapsed vesicles 

in addition to the spherical vesicular pores. V horizons formed in eolian 

material but may be underlain by soil horizons that formed in residuum, 

alluvium, or other transported materials. Because of their eolian origin, 

they are typically enriched in particle-size fractions ranging from 

silt through fine sand. Rarely, the V horizon is massive rather than

structured. The structural arrangement of particles and vesicular porosity 

differentiates this horizon from the loose, unaltered eolian deposits that 

may occur above it. Underlying B horizons commonly have redder hues 

than the V horizon and lack vesicular pores (Turk et al., 2011).

Transitional and combination horizons with V horizon material 

occur in certain circumstances. Although uncommon, an AV or VA 

horizon may occur. It is both enriched in organic matter and contains 

vesicular pores. BV or VB horizons may indicate vesicular horizons that 

contain clay or carbonate coatings, or other properties of the underlying 

B horizon. EV or VE transitional horizons may also occur, especially in 

sodic soils. 

Combination horizons of the V horizon with A, B, or E horizons may 

occur in bioturbated zones, such as shrub islands or areas where surface 
Soil Survey Manual 

cover associated with the vesicular horizon (e.g., desert pavement) 

is patchy. Vesicular pores have been observed to reform quickly after 

physical disruption (Yonovitz and Drohan, 2009).

A Horizons

A horizons are mineral horizons that formed at the soil surface or 

below an O horizon. They exhibit obliteration of all or much of any 

original rock structure and show one or both of the following: 

1. An accumulation of humified organic matter closely mixed

with the mineral fraction and not dominated by properties 

characteristic of V, E, or B horizons; and/or

2. Properties resulting from cultivation, pasturing, or similar kinds 

of disturbance. 

If a surface horizon has properties of both A and E horizons but the 

feature emphasized is an accumulation of humified organic matter, it is

designated as an A horizon. Recent alluvial or eolian deposits that retain 

most of the original rock structure are not considered to be A horizons 

unless they are cultivated.

E Horizons

E horizons are mineral horizons in which the main feature is the 

eluvial loss of silicate clay, iron, aluminum, or some combination of 

these that leaves a concentration of sand and silt particles. They exhibit 

obliteration of all or much of the original rock structure.

An E horizon is commonly differentiated from an underlying B 

horizon in the same sequum by a color of higher value or lower chroma 

(or both), by coarser texture, or by a combination of these properties. 

In some soils the color of the E horizon is that of the sand and silt 

particles, but in many soils coatings of iron oxides or other compounds 

mask the color of the primary particles. An E horizon is most commonly 

differentiated from an overlying A horizon by its lighter color. It generally 

contains less organic matter than the A horizon. It is commonly near 

the soil surface, below an O, V, or A horizon, and above a B horizon. 

However, the symbol E can be used for eluvial horizons that are at the 

soil surface, are within or between parts of the B horizon, or extend to 

depths greater than those of normal observation, if the horizons have 

resulted from pedogenic processes.

B Horizons

B horizons are mineral horizons that typically formed below an A, V, 

E, or O horizon. They exhibit obliteration of all or much of the original 
96 Chapter 

rock structure and show one or more of the following as evidence of 

pedogenesis:

1. Illuvial concentration of silicate clay, iron, aluminum, humus, 

sesquioxides, carbonates, gypsum, salts more soluble than 

gypsum, or silica, alone or in combination;

2. Evidence of the removal, addition, or transformation of 

carbonates, anhydrite, and/or gypsum; 

3. Residual concentration of oxides, sesquioxides, and silicate clay, 

alone or in combination; 

4. Coatings of sesquioxides that make the horizon color conspicuously lower in value, higher in chroma, or redder in hue than 

overlying and underlying horizons, without apparent illuviation 

of iron;

5. Alteration that forms silicate clay or liberates oxides, or both, and 

that forms pedogenic structure if volume changes accompany 

changes in moisture content; 

6. Brittleness; or

7. Strong gleying when accompanied by other evidence of pedogenic change.

All of the different kinds of B horizons are, or originally were, 

subsurface horizons. B horizons include horizons (cemented or not 

cemented) with illuvial concentrations of carbonates, gypsum, or silica 

that are the result of pedogenic processes. They are contiguous to other 

genetic horizons and brittle layers that show other evidence of alteration, 

such as prismatic structure or illuvial accumulation of clay.

B horizons do not include layers in which clay films coat rock

fragments or cover finely stratified unconsolidated sediments, regardless

of whether the films formed in place or by illuviation; layers into which

carbonates have been illuviated but that are not contiguous to an overlying 

genetic horizon; and layers with strong gleying but no other pedogenic 

changes.

C Horizons or Layers

C horizons or layers are mineral horizons or layers, excluding 

strongly cemented and harder bedrock, that are little affected by 

pedogenic processes and lack properties of O, A, V, E, B, and L horizons. 

Their material may be either like or unlike that from which the solum 

presumably formed. The C horizon may have been modified, even if

there is no evidence of pedogenesis.

Included as C layers (and typically designated Cr) are sediment, 

saprolite, bedrock, and other geologic materials that are moderately 
Soil Survey Manual 

cemented or less cemented (see table 3-7). The excavation difficulty

of these materials commonly is low or moderate (see table 3-14). In

descriptions of soils that formed in material that is already highly 

weathered, if this material does not meet the requirements of an A, V, E, 

or B horizon, it is designated by the letter C. Changes are not considered 

pedogenic if they are not related to the overlying horizons. Some 

layers that have accumulations of silica, carbonates, gypsum, or more 

soluble salts are included in C horizons, even if cemented. However, 

if a cemented layer formed through pedogenic processes, rather than 

geologic processes (e.g., lithification), it is considered a B horizon.

R Layers

R layers consist of strongly cemented to indurated bedrock. Granite, 

basalt, quartzite, limestone, and sandstone are examples of bedrock that 

commonly is cemented enough to be designated by the letter R. The 

excavation difficulty of these layers commonly exceeds high. The R

layer is sufficiently coherent when moist to make hand-digging with a

spade impractical, although it may be chipped or scraped. Some R layers 

can be ripped with heavy power equipment. The bedrock may have 

fractures, but these are generally too few or too widely spaced to allow 

root penetration. The fractures may be coated or filled with clay or other

material.

M Layers

M layers are root-limiting layers beneath the soil surface consisting 

of nearly continuous, horizontally oriented, human-manufactured 

materials. Examples of materials designated by the letter M include 

geotextile liners, asphalt, concrete, rubber, and plastic, if they occur as 

continuous, horizontal layers.

W Layers

W layers are used to identify water layers within or beneath the soil 

(fig. 3-3). They are not merely layers of saturated soil material but rather

zones of water between soil layers. The water layer is designated “Wf ” 

if it is permanently frozen (as in a glacic horizon) and “W” if it is not 

permanently frozen (as in a floating bog). The designations W and Wf are

not used for shallow water, ice, or snow above the soil surface.

Transitional and Combination Horizons

In some cases a single master horizon designation does not adequately 

convey information about the layer, such as where the horizon transitions 
98 Chapter 

Figure 3-3

A soil (a Glacistel in Alaska) with a permanently frozen ice layer (designated “Wf”) 

between depths of 60 and 130 cm. (Photo courtesy of John Kelley)

to another layer or where it contains distinct parts from two kinds of 

master horizons.

Transitional Horizons

Transitional horizons are dominated by properties of one master 

horizon but have subordinate properties of another. They are designated 

by two capital-letter symbols, e.g., AB, EB, BE, or BC. The first letter

indicates the horizon whose properties dominate the transitional horizon. 

An AB horizon, for example, has characteristics of both an overlying A 

horizon and an underlying B horizon, but it is more like the A horizon 

than the B.
Soil Survey Manual 

In some cases, a horizon can be designated as transitional even if one 

of the master horizons to which it transitions is not present. For example, 

a BE horizon recognized in a truncated soil has properties similar to 

those of a BE horizon in a soil from which the overlying E horizon has 

not been removed by erosion. A BC horizon may be recognized even 

if no underlying C horizon is present: it transitions to assumed parent 

materials.

Combination Horizons

Combination horizons have two distinct parts that have recognizable 

properties of the two kinds of master horizons. They are designated 

by two capital-letter symbols (master horizons) separated by a virgule 

(/), e.g., E/B, B/E, or B/C. Most of the individual parts of one horizon 

component are surrounded by the other. The designation may be used 

even when horizons similar to one or both of the components are not 

present, provided that the separate components can be recognized in 

the combination horizon. The first letter indicates the horizon with the

greater volume. 

Because single sets of designators do not cover all situations, some 

improvising may be necessary. For example, Lamellic Udipsamments 

have lamellae that are separated from each other by eluvial layers. It is 

generally not practical to describe each lamella and eluvial layer as a 

separate horizon, so the horizons can be combined and the components 

described separately. The horizon with several lamellae and eluvial 

layers can be designated as an “E and Bt” horizon. The complete horizon 

sequence for these soils could be: Ap-Bw-E and Bt1-E and Bt2-C.

Suffix Symbols

Lowercase letters are used assuffixesto designate specific subordinate 

distinctions within master horizons and layers. The term “accumulation,” 

which is used in many of the suffix definitions, indicates that the horizon

has more of the material in question than is presumed to have been 

present in the parent material. The use of a suffix symbol is not restricted

only to those horizons that meet certain criteria for diagnostic horizons 

and other criteria as defined in Soil Taxonomy. If there is any evidence of 

accumulation, a suffix (or suffixes) can be used. The suffix symbols and

their meanings follow:

a Highly decomposed organic material

This symbol is used with O horizons to indicate the most 

highly decomposed organic materials, which have a fiber

content of less than 17 percent (by volume) after rubbing.
100 Chapter 

b Buried genetic horizon

This symbol indicates identifiable buried horizons with

major genetic features that developed before burial. Genetic 

horizons may or may not have formed in the overlying 

material, which may be either like or unlike the assumed 

parent material of the buried horizon. This symbol is not 

used to separate horizons composed of organic soil material 

(that are forming at the soil surface) from underlying 

horizons composed of mineral soil material. It may be used 

in organic soils, but only if they are buried by mineral soil 

materials.

c Concretions or nodules

This symbol indicates a significant accumulation of concretions or nodules. Cementation is required. The cementing 

agent commonly is iron, aluminum, manganese, or titanium. 

It cannot be silica, dolomite, calcite, gypsum, anhydrite, or 

soluble salts.

co Coprogenous earth

This symbol, used only with L horizons, indicates a limnic 

layer of coprogenous earth (sedimentary peat). 

d Physical root restriction

This symbol indicates non-cemented, root-restricting layers in naturally occurring or human-made sediments or 

materials. Examples of natural layers are dense till and 

some non-cemented shales and siltstones. Examples of 

human-made dense layers are plowpans and mechanically 

compacted zones in human-transported material.

di Diatomaceous earth

This symbol, used only with L horizons, indicates a limnic 

layer of diatomaceous earth.

e Organic material of intermediate decomposition

This symbol is used with O horizons to indicate organic 

materials of intermediate decomposition. The fiber content

of these materials is 17 to less than 40 percent (by volume) 

after rubbing.

f Frozen soil or water

This symbol indicates that a horizon or layer contains 

permanent ice. It is not used for seasonally frozen layers or 

for dry permafrost. 
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ff Dry permafrost

This symbol indicates a horizon or layer that is continually 

colder than 0 °C and does not contain enough ice to be 

cemented by ice. It is not used for horizons or layers that 

have a temperature warmer than 0 °C at some time during 

the year.

g Strong gleying

This symbol indicates either that iron has been reduced 

and removed during soil formation or that saturation with 

stagnant water has preserved iron in a reduced state. Most 

of the affected layers have chroma of 2 or less, and many 

have redox concentrations. The low chroma can represent 

either the color of reduced iron or the color of the uncoated 

sand and silt particles from which iron has been removed. 

The symbol is not used for soil materials of low chroma 

that have no history of wetness, such as some shales or E 

horizons. If it is used with B horizons, pedogenic change 

(e.g., soil structure) in addition to gleying is implied. If no 

other pedogenic change besides gleying has taken place, the 

horizon is designated “Cg.”

h Illuvial accumulation of organic matter

This symbol is used with B horizons to indicate the accumulation of illuvial, amorphous, dispersible complexes of 

organic matter and sesquioxides. The sesquioxide component is dominated by aluminum and is present only in very 

small quantities. The organo-sesquioxide material coats 

sand and silt particles. In some horizons these coatings 

have coalesced, filled pores, and cemented the horizon.

The symbol h is also used in combination with the symbol 

s (e.g., Bhs) if the amount of the sesquioxide component is 

significant but the value and chroma, moist, of the horizon

are 3 or less.

i Slightly decomposed organic material

This symbol is used with O horizons to indicate the least 

decomposed of the organic materials. The fiber content

of these materials is 40 percent or more (by volume) after 

rubbing.

j Accumulation of jarosite

This symbol indicates an accumulation of jarosite, which 

is a potassium (ferric) iron hydroxy sulfate mineral, 
102 Chapter 

KFe3(SO4)2(OH)6. Jarosite is commonly the product of 

pyrite that has been exposed to an oxidizing environment. 

It has hue of 2.5Y or yellower and normally has chroma 

of 6 or more, although chroma as low as 3 or 4 has been

reported. It forms in preference to iron (hydr)oxides in 

active acid sulfate soils at pH of 3.5 or less and can be stable

for long periods of time in post-active acid sulfate soils with 

higher pH.

jj Evidence of cryoturbation

This symbol indicates evidence of cryoturbation, which 

includes irregular and broken horizon boundaries, sorted 

rock fragments, and organic soil materials occurring as 

bodies and broken layers within and/or between mineral soil 

layers. The organic bodies and layers are most commonly at 

the contact between an active layer and the permafrost.

k Accumulation of secondary carbonates

This symbol indicates an accumulation of visible pedogenic 

calcium carbonate (less than 50 percent, by volume). Carbonate accumulations occur as carbonate filaments, coatings, masses, nodules, disseminated carbonate, or other 

forms.

kk Engulfment of horizon by secondary carbonates

This symbol indicates major accumulations of pedogenic 

calcium carbonate. It is used when the soil fabric is plugged 

with fine grained pedogenic carbonate (50 percent or more,

by volume) that occurs as an essentially continuous medium. 

It corresponds to the Stage III (or higher) plugged horizon of 

the carbonate morphogenetic stages (Gile et al., 1966).

m Pedogenic cementation

This symbol indicates continuous or nearly continuous 

pedogenic cementation. It is used only for horizons that 

are more than 90 percent cemented but may be fractured. 

The cemented layer is physically root-restrictive. The 

predominant cementing agent (or the two dominant ones) 

can be indicated by letter suffixes, singly or in pairs. The

horizon suffix kkm (and the less commonly used km) indicates 

cementation by carbonates; qm, cementation by silica; sm, 

cementation by iron; yym, cementation by gypsum; kqm, 

cementation by carbonates and silica; and zm, cementation 
Soil Survey Manual 1

by salts more soluble than gypsum. The symbol m is not 

used for permanently frozen layers impregnated by ice.

ma Marl

This symbol, used only with L horizons, indicates a limnic 

layer of marl.

n Accumulation of sodium

This symbol indicates an accumulation of exchangeable sodium.

o Residual accumulation of sesquioxides

This symbol indicates a residual accumulation of sesquioxides.

p Tillage or other disturbance

This symbol indicates disturbance of a horizon by mechanical means, pasturing, or similar uses. A disturbed organic 

horizon is designated “Op.” A disturbed mineral horizon is 

designated “Ap” even if it is clearly a former E, B, or C 

horizon.

q Accumulation of silica

This symbol indicates an accumulation of secondary silica.

r Weathered or soft bedrock

This symbol is used with C horizons to indicate layers of 

bedrock that are moderately cemented or less cemented. 

Examples are weathered igneous rock and partly consolidated 

sandstone, siltstone, or shale. The excavation difficulty is

low to high.

s Illuvial accumulation of sesquioxides and organic matter

This symbol is used with B horizons to indicate an 

accumulation of illuvial, amorphous, dispersible complexes 

of organic matter and sesquioxides, if both the organic matter 

and sesquioxide components are significant and if either the

value or chroma, moist, of the horizon is 4 or more. The 

symbol is also used in combination with h (e.g., Bhs) if 

both the organic matter and sesquioxide components are 

significant and if the value and chroma, moist, are 3 or less.

se Presence of sulfides

This symbol indicates the presence of sulfides in mineral or

organic horizons. Horizons with sulfides typically have dark
104 Chapter 

colors (e.g., value of 4 or less, chroma of 2 or less). These 

horizons typically form in soils associated with coastal 

environments that are permanently saturated or submerged 

(i.e., tidal marshes or estuaries). Soil materials which have 

sulfidization actively occurring emanate hydrogen sulfide

gas, which is detectable by its odor (Fanning and Fanning, 

1989; Fanning et al., 2002). Sulfides may also occurin upland

environments that have a source of sulfur. Soils in such 

environments are commonly of geologic origin and may not 

have a hydrogen sulfide odor. Examples include soils that

formed in parent materials derived from coal deposits, such 

as lignite, or soils that formed in coastal plain deposits, such 

as glauconite, that have not been oxidized because of thick 

layers of overburden.

ss Presence of slickensides

This symbol indicates the presence of pedogenic slickensides. Slickensides result directly from the swelling of clay 

minerals and shear failure, commonly at angles of 20 to 60 

degrees above horizontal. They are indicators that other 

vertic characteristics, such as wedge-shaped peds and surface cracks, may be present.

t Accumulation of silicate clay

This symbol indicates an accumulation of silicate clay that 

either has formed within a horizon and subsequently has 

been translocated within the horizon or that has been moved 

into the horizon by illuviation, or both. At least some part of 

the horizon shows evidence of clay accumulation, either as 

coatings on surfaces of peds or in pores, as lamellae, or as 

bridges between mineral grains.

u Presence of human-manufactured materials (artifacts)

This symbol indicates the presence of objects or materials 

that have been created or modified by humans, typically for

a practical purpose in habitation, manufacturing, excavation, 

or construction activities. Examples of artifacts are bitumen 

(asphalt), boiler slag, bottom ash, brick, cardboard, carpet, 

cloth, coal combustion by-products, concrete (detached 

pieces), debitage (i.e., stone tool flakes), fly ash, glass, metal,

paper, plasterboard, plastic, potsherd, rubber, treated wood, 

and untreated wood products.
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v Plinthite

This symbol is used to indicate the presence of iron-rich, 

humus-poor, reddish material that is firm or very firm when

moist and is less than strongly cemented. Plinthite hardens 

irreversibly when exposed to the atmosphere and to repeated 

wetting and drying. 

w Development of color or structure

This symbol is used only with B horizons to indicate the 

development of color or structure, or both, with little or no 

apparent illuvial accumulation of material. Note: It is not 

used to indicate a transitional horizon.

x Fragipan character

This symbol indicates a genetically developed layer that has 

a combination of firmness and brittleness and commonly a

higher bulk density than adjacent layers. Some part of the 

layer is physically root-restrictive.

y Accumulation of gypsum

This symbol indicates an accumulation of gypsum. It is used 

when the horizon fabric is dominated by soil particles or 

minerals other than gypsum. Gypsum is present in amounts 

that do not significantly obscure or disrupt other features of

the horizon. This symbol is also used to indicate the presence 

of anhydrite.

yy Dominance of horizon by gypsum

This symbol indicates a horizon that is dominated by the 

presence of gypsum. The gypsum content may be due to 

an accumulation of secondary gypsum, the transformation 

of primary gypsum inherited from parent material, or other 

processes. This symbol is used when the horizon fabric 

has such an abundance of gypsum (generally 50 percent or 

more, by volume) that pedogenic and/or lithologic features 

are obscured or disrupted by growth of gypsum crystals. 

Horizons that have this suffix typically are highly whitened

(e.g., value of 7 through 9.5 and chroma of 4 or less). This 

symbol is also used to connote the presence of anhydrite.

z Accumulation of salts more soluble than gypsum

This symbol indicates an accumulation of salts that are more 

soluble than gypsum.
106 Chapter 

Conventions for Using Horizon Designation Symbols

The following guidelines can be used in assigning horizon designation 

symbols to soil horizons and layers.

Letter Suffixes

Many master horizons and layers that are symbolized by a single 

capital letter can have one or more lowercase-letter suffixes. The

following rules apply:

1. Letter suffixes directly follow the capital letter of the master

horizon or layer, or the prime symbol, if used.

2. More than three suffixes are rarely used.

3. If more than one suffix is needed, the following letters (if used)

are written first: a, d, e, h, i, r, s, t, and w. None of these letters 

are used in combination for a single horizon, except to designate 

a Bhs horizon or Crt layer. 

4. If more than one suffix is needed and the horizon is not buried,

the following symbols, if used, are written last: c, f, g, m, v, and

x. Examples are Bjc and Bkkm. If any of these suffixes are used

together in the same horizon, symbols c and g are written last 

(e.g., Btvg), with one exception. If the symbol f (frozen soil or 

water) is used together with any of the other symbols in this rule, 

it is written last, e.g., Cdgf. 

5. If a genetic horizon is buried, the suffix b is written last, e.g., 

Oab. 

6. Suffix symbols h, s, and w are not used with g, k, kk, n, o, q, y, 

yy, or z.

7. If the above rules do not apply to certain suffixes, such as k, kk, 

q, y, or yy, the suffixes may be listed together in order of assumed

dominance or alphabetically if dominance is not a concern.

A B horizon that has a significant accumulation of clay and also

shows development of color or structure, or both, is designated “Bt” 

(suffix symbol t has precedence over symbols w, s, and h). A B horizon 

that is gleyed or that has accumulations of carbonates, sodium, silica, 

gypsum, salts more soluble than gypsum, or residual accumulations of 

sesquioxides carries the appropriate symbol: g, k, kk, n, q, y, yy, z, or o. 

If illuvial clay is also present, the symbol t precedes the other symbol, 

e.g., Bto.

Vertical Subdivisions

Commonly, a horizon or layer designated by a single letter or a 

combination of letters has to be subdivided. For this purpose, numbers 
Soil Survey Manual 1

are added to the letters of the horizon designation. These numbers follow 

all the letters. Within a sequence of C horizons, for example, successive 

layers may be designated C1, C2, C3, etc. If the lower horizons are

strongly gleyed and the upper horizons are not strongly gleyed, they may 

be designated C1-C2-Cg1-Cg2 or C-Cg1-Cg2-R.

These conventions apply regardless of the purpose of the subdivision. 

In many soils a horizon that could be identified by a single set of letters

is subdivided to recognize differences in morphological features, such as 

structure, color, or texture. These divisions are numbered consecutively, 

but the numbering starts again at 1 when any letter of the horizon 

symbol changes, e.g., Bt1-Bt2-Btk1-Btk2 (not Bt1-Bt2-Btk3-Btk4).

The numbering of vertical subdivisions within consecutive horizons is 

not interrupted at a discontinuity (indicated by a numerical prefix) if the

same letter combination is used in both materials, e.g., Bs1-Bs2-2Bs3-

2Bs4 (not Bs1-Bs2-2Bs1-2Bs2).

During sampling for laboratory analyses, thick soil horizons are 

sometimes subdivided even though differences in morphology are 

not evident in the field. These subdivisions are identified by numbers

that follow the respective horizon designations. For example, four 

subdivisions of a Bt horizon sampled by 10-cm increments are designated 

Bt1, Bt2, Bt3, and Bt4. If the horizon has already been subdivided

because of differences in morphological features, the set of numbers that 

identifies the additional sampling subdivisions follows the first number.

For example, three subdivisions of a Bt2 horizon sampled by 10-cm 

increments are designated Bt21, Bt22, and Bt23. The descriptions for

each of these sampling subdivisions can be the same, and a statement 

indicating that the horizon has been subdivided only for sampling 

purposes can be added.

Discontinuities

Numbers are used as prefixes to horizon designations (specifically,

A, V, E, B, C, and R) to indicate discontinuities in mineral soils. These 

prefixes are distinct from the numbers that are used as suffixes denoting

vertical subdivisions.

A discontinuity that can be identified by a number prefix is a

significant change in particle-size distribution or mineralogy that

indicates a difference in the parent material from which the horizons have 

formed and/or a significant difference in age, unless the difference in

age is indicated by the suffix b. Symbols that identify discontinuities are 

used only when they can contribute substantially to an understanding of 

the relationships among horizons. The stratification common to soils that

formed in alluvium is not designated as a discontinuity, unless particle-
108 Chapter 

size distribution differs markedly from layer to layer (i.e., particle-size 

classes are strongly contrasting) even though genetic horizons may have 

formed in the contrasting layers.

If a soil formed entirely in one kind of material, the whole profile

is understood to be material 1 and the number prefix is omitted from

the symbol. Similarly, the uppermost material in a profile consisting of

two or more contrasting materials is understood to be material 1 and the 

number is omitted. Numbering starts with the second layer of contrasting 

material, which is designated 2. Underlying contrasting layers are 

numbered consecutively. Even when the material of a layer below 

material 2 is similar to material 1, it is designated 3 in the sequence; the

numbers indicate a change in materials, not types of material. Where two 

or more consecutive horizons have formed in the same kind of material, 

the same prefix number indicating the discontinuity is applied to all the

designations of horizons in that material, for example, Ap-E-Bt1-2Bt2-

2Bt3-2BC. The suffix numbers designating verticalsubdivisions of the Bt

horizon continue in consecutive order across the discontinuity. However, 

vertical subdivisions do not continue across lithologic discontinuities if 

the horizons are not consecutive or contiguous to each other. If other 

horizons intervene, another vertical numbering sequence begins for the 

lower horizons, for example, A-C1-C2-2Bw1-2Bw2-2C1-2C2. 

If an R layer is below a soil that formed in residuum and if it is similar 

to the material from which the soil developed, the number prefix is not

used. The prefix is used, however, if it is thought that the R layer would

weather to material unlike that in the solum, e.g., A-Bt-C-2R or A-Bt-2R. 

If part of the solum has formed in residuum, the symbol R is given the 

appropriate prefix, for example, Ap-Bt1-2Bt2-2Bt3-2C1-2C2-2R.

A buried genetic horizon (designated by the suffix b) requires special 

consideration. It is obviously not in the same deposit as the overlying 

horizons. Some buried horizons, however, formed in material that is 

lithologically like the overlying deposit. In this case, a prefix is not used

to distinguish material of the buried horizon. If the material in which 

a horizon of a buried soil formed is lithologically unlike the overlying 

material, the discontinuity is indicated by a number prefix and the

symbol for the buried horizon also is used, for example, Ap-Bt1-Bt2-

BC-C-2ABb-2Btb1-2Btb2-2C.

Discontinuities between different kinds of layers in organic soils 

are not identified. In most cases, such differences are identified by letter

suffixes if the different layers are organic materials (e.g., Oe vs. Oa) or

by the master horizon symbol if the different layers are mineral or limnic 

materials (e.g., Oa vs. Ldi).
Soil Survey Manual 1

The Prime Symbol

If two or more horizons with identical number prefixes and letter

combinations are separated by one or more horizons with a different 

horizon designation, identical letter and number symbols can be used for 

those horizons with the same characteristics. For example, the sequence 

A-E-Bt-E-Btx-C identifies a soil that has two E horizons. To emphasize

this characteristic, the prime symbol ( ́) is added after the symbol of the 

lower of the two horizons that have identical designations, e.g., A-E-BtE ́-Btx-C. The prime symbol is placed after the master horizon symbol 

and before the suffix letter symbol or symbols (if used), for example, B ́t.

The prime symbol is not used unless all letter and number prefixes

are completely identical. The sequence A-Bt1-Bt2-2E-2Bt1-2Bt2 is an 

example. Because it has two Bt master horizons of different lithologies, 

the Bt horizons are not identical and the prime symbol is not needed. The 

prime symbol is used for soils with lithologic discontinuities if horizons 

have identical designations. For example, a soil with the sequence A-C2Bw-2Bc-2B ́w-3Bc has two identical 2Bw horizons but two different

Bc horizons (2Bc and 3Bc); the prime symbol is used only with the lower

2Bw horizon (2B ́w). In the rare cases where three layers have identical 

letter symbols, double prime symbols can be used for the lowest of these 

horizons, for example, E ́ ́.

Vertical subdivisions of horizons or layers (number suffixes) are not

taken into account when the prime symbol is assigned. The sequence 

A-E-Bt-E ́-B ́t1-B ́t2-B ́t3-C is an example.

These same principles apply in designating layers of organic soils. 

The prime symbol is used only to distinguish two or more horizons that 

have identical symbols. For example, Oi-C-O ́i-C ́ indicates a soil with 

two identical Oi and C layers and Oi-C-Oe-C ́ indicates a soil with two 

identical C layers. The prime symbol is added to the lower layers to 

differentiate them from the upper layers. 

The Caret Symbol

The caret symbol (^) is used as a prefix to indicate horizons and

layers that formed in human-transported material. This material has 

been moved horizontally onto a pedon from a source area outside of that 

pedon by purposeful human activity, usually with the aid of machinery 

or hand tools. Number prefixes may be used before the caret symbol

to indicate the presence of discontinuities within the human-transported 

material (e.g., ^Au-^Bwu-^BCu-2^Cu1-2^Cu2) or between the humantransported material and underlying horizons formed in other parent 

materials (e.g., ^A-^C1-2^C2-3Bwb).
110 Chapter 

Sample Horizons and Sequences

The following examples illustrate some common horizon and layer 

sequences of important soils (subgroup taxa) and the use of numbers to 

identify vertical subdivisions and discontinuities. Transitional horizons, 

combination horizons, and the use of the prime and caret symbols are 

also illustrated. 

Mineral Soils

Typic Hapludoll: A1-A2-Bw-BC-C 

Typic Haplustoll: Ap-A-Bw-Bk-Bky1-Bky2-C 

Cumulic Haploxeroll: Ap-A-Ab-C-2C-3C

Typic Argialboll: Ap-A-E-Bt1-Bt2-BC-C 

Typic Argiaquoll: A-AB-BA-Btg-BCg-Cg 

Alfic Udivitrand: Oi-A-Bw1-Bw2-2E/Bt-2Bt/E1-2Bt/E2-2Btx1-

2Btx2 

Entic Haplorthod: Oi-Oa-E-Bs1-Bs2-BC-C 

Typic Haplorthod: Ap-E-Bhs-Bs-BC-C1-C2 

Typic Fragiudalf: Oi-A-E-BE-Bt1-Bt2-B/E-Btx1-Btx2-C 

Typic Haploxeralf: A1-A2-BAt-2Bt1-2Bt2-2Bt3-2BC-2C

Glossic Hapludalf: Ap-E-B/E-Bt1-Bt2-C 

Typic Paleudult: A-E-Bt1-Bt2-B/E-B ́t1-B ́t2-B ́t3

Typic Hapludult: Oi-A1-A2-BA-Bt1-Bt2-BC-C 

Arenic Plinthic Paleudult: Ap-E-Bt-Btc-Btv1-Btv2-BC-C 

Xeric Haplodurid: A-Bw-Bkq-2Bkqm 

Vertic Natrigypsid: A-Btn-Btkn-Bky-2By-2BCy-2Cr 

Typic Calciargid: A-Bt-Btk1-Btk2-C 

Typic Dystrudept: Ap-Bw1-Bw2-C-R 

Typic Fragiudept: Ap-Bw-E-Bx1-Bx2-C 

Typic Endoaquept: Ap-AB-Bg1-Bg2-BCg-Cg 

Typic Haplustert: Ap-A-Bss-BCss-C 

Typic Hapludox: Ap-A/B-Bo1-Bo2-Bo3-Bo4-Bo5

Typic Udifluvent: Ap-C-Ab-C ́

Glacic Histoturbel: Oi-OA-Bjjg-Wf-Cgf 

Organic Soils

Typic Haplosaprist: Oap-Oa1-Oa2-Oa3-C

Typic Sphagnofibrist: Oi1-Oi2-Oi3-Oe

Limnic Haplofibrist: Oi-Lco-O ́i1-O ́i2-L ́co-Oe-C

Lithic Cryofolist: Oi-Oa-R 

Typic Hemistel: Oi-Oe-Oef 
Soil Survey Manual 1

Human-Altered Soils

Anthrodensic Ustorthent: ^Ap-^C/B-^Cd-2C 

Anthroportic Udorthent: ^Ap-^Cu-Ab-Btb-C 

Subaqueous Soils

Psammentic Frasiwassents: A1-A2-CA-Cg1-Cg2-Cg3-Cg4

Thapto-Histic Sulfiwassents: Ase-Cse1-Cse2-Oase1-Oa1-Oa2

Sulfic Psammowassents: A-Cg1-Cg2-Aseb-C ́g-A ́seb-C ́ ́g1-

C ́ ́g2-C ́ ́g3

Cyclic and Intermittent Horizons and Layers

Soils with cyclic or intermittent horizons pose special challenges 

in describing soil profiles. The profile of a soil having cyclic horizons

exposes layers whose boundaries are near the surface at one point and 

extend deep into the soil at another. The aggregate horizon thickness may 

be only 50 cm at one place but more than 125 cm at a place 2 meters 

away. The cycle repeats. It commonly has considerable variation in both 

depth and horizontal interval but still has some degree of regularity. 

When the soil is visualized in three dimensions instead of two, some 

cyclic horizons extend downward in inverted cones. The cone of the 

lower horizon fits around the cone of the horizon above. Other cyclic

horizons appear wedge-shaped.

The profile of a soil having an intermittent horizon shows that the

horizon extends horizontally for some distance, ends, and reappears 

again some distance away. For example, the horizons of Turbels, which 

by definition are subject to cryoturbation, are irregular, intermittent, and

distorted. A B horizon interrupted at intervals by upward extensions of 

bedrock into the A horizon is another example. The distance between 

places where the horizon is absent is commonly variable but has some 

degree of regularity. It ranges from less than 1 meter to several meters.

For soils with cyclic or intermittent horizons or layers, a soil profile

at one place may be unlike a profile only a few meters away. Standardized

horizon nomenclature and pedon description forms are not well suited to 

soil profiles with such variability. When describing these types of soils, it

is important to make notes on the individual horizons to record the nature 

of the variations. Photographs and diagrams can also be used to convey 

the information. Descriptions of the order of horizontal variation as well 

as vertical variation within a pedon include the kind of variation, the 

spacing of cycles or interruptions, and the amplitude of depth variation 

of cyclic horizons.
112 Chapter 

Boundaries of Horizons and Layers

A boundary is a relatively sharp plane-like division or a more 

gradual transitional layer between two adjoining horizons or layers. Most 

boundaries are zones of transition rather than sharp lines of division. 

Boundaries vary in distinctness and topography.

Distinctness

Distinctness refers to the thickness of the zone within which the 

boundary can be located. The distinctness of a boundary depends partly 

on the degree of contrast between the adjacent layers and partly on the 

thickness of the transitional zone between them. Distinctness is defined

in terms of thickness of the transitional zone as follows:

Very abrupt ............ less than 0.5 cm

Abrupt .................... 0.5 to less than 2 cm

Clear ...................... 2 to less than 5 cm

Gradual .................. 5 to less than 15 cm

Diffuse ................... 15 cm or more

Very abrupt boundaries occur at some lithologic discontinuities, such 

as geogenic deposits or strata (tephras, alluvial strata, etc.). They can 

also occur at the contacts of root-limiting layers. Examples are duripans; 

fragipans; petrocalcic, petrogypsic, and placic horizons; continuous 

ortstein; and densic, lithic, paralithic, and petroferric contacts. See 

Soil Taxonomy (Soil Survey Staff, 1999) for more information and 

definitions.

Abrupt soil boundaries, such as those between the E and Bt horizons 

of many soils, are easily determined. Some boundaries are not readily 

seen but can be located by testing the soil above and below the boundary. 

Diffuse boundaries, such as those in many old soils in tropical areas, 

are very difficult to locate. They require time-consuming comparisons

of small specimens of soil from various parts of the profile to determine

the midpoint of the transitional zone. For soils that have nearly uniform 

properties or that change very gradually as depth increases, horizon 

boundaries are imposed more or less arbitrarily without clear evidence 

of differences. 

Topography

Topography refers to the irregularities of the surface that divides the 

horizons(fig. 3-4). Termsfor topography describe the shape of the contact

between horizons as seen in a vertical cross-section. Even though soil 

layers are commonly seen in vertical section, they are three-dimensional. 

Terms describing topography of boundaries are:
Soil Survey Manual 1

Smooth.—The boundary is a plane with few or no irregularities.

Wavy.—The boundary has undulations in which depressions are 

wider than they are deep.

Irregular.—The boundary has pockets that are deeper than they are 

wide.

Broken.—One or both of the horizons or layers separated by the 

boundary are discontinuous and the boundary is interrupted.

Figure 3-4

A

B

Irregular

A

B

Wavy

A

B 

Smooth

Bt

B

A

E

Broken

Examples of topography classes for horizon boundaries (adapted from Schoeneberger 

et al., 2012).

Thickness

The thickness of the horizon or layer is recorded by entering depths 

for the upper and lower boundaries. For horizons or layers with significant

lateral variation in thickness, the average horizon thickness may also be 

noted.
114 Chapter 

Near Surface Subzones

Background Information

In many soils, the morphology of the uppermost few centimeters 

(generally from less than 1 to about 18 cm) is strongly controlled by 

antecedent weather and by soil use. A soil may be freshly tilled and have 

a loose surface one day and have a strong crust because of a heavy rain 

the next day. A soil may be highly compacted by livestock and have a 

firm near surface in one place but have little disturbance to the uppermost

few centimeters and be very friable in most other places. These affected 

soils properties are referred to as “use-dependent” or “dynamic.” See 

chapter 9 for information about studying dynamic soil properties in the 

field.

The following discussion provides a set of terms for describing 

subzones of the near surface and, in particular, the near surface of tilled 

soils. The horizon designations or symbols for describing these near 

surface subzones are limited. The suffix d is used for root-restrictive 

compacted layers; master horizon symbol V may be used to designate 

some layers with a dominance of vesicular pores. Surface horizons can be 

subdivided using standard horizon designations to record the subzones. 

An example horizon sequence could include Ap1 (a mechanically bulked 

subzone), Ap2 (a water-compacted subzone), and Bd (a mechanically 

compacted subzone). Descriptions of these separations should also 

identify the kind of subzone described. Very thin surface crusts (less than 

about 1 cm thick) are generally described as a special surface feature 

rather than as a separate layer. 

Kinds of Near Surface Subzones

In this section, five kinds of near surface subzones are presented and

the general processes leading to their formation are described. The five

kinds of subzones are: mechanically bulked, mechanically compacted, 

water compacted, surficial bulked, and crust (either biological or chemical). 

Figure 3-5 shows stylized profiles depicting various combinations of

these subzones.

Identification of subzones is not clear cut. Morphological expression

of bulking and compaction may be quite different among soils depending 

on particle-size distribution, organic matter content, clay mineralogy, 

water regime, or other factors.

The distinction between a bulked and compacted state for soil 

material with appreciable shrink-swell potential is partly based on the 
Soil Survey Manual 1

Figure 3-5

Surficial Bulked

Mechanically Bulked

Water Compacted

Crust

Mechanically Compacted

a b c d e

Five kinds of near surface subzones (scale is approximately 18 cm).

potential for the transmission of strain on drying over distances greater 

than the horizontal dimensions of the larger structural units. In a bulked 

subzone, little or no strain is propagated; in a compacted subzone, the 

strain is propagated over distances greater than the horizontal dimensions 

of the larger structural units. Many soils have low shrink-swell potential 

because of texture, clay mineralogy, or both. For these soils, the 

expression of cracks cannot be used to distinguish between a bulked state 

and a compacted state.

The distinction between compaction and bulking is subjective. It is 

useful to establish a concept of a normal degree of compaction of the 

near surface and then compare the actual degree of compaction to this. 

The concept for tilled soils should be the compaction of soil material on 

level or convex parts of the tillage-determined relief. The soil should 

have been subject to the bulking action of conventional tillage without 

the subsequent mechanical compaction. The subzone in question should 

have been brought to a wet or very moist water state from an appreciably 
116 Chapter 

drier condition and then dried to slightly moist or drier at least once. It 

should not have been subject, however, to a large number of wetting and 

drying cycles where the maximum wetness involved the presence of free 

water. If the soil material has a degree of compaction similar to what 

would be expected, then the term normal compaction is used.

Mechanically Bulked Subzone 

The mechanically bulked subzone has undergone, through 

mechanical manipulation, a reduction in bulk density and an increase in 

discreteness of structural units, if present. The mechanical manipulation 

is commonly due to tillage operations. Rupture resistance of the mass 

overall, inclusive of a number of structural units, is typically loose or 

very friable and is occasionally friable. Individual structural units may 

be friable or even firm. Mechanical continuity among structural units 

is low. Structure grade, if the soil material exhibits structural units 

less than 20 mm across, is moderate or strong. Strain that results from 

contraction on drying of individual structural units may not extend across 

the structural units. Hence, internally initiated desiccation cracks may be 

weak or absent even though the soil material in a consolidated condition 

has considerable shrink-swell potential. Cracks may be present, however, 

if they initiate deeper in the soil. The mechanically bulked subzone is 

depicted in figure 3-5 as the first layer in profile a and the second layer

in profiles b and c.

Mechanically Compacted Subzone

The mechanically compacted subzone has been subject to compaction, usually due to tillage operations but also by animals. Commonly, 

mechanical continuity of the fabric and bulk density are increased. Rupture resistance depends on texture and degree of compaction. Generally, 

friable is the minimum class. Mechanical continuity of the fabric permits 

propagation of strain (that results on drying) only over several centimeters. Internally initiated cracks appear if the soil material has appreciable 

shrink-swell potential and drying was sufficient. In some soils this subzone restricts root growth. The suffix d may be used if compaction results 

in a strong plow pan. The mechanically compacted subzone is the lowest 

layer of all profiles shown in figure 3-5.

Water-Compacted Subzone

The water-compacted subzone has been compacted by repetitive 

large changes in water state without mechanical load, except for the 

weight of the soil. Repetitive occurrence of free water is particularly 

conducive to compaction. Depending on texture, moist rupture resistance 
Soil Survey Manual 1

ranges from very friable through firm. Structural units, if present, are 

less discrete than those in the same soil material if mechanically bulked. 

The subzone generally has weak structure or is massive. Mechanical 

continuity of the fabric is sufficient for strain that originates on drying

to propagate appreciable distances. As a consequence, if shrink-swell 

potential is sufficient, cracks develop on drying. In many soils, the watercompacted subzone replaces the mechanically bulked subzone over time. 

The replacement can occur in a single year if the subzone is subject to 

periodic occurrence of free water with intervening periods of being 

slightly moist or dry. The presence of a water-compacted subzone and the 

absence of a mechanically bulked subzone is an important consequence 

of no-till farming systems. The water-compacted subzone is depicted in 

figure 3-5 as the second layer of profiles d and e.

Surficial Bulked Subzone

The surficial bulked subzone occurs in the very near surface.

Continuity of the fabric is low. Cracks are not initiated in this subzone but 

may be present (they may initiate in underlying, more compacted soil). 

The subzone forms by various processes. Frost action under conditions 

where the soil is drier than wet is one process. Pronounced shrinking and 

swelling in response to drying and wetting (which is characteristic of 

Vertisols) is another process. The surficial bulked subzone is depicted in

figure 3-5 as the first layer of profiles c and e.

Crust 

A crust is a surficial subzone, typically less than 50 mm thick but

ranging to as much as 100 mm thick, that exhibits markedly more 

mechanical continuity of the soil fabric than the zone immediately 

beneath. Commonly, the original soil fabric has been reconstituted by 

water action and the original structure has been replaced by a massive 

condition. While the material is wet, raindrop impact (including sprinkler 

irrigation) and freeze-thaw cycles can lead to reconstitution. The crust is 

depicted in figure 3-5 as the first layer of profiles b and d.

Crusts may be described in terms of thickness in millimeters, 

structure and other aspects of the fabric, and consistence, including 

rupture resistance while dry and micropenetration resistance while wet. 

Thickness pertains to the zone where reconstitution of the fabric has 

been pronounced. The distance between surface-initiated cracks (described 

later in this chapter) may be a useful observation for seedling emergence 

considerations. If the distance is short, the weight of the crust slabs is low.

Soil material with little apparent reconstitution commonly adheres 

beneath the crust and is removed with the crust. This soil material, which 
118 Chapter 

shows little or no reconstitution, is not part of the crust and does not 

contribute to the thickness.

Recognized types of soil crusts include biological, chemical and 

structural.

Biological crusts, which consist of algae, lichens, or mosses, occur 

on the surface of some soils, especially in some relatively undisturbed 

settings, such as rangelands. These crusts are easily diminished or 

destroyed by disturbance. 

Chemical crusts commonly occur in arid environments where salty 

evaporites accumulate at the surface. They include crusts consisting of 

mineral grains cemented by salts.

Structural crusts form from local transport and deposition of soil 

material, commonly in tilled fields. They have weaker mechanical

continuity than other crusts. The rupture resistance is lower, and the 

reduction in infiltration may be lessthan that of crusts with similartexture.

Raindrop impact and freeze-thaw cycles contribute to the formation of 

structural crusts.

Root-Restricting Depth

The root-restricting depth is the depth at which physical (including 

soil temperature) and/or chemical characteristics strongly inhibit root 

penetration. Restriction means the incapability to support more than a few 

fine or very fine roots if the depth from the soil surface and the water state 

(other than the occurrence of frozen water) are not limiting. For cotton, 

soybeans, and other crops that have less abundant roots than grasses 

have, the very few class is used instead of the few class. The restriction 

may be below where plant roots normally occur because of limitations in 

water state, temperatures, or depth from the surface. The root-restricting 

depth should be evaluated for the specific plants important to the use

of the soil. These plants are indicated in the soil description. The rootrestriction depth may differ depending on the plant. 

Morphology and Root Restriction

Root-depth observations should be used to make the generalization 

of root-restricting depth. If these are not available (commonly because 

roots do not extend to the depth of concern) then inferences may be 

made from morphology. A change in particle-size distribution alone 

(e.g., loamy sand over gravel) is not typically a basis for physical root 

restriction. Some guidelines for inferring physical restriction are given 
Soil Survey Manual 1

below. Chemical restrictions, such as high levels of extractable aluminum 

and/or low levels of extractable calcium, are not considered; these are 

generally not determinable by field examination alone.

Physical root restriction is assumed:

1. At the contact with bedrock and other continuously cemented 

materials, regardless of the rupture resistance class or thickness; 

2. For certain horizons or layers, such as fragipans or those 

consisting of densic materials, that, although non-cemented, are 

root restrictive by definition; and

3. For layers with a combination of structure, consistence, and/or 

penetration resistance that suggests that the resistance of the soil 

fabric to root entry is high and that vertical cracks and planes of 

weakness for root entry are absent or widely spaced (i.e., more 

than 10 cm apart) as follows:

a. For a zone more than 10 cm thick that when very moist

or wet is very firm (firm, if sandy) or firmer or that has a

penetration resistance class of large (i.e., high or higher), 

and is massive or platy or has weak structure of any type.

b. For a zone that has structural units of any grade with a 

vertical repeat distance of more than 10 cm and while very 

moist or wet is very firm (firm, if sandy) or extremely firm, 

or has a large (i.e., high or higher) penetration resistance.

Classes of Root-Restricting Depth

Terms describing depth to physical restriction for roots are:

Very shallow ......... less than 25 cm

Shallow .................. 25 to less than 50 cm

Moderately deep .... 50 to less than 100 cm

Deep ....................... 100 to less than 150 cm

Very deep ............... 150 cm or more

Particle-Size Distribution

This section discusses particle-size distribution of mineral soil 

separates. Fine earth indicates particles smaller than 2 mm in diameter. 

Fragments 2 mm or larger consist of rock fragments, pieces of geologic 

or pedogenic material with a strongly cemented or more cemented 

rupture-resistance class; pararock fragments, pieces of geologic or 

pedogenic material with an extremely weakly cemented to moderately 
120 Chapter 

cemented rupture-resistance class; and discrete artifacts, pieces of 

human-manufactured material. Particle-size distribution of fine earth is

determined in the field mainly by feel. The content of rock fragments,

pararock fragments, and discrete artifacts is an estimate of the proportion 

of the soil volume that they occupy.

Soil Separates

After pretreatment to remove organic matter, carbonates, soluble 

salts, and other cementing agents and after dispersion to physically 

separate individual soil particles, the U.S. Department of Agriculture 

uses the following size separates for fine-earth fraction:

Very coarse sand .... < 2.0 to > 1.0 mm

Coarse sand ............ 1.0 to > 0.5 mm

Medium sand ......... 0.5 to > 0.25 mm

Fine sand ................ 0.25 to > 0.10 mm

Very fine sand ........ 0.10 to > 0.05 mm

Coarse silt .............. 0.05 to > 0.02 mm 

Fine silt .................. 0.02 to > 0.002 mm 

Coarse clay ............ 0.002 to > 0.0002 mm 

Fine clay ................ less than or equal to 0.0002 mm

Figure 3-6 compares the USDA system for naming various sizes of

soil separates with four other systems: International (Soil Survey Staff, 

1951); Unified (ASTM, 2011); AASHTO (AASHTO, 1997a, 1997b);

and Modified Wentworth (Ingram, 1982).

Soil Texture

Soil texture refers to the weight proportion of the separates for 

particles less than 2 mm in diameter as determined from a laboratory 

particle-size distribution. The pipette method is the preferred standard, 

but the hydrometer method also is used in field labs (Soil Survey Staff,

2009). If used, the hydrometer method should be noted with the results. 

Field estimates of soil texture class are based on qualitative criteria, 

such as how the soil feels (gritty, smooth, sticky) and how it responds to 

rubbing between the fingersto form a ribbon. Estimated field texture class

should be checked against laboratory determinations, and the field criteria

used to estimate texture class should be adjusted as necessary to reflect

local conditions. Sand particles feel gritty and can be seen individually 

with the naked eye. Silt particles have a smooth feel to the fingers when
Relationships among particle-size classes of the USDA system and four other systems.
122 Chapter 

dry or wet and cannot be seen individually without magnification. Clay

soils are sticky in some areas and not sticky in others. For example, soils 

dominated by smectitic clays feel different from soils that contain similar 

amounts of micaceous or kaolinitic clay. The relationships that are useful 

for judging texture of one kind of soil may not apply as well to another 

kind. 

Some soils are not dispersed completely in the standard laboratory 

particle-size analysis. Examples include soils with andic soil properties 

(high amounts of poorly crystalline, amorphous minerals) and soils 

with high contents of gypsum (more than about 25 percent). For soils 

like these, for which the estimated field texture class and the laboratory

measured particle-size distribution differ markedly, the field texture is

referred to as apparent because it is not an estimate that correlates well 

with the results of a laboratory test. Apparent field texture is only a tactile

evaluation and does not infer laboratory test results. The twelve texture 

classes (fig. 3-7) are sands, loamy sands, sandy loams, loam, silt loam,

silt, sandy clay loam, clay loam, silty clay loam, sandy clay, silty clay, 

and clay. Subclasses of sand are coarse sand, sand, fine sand, and very

fine sand. Subclasses of loamy sands and sandy loams that are based on 

sand size are named similarly. 

Definitions of Soil Texture Classes and Subclasses

Sands.—Material has more than 85 percent sand, and the percentage 

of silt plus 1.5 times the percentage of clay is less than 15.

Coarse sand.—Material has a total of 25 percent or more very coarse 

and coarse sand and less than 50 percent any other single grade 

of sand. 

Sand.—Material has a total of 25 percent or more very coarse, coarse, 

and medium sand, a total of less than 25 percent very coarse and 

coarse sand, and less than 50 percent fine sand and less than 50

percent very fine sand; OR material has 25 percent or more very

coarse and coarse sand and 50 percent or more medium sand.

Fine sand.—Material has 50 percent or more fine sand, and fine sand

exceeds very fine sand; OR material has a total of less than 25

percent very coarse, coarse, and medium sand and less than 50 

percent very fine sand.

Very fine sand.—Material has 50 percent or more very fine sand.

Loamy sands.—Material has between 70 and 90 percent sand, the 

percentage of silt plus 1.5 times the percentage of clay is 15 or more, and 

the percentage of silt plus twice the percentage of clay is less than 30.
Soil Survey Manual 1

Loamy coarse sand.—Material has a total of 25 percent or more very 

coarse and coarse sand and less than 50 percent any other single 

grade of sand. 

Loamy sand.—Material has a total of 25 percent or more very coarse, 

coarse, and medium sand, a total of less than 25 percent very 

coarse and coarse sand, and less than 50 percent fine sand and

less than 50 percent very fine sand; OR material has a total of 25

percent or more very coarse and coarse sand and 50 percent or 

more medium sand.

Loamy fine sand.—Material has 50 percent or more fine sand or less

than 50 percent very fine sand and a total of less than 25 percent

very coarse, coarse, and medium sand. 

Loamy very fine sand.—Material has 50 percent or more very fine

sand. 

Sandy loams.—Material has 7 to less than 20 percent clay and more 

than 52 percent sand, and the percentage of silt plus twice the percentage 

of clay is 30 or more; OR material has less than 7 percent clay and less

than 50 percent silt, and the percentage of silt plus twice the percentage 

of clay is 30 or more.

Coarse sandy loam.—Material has a total of 25 percent or more very 

coarse and coarse sand and less than 50 percent any other single 

grade of sand; OR material has a total of 30 percent or more very

coarse, coarse, and medium sand, and very fine sand is 30 to less

than 50 percent.

Sandy loam.—Material has a total of 30 percent or more very coarse,

coarse, and medium sand but a total of less than 25 percent very 

coarse and coarse sand, less than 30 percent fine sand, and less

than 30 percent very fine sand; OR material has a total of 15

percent or less very coarse, coarse, and medium sand, less than 

30 percent fine sand, and less than 30 percent very fine sand with

a total of 40 percent or less fine and very fine sand; OR material

has a total of 25 percent or more very coarse and coarse sand and 

50 percent or more medium sand.

Fine sandy loam.—Material has 30 percent or more fine sand, less

than 30 percent very fine sand, and a total of less than 25 percent

very coarse and coarse sand; OR material has a total of 15 to less 

than 30 percent very coarse, coarse, and medium sand and a total

of less than 25 percent very coarse and coarse sand; OR material 

has a total of 40 percent or more fine and very fine sand (and fine

sand equals or exceeds very fine sand) and a total of 15 percent

or less very coarse, coarse, and medium sand; OR material has 
124 Chapter 

a total of 25 percent or more very coarse and coarse sand and 50 

percent or more fine sand.

Very fine sandy loam.—Material has 30 percent or more very fine

sand and a total of less than 15 percent very coarse, coarse, and 

medium sand, and very fine sand exceeds fine sand; OR material

has 40 percent or more fine and very fine sand (and very fine

sand exceeds fine sand) and a total of less than 15 percent very

coarse, coarse, and medium sand; OR material has 50 percent or 

more very fine sand and a total of 25 percent or more very coarse

and coarse sand; OR material has a total of 30 percent or more

very coarse, coarse, and medium sand and 50 percent or more 

very fine sand.

Loam.—Material has 7 to less than 27 percent clay, 28 to less than 

50 percent silt, and 52 percent or less sand.

Silt loam.—Material has 50 percent or more silt and 12 to less than 

27 percent clay; OR material has 50 to less than 80 percent silt and less 

than 12 percent clay. 

Silt.—Material has 80 percent or more silt and less than 12 percent 

clay.

Sandy clay loam.—Material has 20 to less than 35 percent clay, less

than 28 percent silt, and more than 45 percent sand.

Clay loam.—Material has 27 to less than 40 percent clay and more 

than 20 to 45 percent sand.

Silty clay loam.—Material has 27 to less than 40 percent clay and 20 

percent or less sand.

Sandy clay.—Material has 35 percent or more clay and more than

45 percent sand.

Silty clay.—Material has 40 percent or more clay and 40 percent or 

more silt.

Clay.—Material has 40 percent or more clay, 45 percent or less sand, 

and less than 40 percent silt.

The USDA textural triangle is shown in figure 3-7. A soil sample

is assigned to one of the twelve soil texture classes according to the 

values for the proportions of sand, silt, and clay, which are located along 

each of the three axes. The eight subclasses in the sand and loamy sand 

groups provide refinement that in some cases may be greater than can be

consistently determined by field techniques. Only those distinctions that

are significant to use and management and that can be consistently made

in the field should be applied when determinations of texture are based

on field estimates alone.
Soil Survey Manual 1

Figure 3-7

USDA textural triangle showing the percentages of clay, silt, and sand in the 12 basic 

texture classes.

Groupings of Soil Texture Classes

The need for fine distinctions in the texture of the soil layers results

in a large number of classes and subclasses of soil texture. It commonly 

is convenient to speak generally of broad groups or classes of texture. 

Table 3-1 provides an outline of three general soil texture groups and five

subgroups. In some areas where soils have a high content of silt, a fourth 

general class, silty soil materials, may be used for silt and silt loam.

Terms Used in Lieu of Soil Texture

There are some horizons or layers for which soil texture class terms 

are not applicable. These include bedrock and other cemented horizons 

(such as petrocalcic horizons, duripans, etc.), those composed of organic 

soil materials, and those composed of water, either liquid or frozen, below 

a mineral or organic soil surface layer. Other exceptions include layers 

composed of more than 90 percent rock fragments or artifacts and horizons 

or layers composed of 40 percent or more gypsum in the fine-earth fraction

(and that are not cemented). These exceptions are discussed below.
126 Chapter 

Table 3-1

General Soil Texture Groups

General texture groups 

and subgroups* Texture classes

Sandy soil materials

Coarse textured Sands (coarse sand, sand, fine sand, very

fine sand); loamy sands (loamy coarse

sand, loamy sand, loamy fine sand,

loamy very fine sand)

Loamy soil materials

Moderately coarse textured Coarse sandy loam, sandy loam, fine

sandy loam

Medium textured Very fine sandy loam, loam, silt loam, silt

Moderately fine textured Clay loam, sandy clay loam, silty clay 

loam

Clayey soil materials

Fine textured Sandy clay, silty clay, clay

* Note: These are not the sandy, loamy, and clayey family particle-size classes defined

in Soil Taxonomy.

Soil Materials with a High Content of Gypsum

For soil materials with 40 percent or more, by weight, gypsum in 

the fine-earth fraction, gypsum dominates the physical and chemical

properties of the soil to the extent that particle-size classes are not 

meaningful. Two terms in lieu of texture are used: 

Coarse gypsum material.—50 percent or more of the fine-earth

fraction is comprised of particles ranging from 0.1 to 2.0 mm in 

diameter. 

Fine gypsum material.—Less than 50 percent of the fine-earth

fraction is comprised of particles ranging from 0.1 to 2.0 mm in 

diameter.

Bedrock and Cemented Horizons

These horizons or layers are described as bedrock or cemented 

material. Additional information about the kind of rock, degree of 

cementation, and kind of cementing agent can also be provided.
Soil Survey Manual 1

Water Layers

These layers are described as water or ice. They only refer to subsurface 

layers, such as in a floating bog. Figure 3-3 shows a subsoil layer of ice.

Soil Materials with a High Content of Rock or Pararock 

Fragments

For soil materials with more than 90 percent rock or pararock 

fragments, there is not enough fine earth to determine the texture class.

In these cases, the terms gravel, cobbles, stones, boulders, channers, and 

flagstones or their pararock fragment equivalents are used. Size range 

and shape for these terms are described under “Rock Fragments and 

Pararock Fragments” and are summarized in table 3-2.

Soil Materials with a High Content of Artifacts

For soil materials with more than 90 percent artifacts, the term 

artifacts is used.

Organic Soils

Layers that are not saturated with water for more than a few days 

at a time are organic if they have 20 percent or more organic carbon. 

Layers that are saturated for longer periods, or were saturated before 

being drained, are organic if they have 12 percent or more organic 

carbon and no clay, 18 percent or more organic carbon, and 60 percent 

or more clay or have a proportional amount of organic carbon, between 

12 and 18 percent, if the clay content is between 0 and 60 percent. The 

required organic carbon content for saturated soils having between 0 and 

60 percent clay can be calculated as: OCrequired = 12+(0.1 * percent clay). 

Soils with more than 60 percent clay need an organic carbon content of 

at least 18 percent.

The kind and amount of the mineral fraction, the kind of organisms 

from which the organic material was derived, and the state of 

decomposition affect the properties of the soil material. Descriptions 

include the percentage of undecomposed fibers and the solubility in

sodium pyrophosphate of the humified material. Attention should be

given to identifying and estimating the volume occupied by sphagnum

fibers, which have extraordinary high water retention. When squeezed

firmly in the hand to remove as much water as possible, sphagnum fibers

are lighter in color than fibers of hypnum and most other mosses.

Fragments of wood more than 20 mm across and so undecomposed 

that they cannot be crushed by the fingers when moist or wet are called

wood fragments. They are comparable to rock fragments in mineral soils 

and are described in a comparable manner. 
128 Chapter 

Saturated organic soil materials.—The types of organic soil 

materials that are described in saturated organic soil materials are:

Muck.—Well decomposed organic soil material with a low content of 

fibers (plant tissue excluding live roots).

Peat.—Slightly decomposed organic soil material with a high content 

of original fibers.

Mucky peat.—Organic soil material that is intermediate in degree 

of decomposition, fiber content, bulk density, and water content

between muck and peat. 

Muck, peat, and mucky peat may be described in both organic 

and mineral soils provided the soils are saturated with water for 30 or

more cumulative days in normal years or are artificially drained. These

materials only qualify for the diagnostic sapric, fibric, and hemic soil 

material of Soil Taxonomy when they occur in organic soils (i.e., the soil 

of the order Histosols and the suborder Histels).

Non-saturated organic soil materials.—The types of organic 

soil materials that are described in layers not saturated for 30 or more

cumulative days are: 

Highly decomposed plant material.—Well decomposed, organic soil 

material with a low content of fibers (plant tissue excluding live

roots).

Moderately decomposed plant material.—Material intermediate in 

degree of decomposition, fiber content, bulk density, and water

content between highly decomposed and slightly decomposed 

plant material. 

Slightly decomposed plant material.—Slightly decomposed organic 

soil material with a high content of original fibers.

Modifiers for Terms Used in Lieu of Texture

Modifiers may be needed to better describe the soil material making

up the horizon or layer. These include terms for significant amounts

of particles 2.0 mm or larger (rock fragments, pararock fragments, or 

artifacts) and terms that indicate the composition of the soil material. 

Soil Materials with Rock Fragments, Pararock Fragments, or 

Artifacts

To describe soils with 15 percent or more, by volume, rock fragments, 

pararock fragments, or artifacts, the texture terms are modified with

terms indicating the amount and kind of fragments. Examples include 

very gravelly loam, extremely paracobbly sand, and very artifactual 
Soil Survey Manual 1

sand. The conventions for use of these terms and the definitions of class

terms are discussed in the following sections on rock fragments, pararock 

fragments, and artifacts.

Class Modifiers Indicating Soil Material Composition

Soil composition modifiers are used for some soils that have andic

properties or formed in volcanic materials, soils that have a high content 

of gypsum, some organic soil materials, and mineral soil materials with 

a high content of organic matter. Terms are also provided for limnic soil 

materials and permanently frozen layers (permafrost). 

Soil Materials with Andic Properties or Volcanic Origin

Hydrous.—Material that has andic soil properties and an undried 15 

bar (1500 kPa) water content of 100 percent or more of the dry 

weight (e.g., hydrous clay).

Medial.—Material that has andic soil properties and has a 15 bar 

(1500 kPa) water content of less than 100 percent on undried 

samples and of 12 percent or more on air-dried samples (e.g., 

medial silt loam).

Ashy.—Material that has andic soil properties and is neither hydrous 

nor medial, or material that does not have andic soil properties 

and the chemistry and physical makeup of its fine-earth fraction

reflects the weathering processes of volcanic materials (e.g.,

ashy loam). The weathering processes of volcanic materials are 

evidenced by 30 percent or more particles 0.02 to 2.0 mm in

diameter, of which 5 percent or more is composed of volcanic 

glass and the [(aluminum plus 1⁄2 iron percent by ammonium 

oxalate) times 60] plus the volcanic glass percent is equal to or 

more than 30.

Soil Materials with Gypsum

Gypsiferous.—Material that contains 15 to less than 40 percent, by 

weight, gypsum (e.g., gypsiferous fine sandy loam).

For material that has 40 percent or more gypsum, a term in lieu of 

texture is used (e.g., fine gypsum material or coarse gypsum material, 

defined above).

Organic Soil Materials

Modifiers are only used with the “in lieu of texture” terms muck, 

peat, or mucky peat. The following modifiers are used only for organic
130 Chapter 

soil materials that are saturated with water for 30 or more cumulative

days in normal years or are artificially drained.

Woody.—Material contains 15 percent or more wood fragments 

larger than 20 mm in size or contains 15 percent or more fibers

that can be identified as wood origin and has more wood fibers

than any other kind of fiber (e.g., woody muck).

Grassy.—Material contains more than 15 percent fibers that can be

identified as grass, sedges, cattails, and other grasslike plants and

contains more grassy fibers than any other kind of fiber (e.g.,

grassy mucky peat).

Mossy.—Material contains more than 15 percent fibers that can be

identified as moss and contains more moss fibers than any other

kind of fiber (e.g., mossy peat).

Herbaceous.—Material contains more than 15 percent fibers that can

be identified as herbaceous plants other than moss and grass or

grasslike plants and has more of these fibers than any other kind

of fiber (e.g., herbaceous muck).

Mineral Soil Materials with a High Content of Organic Matter

Highly organic.—Term indicates near surface horizons of mineral 

soils that are saturated with water for less than 30 cumulative

days in normal years and are not artificially drained (e.g., highly

organic loam). Excluding live roots, the horizon has organic 

carbon content (by weight) of one of the following: 

• 5 to < 20 percent if the mineral fraction contains no 

clay, 

• 12 to < 20 percent if the mineral fraction contains 60 

percent or more clay, or

• [5 + (clay percentage multiplied by 0.12)] to < 20 

percent if the mineral fraction contains less than 60 

percent clay.

Mucky.—Term indicates near surface horizons of mineral soils that 

are saturated with water for 30 or more cumulative days in

normal years or are artificially drained (e.g., mucky silt loam).

Excluding live roots, the horizon has more than 10 percent 

organic matter and less than 17 percent fibers.

Peaty.—Term indicates near surface horizons of mineral soils that are 

saturated with water for 30 or more cumulative days in normal

years or are artificially drained (e.g., peaty clay loam). Excluding

live roots, the horizon has more than 10 percent organic matter 

and 17 percent or more fibers.
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Limnic Soil Materials

Limnic soil materials occur in layers underlying some soils 

of the soil order Histosols. By definition (see Soil Taxonomy) they 

are not recognized in mineral soils. They are mineral or organic 

soil materials originating from aquatic organisms or from aquatic 

plants that were later altered by aquatic organisms. The following 

terms are used to describe the origin of the limnic materials:

Coprogenous.—Material contains many very small (0.1 to 0.001 

mm) fecal pellets (e.g., coprogenous sandy loam).

Diatomaceous.—Material is composed dominantly of diatoms (e.g., 

diatomaceous silt loam). 

Marly.—Material is composed dominantly of calcium carbonate 

“mud” (e.g., marly silty clay).

Layers for which these terms are used may or may not also meet the 

definition for coprogenous earth, diatomaceous earth, or marl as defined

in Soil Taxonomy.

Permafrost 

Layers of permafrost are described as permanently frozen (e.g., 

permanently frozen loamy sand).

Rock Fragments and Pararock Fragments

Rock fragments are unattached pieces of geologic or pedogenic 

material 2 mm in diameter or larger that have a strongly cemented

or more cemented rupture-resistance class. Pararock fragments are 

unattached pieces of geologic or pedogenic material 2 mm in diameter or 

larger that are extremely weakly cemented through moderately cemented. 

Pararock fragments are not retained on sieves because they are crushed 

by grinding during the preparation of samples for particle-size analysis 

in the laboratory. Rock fragments and pararock fragments include all 

sizes between 2.0 mm and horizontal dimensions smaller than the size 

of a pedon. The words “rock” and “pararock” are used here in the broad 

sense and connote more than just natural fragments of geologic material. 

Thus, rock and pararock fragments may be discrete, cemented pieces 

of bedrock, bedrock-like material, durinodes, concretions, nodules, or 

pedogenic horizons (e.g., petrocalcic fragments). Artifacts, however, are 

not included as rock or pararock fragments. They are described separately.

Rock fragments and pararock fragments are described by size, shape, 

hardness, roundness, and kind of fragment. The classes are gravel, 
132 Chapter 

cobbles, channers, flagstones, stones, and boulders and their pararock 

counterparts (i.e., paragravel, paracobbles, etc.) (table 3-2). If a size or

range of sizes predominates, the class is modified (e.g., “fine gravel,”

“cobbles 100 to 150 mm in diameter,” “channers 25 to 50 mm in length”).

Gravel and paragravel are a collection of fragments that have 

diameters ranging from 2 to 76 mm. Individual fragments in this size 

range are properly referred to as “pebbles,” not “gravels.” The term gravel 

as used here indicates the collection of pebbles in a soil horizon and does 

not imply a geological formation. The terms “pebble” and “cobble” are 

typically restricted to rounded or subrounded fragments; however, they 

can be used to describe angular fragments that are not flat. Words such

as “chert,” “limestone,” and “shale” refer to a kind or lithology of rock, 

not a piece of rock. The composition of the fragments can be given, for 

example: “chert gravel,” “limestone channers,” “siltstone parachanners.” 

The upper size limit of gravel and paragravel is 76 mm (3 inches).

This coincides with the upper limit used by many engineers for grainsize distribution computations. The 5-mm and 20-mm divisions for the 

separation of fine, medium, and coarse gravel coincide with the sizes of

openings in the number 4 screen (4.76-mm) and the 3⁄4-inch (19.05-mm) 

screen used in engineering.

The 76-mm (3-inch) limitseparates gravel from cobbles, the 250-mm

(10-inch) limit separates cobbles from stones, and the 600-mm (24-inch) 

limit separates stones from boulders. The 150-mm (6-inch) and 380-mm

(15-inch) limits for thin, flat channers and flagstones, respectively, follow

conventions used for many years to provide class limits for plate-shaped 

and crudely spherical rock fragments that have about the same soil use 

implications as the 250-mm limit for spherical shapes.

Estimating Rock Fragments in the Soil

Rock fragmentsin the soil can greatly influence use and management.

It is important to not only consider the total amount of rock fragments, 

but also the proportions of the various size classes (gravel, cobbles, 

stones, etc.). A soil with 10 percent stones is quite different from one with 

10 percent gravel. When developing interpretive criteria, a distinction 

must be made between volume and weight percent of rock fragments. 

Field descriptions generally record estimates of volume, while laboratory 

measurements of rock fragments are given as weight for the various size 

classes.

The National Cooperative Soil Survey in the United States uses 

interpretive algorithms based on weight percent of the > 250, > 76-250, 

> 5-76, and 2-5 mm fractions when rating soils for various potential uses. 
Soil Survey Manual 1

The first two size ranges are on a whole soil basis, and the latter two are

on a < 76 mm basis. For the > 250 and > 76-250 mm fractions, weighing 

is generally impracticable and volume percentage estimates are made 

from areal percentage measurements by point-count or line-intersect 

methods. Length of the transect or area of the exposure should be at least 

50 times, and preferably 100 times, the area or dimensions of the rock 

fragment size that encompasses about 90 percent of the rock fragment 

volume. For the < 76 mm weight, measurements are feasible but may 

require 50 to 60 kg of sample if appreciable rock fragments near 76 mm 

are present. An alternative is to obtain volume estimates for the 20-76 

mm fraction and weight estimates for the < 20 mm fraction. This method 

is preferred because of the difficulty in visual evaluation of the 2 to 5

mm size separations. The weight percentages of > 5-20 mm and 2-5 mm 

fractions may be converted to volume estimates and placed on a < 76 mm 

base by computation. 

Terms for Rock Fragments and Their Use in Modifying 

Texture Classes

The adjectival form of a class name of rock fragments or pararock 

fragments (table 3-2) is used as a modifier of the texture class name, e.g.,

paragravelly loam, very cobbly sandy loam. Table 3-3 provides rules for

determining the proper texture modifier term for material with a mixture

of rock fragment sizes. This section also provides rules for assigning 

terms for soils with a mixture of rock and pararock fragments.

The following classes, based on volume percentages, are used: 

Less than 15 percent.—No texture modifier terms are used with soils

having less than 15 percent gravel, paragravel, cobbles, paracobbles, channers, parachanners, flagstones, or paraflagstones.

15 to less than 35 percent.—The adjectival term of the dominant 

kind of fragment is used as a modifier of the texture class, e.g.,

gravelly loam, parachannery silt loam, cobbly sandy loam.

35 to less than 60 percent.—The adjectival term of the dominant kind 

of rock fragment is used with the word “very” as a modifier of

the texture class, e.g., very gravelly loam, very parachannery silt 

loam, very cobbly loamy sand (fig. 3-8).

60 to less than 90 percent.—The adjectival term of the dominant 

kind of rock fragment is used with the word “extremely” as 

a modifier of the texture class, e.g., extremely gravelly loam,

extremely parachannery silt loam, extremely cobbly sandy 

loam. 
134 Chapter 

Table 3-2

Terms for Rock Fragments and Pararock Fragments

Shape and size Noun* Adjective*

Nonflat fragments (spherical or cubelike):

2–76 mm diameter Gravel Gravelly 

 2–5 mm diameter Fine gravel Fine gravelly 

 > 5–20 mm diameter Medium gravel Medium gravelly 

 > 20–76 mm diameter Coarse gravel Coarse gravelly

> 76–250 mm diameter Cobbles Cobbly

> 250–600 mm diameter Stones Stony

> 600 mm diameter Boulders Bouldery

Flat fragments:

2–150 mm long Channers Channery

> 150–380 mm long Flagstones Flaggy

> 380–600 mm long Stones Stony

> 600 mm long Boulders Bouldery

* For fragments that are less than strongly cemented, the prefix “para” is added to the

terms in this table to form either a descriptive noun or the adjective for the texture 

modifier (e.g., paracobbles, paragravelly).

90 percent or more.—No texture modifier terms are used. If there is

too little fine earth to determine the texture class (less than about

10 percent, by volume) a term in lieu of texture (i.e., gravel, 

cobbles, stones, boulders, channers, flagstones, or their pararock

fragment equivalents) is used as appropriate.

The class limits apply to the volume of the layer occupied by all 

rock fragments 2 mm in diameter or larger. The soil generally contains 

fragmentssmaller or larger than those identified by the term. For example,

very cobbly sandy loam typically contains gravel but “gravelly” is not in 

the name. The use of a term for larger pieces of rock, such as boulders, 

does not imply that the pieces are entirely within a given soil layer. A 

single boulder may extend through several layers.

Table 3-3 can be used to determine the proper modifier if there is

a mixture of rock fragment sizes. To use the table, first choose the row

with the appropriate total rock fragments. Then read the criteria in the 

columns under “Gravel, cobbles, stones, and boulders,” starting from the 
Soil Survey Manual 1

Figure 3-8

A soil in which the layers below a depth of about 20 cm are very cobbly loamy sand. 

Left side of scale is in 20-cm increments.

Table 3-3

Guide for Determining Rock Fragment Modifier of Texture 

for Soils with a Mixture of Rock Fragment Sizes

Total rock

fragments

(Vol. %)

Gravel (GR), cobbles (CB), stones (ST), and 

boulders (BY)

(Substitute channers for gravel and 

flagstones for cobbles, where applicable)

If GR ≥ 

1.5 CB + 

2 ST + 

2.5 BY

If CB ≥ 

1.5 ST + 

2 BY

If ST ≥ 

1.5 BY

If ST < 

1.5 BY

≥ 15 < 35 Gravelly Cobbly Stony Bouldery

≥ 35 < 60 Very gravelly Very cobbly Very stony Very 

bouldery

≥ 60 < 90 Extremely 

gravelly

Extremely 

cobbly

Extremely 

stony

Extremely 

bouldery

≥ 90 Gravel Cobbles Stones Boulders
136 Chapter 

left-most column and proceeding to the right. Stop in the first column in

which a criterion is met.

More precise estimates of the amounts of rock fragments than are 

provided by the defined classes are needed for some purposes. For more

precise information, estimates of percentages of each size class or a 

combination of size classes are included in the description, e.g., “very 

cobbly sandy loam,” “30 percent cobbles and 15 percent gravel or silt

loam,” “about 10 percent gravel.” If loose pieces of rock are significant to

the use and management of a soil, they are the basis of phase distinctions 

among map units. Exposed bedrock is not soil and is identified separately

in mapping as a kind of miscellaneous area (i.e., Rock outcrop).

The volume occupied by individual pieces of rock can be seen, and 

their aggregate volume percentage can be calculated. For some purposes, 

volume percentage must be converted to weight percentage.

The following rules are used to select texture modifiers if a horizon

includes both rock and pararock fragments:

1. Describe the individual kinds and amounts of rock and pararock 

fragments.

2. Do not use a fragment texture modifier if the combined volume

of rock and pararock fragments is less than 15 percent.

3. If the combined volume of rock and pararock fragments is more 

than 15 percent and the volume of rock fragments is less than 

15 percent, assign pararock fragment modifiers based on the

combined volume of fragments. For example, use “paragravelly” 

as a texture modifier forsoils with 10 percent rock and 10 percent

pararock gravel-sized fragments. 

4. If the volume of rock fragments is 15 percent or more, use the 

appropriate texture modifier for rock fragments regardless of the

volume of pararock fragments.

Rock Fragment Hardness, Roundness, and Kind

Fragment hardness is equivalent to the rupture resistance class for 

a cemented fragment of specified size that has been air dried and then

submerged in water. The hardness of a fragment is significant where the

rupture resistance class is strongly cemented or greater. See the section on 

rupture resistance later in this chapter for details describing the fragment 

hardness classes and their test descriptions. 

Fragment roundness is an expression of the sharpness of the edges 

and corners of rock fragments and pararock fragments. The roundness 

of fragments impacts water infiltration, root penetration, and macropore

space. The following roundness classes are used:
Soil Survey Manual 1

Very angular .......... Strongly developed faces and very sharp, 

broken edges

Angular .................. Strongly developed faces and sharp edges

Subangular ............. Detectable flat faces and slightly rounded

corners

Subrounded ............ Detectable flat faces and well rounded corners

Rounded ................. Flat faces absent or nearly absent and all 

corners rounded

Well rounded ......... Flat faces absent and all corners rounded

Fragment kind is the lithology or composition of the 2 mm or larger 

fraction of the soil. Kinds of fragments are varied based on whether 

their origin is from a geologic source or a pedogenic source. Examples 

of kinds of fragments are basalt fragments, durinodes, iron-manganese 

concretions, limestone fragments, petrocalcic fragments, tuff fragments, 

and wood fragments.

Artifacts

Artifacts are discrete water-stable objects or materials created, 

modified, or transported from their source by humans, usually for a

practical purpose in habitation, manufacturing, excavation, agriculture, 

or construction activities. Examples are processed wood products, coal 

combustion by-products, bitumen (asphalt), fibers and fabrics, bricks,

cinder blocks, concrete, plastic, glass, rubber, paper, cardboard, iron 

and steel, altered metals and minerals, sanitary and medical waste, 

garbage, and landfill waste. Artifacts also include natural materials

which were mechanically abraded by human activities (as evidenced by 

scrapes, gouges, tool marks, etc.), such as shaped or carved stone work, 

grindstones, and shaped stones and debitage (e.g., stone tool flakes).

Artifacts are generally categorized as either particulate or discrete. 

The distinction is based on size: particulate artifacts have a diameter of 

less than 2 mm and discrete artifacts have a diameter of 2 mm or more. 

Discrete artifacts are easier to identify and are essentially fragments of 

human origin. Particulate artifacts are sometimes difficult to discern from

naturally occurring fine-earth soil material.

Describing Artifacts in Soil

Artifacts are described if they are judged to be durable enough to 

persist in the soil (resist weathering and leaching) for a few decades 

or more. Descriptions of artifacts generally include quantity, cohesion, 
138 Chapter 

persistence, size, and safety classes. They may also include shape, kind, 

penetrability by roots, and roundness. Additional attributes (such as those 

discussed below under the heading “Consistence”) may be described to 

help understand and interpret the soil. The conventions for describing 

artifacts are explained in the following paragraphs.

Quantity refers to the estimated volume percent of a horizon or 

other specified unit occupied by discrete artifacts. If classes (rather than

quantitative estimates) are given, they are the same as those described in 

this chapter for mottles.

Cohesion refers to the relative ability of the artifact to remain intact 

after significant disturbance. The cohesion classes are:

Cohesive.—Artifacts adhere together sufficiently so that they cannot

be easily broken into pieces < 2 mm either by hand or with a 

simple crushing device, such as a mortar and pestle.

Noncohesive.—Artifacts are easily broken into pieces < 2 mm either 

by hand or with a simple crushing device, such as a mortar and 

pestle. Noncohesive artifacts are similar to pararock fragments 

and will be incorporated into the fine-earth fraction of the soil

during routine laboratory sample preparation.

Penetrability describes the relative ease with which roots can penetrate the artifact and potentially extract any stored moisture, nutrients, or 

toxic elements. The penetrability classes are:

Nonpenetrable.—Roots cannot penetrate through the solid parts of 

the artifact or between the component parts of the artifact.

Penetrable.—Roots can penetrate through the solid parts of the 

artifact or between the component parts of the artifact.

Persistence describes the relative ability of solid artifacts to withstand weathering and decay over time. Local conditions, such as 

temperature and moisture,significantly impact the persistence of artifacts

in the soil. The persistence classes are:

Nonpersistent.—The artifact is susceptible to relatively rapid 

weathering or decay and is expected to be lost from the soil in less 

than a decade. Loss of soil mass and eventually subsidence result.

Persistent.—The artifact is expected to remain intact in the soil for a 

decade or more. 

Roundness indicates the sharpness of edges and corners of natural 

objects, such as rock fragments, and human-manufactured objects, such 

as artifacts. The artifact roundness classes are the same as those used for 

fragment roundness (above).
Soil Survey Manual 1

Safety describes the degree of risk to humans from contact with soils 

that contain artifacts. Physical contact with soils containing dangerous 

or harmful artifacts should be avoided unless proper training is provided 

and protective clothing is available. The safety classes are:

Innocuous.—The artifacts are considered to be harmless to living 

beings. Examples include untreated wood products, iron, bricks, 

cinder blocks, concrete, plastic, glass, rubber, organic fibers,

inorganic fibers, unprinted paper and cardboard, and some

mineral and metal products. Sharp innocuous artifacts can 

cause injury, but the materials themselves are still considered 

innocuous.

Noxious.—The artifacts are potentially harmful or destructive to living 

beings unless dealt with carefully. The harm may be immediate 

or long-term and through direct or indirect contact. Examples 

include arsenic-treated wood products, batteries, waste and 

garbage, radioactive fallout, liquid petroleum products, asphalt, 

coal ash, paper printed with metallic ink, and some mineral and 

metal products. 

Shape is variable among kinds of artifacts. The shape classes are:

Elongated.—One dimension is at least three times longer than both 

of the others. 

Equidimensional.—Dimensions in length, width, and height are approximately similar. 

Flat.—One dimension is less than one third that of both of the others, 

and one dimension is less than three times that of the intermediate 

dimension. 

Irregular.—The form is branching and convoluted.

Size may be measured and reported directly or given as a class. The 

dimension to which size-class limits apply depends on the shape of the 

artifact described. If the shape is nearly uniform, size is measured in 

the shortest dimension, such as the effective diameter of a cylinder or 

the thickness of a plate. For elongated or irregular bodies, size generally 

refers to the longest dimension but direct measurements for 2 or 3

dimensions can be given for clarification. The size classes for discrete

artifacts are:

Fine ........................ 2 to < 20 mm

Medium ................. 20 to < 75 mm

Coarse .................... 75 to < 250 mm

Very coarse ............ ≥ 250 mm
140 Chapter 

Kinds of Artifacts

There are too many varieties of artifacts to provide a comprehensive 

list. The most common types include:

• Noxious and innocuous artifacts

• Treated and untreated wood products

• Liquid petroleum products

• Coal combustion by-products

• Paper (printed and unprinted) and cardboard

• Sanitary and medical waste

• Garbage and landfill waste

• Asphalt

• Organic and inorganic fibers

• Bricks

• Cinder blocks

• Concrete

• Plastic

• Glass 

• Rubber products

• Iron and steel

Texture Modifier Terms for Soils with Artifacts

The texture of soils with artifacts is described according to the 

content of artifacts: 

Less than 15 percent.—No texture modifier terms are used.

15 to less than 35 percent.—The term “artifactual” is used, e.g., 

artifactual loam.

35 to less than 60 percent.—The term “very artifactual” is used, e.g., 

very artifactual loam. 

60 to less than 90 percent.—The term “extremely artifactual” is used, 

e.g., extremely artifactual loam. 

90 percent or more.—No texture modifier terms are used. If there is

not enough fine earth to determine the texture class (less than

about 10 percent, by volume) the term “artifacts” is used. 

Compound Texture Modifiers

In some cases, the mineral soil may contain a combination of fragment 

or composition types for which the use of compound texture modifiers is

useful. For example, a soil horizon may contain both artifacts and other 
Soil Survey Manual 1

fragments, such as rock fragments and pararock fragments. In these cases, 

the rock fragments, pararock fragments, and artifacts are each described 

separately. Modifiers for both artifacts and rock or pararock fragments

can be combined. The modifier for artifacts comes before the modifier

for rock or pararock fragments, e.g., artifactual very gravelly sandy loam. 

Modifiers for composition and rock fragments can also be combined. For

example, a horizon of channery mucky clay or one of gravelly gypsiferous 

sandy loam contains rock fragments and also a content of high organic 

matter or gypsum. There are many possible combinations.

Fragments on the Surface

This section discusses the description of rock fragments (especially 

stones and boulders) that are on the soil as opposed to in the soil. 

The description of gravel, cobbles, and channers (≥ 2 but < 250 mm

in diameter) differs from that for stones and boulders (≥ 250 mm in

diameter) because an important aspect of gravel, cobbles, and channers is 

their areal percent cover on the ground surface. This cover provides some 

protection from wind and water erosion. It may also interfere with seed 

placement and emergence after germination. For stones and boulders, 

the percent of cover is not of itself as important as the interference with 

mechanical manipulation of the soil. For example, a very small areal 

percentage of large fragments, insignificant for erosion protection, may

interfere with tillage, tree harvesting, and other operations involving 

machinery.

The areal percentage of the ground surface is determined using pointcount and/or line-intersect procedures. If the areal percentage equals or 

exceeds 80 percent, the top of the soil is considered to be the mean height 

of the top of the rock or pararock fragments. The volume proportions of 

the 2 to 5 mm, 5 to 75 mm, and 75 to 250 mm fragments should be recorded. This can be done from areal measurements in representative areas.

The number, size, and spacing of stones and boulders (≥ 250 mm

in diameter) on the surface of a soil, including both those that lie on 

the surface and those that are partly within the soil, have important 

effects on soil use and management. The classes are given in terms of 

the approximate amount of rock fragments of stone and boulder size at 

the surface:

Class 1.—Stones or boulders cover 0.01 to less than 0.1 percent of 

the surface. The smallest stones are at least 8 meters apart; the 

smallest boulders are at least 20 meters apart (fig. 3-9).
142 Chapter 

Class 2.—Stones or boulders cover 0.1 to less than 3 percent of the

surface. The smallest stones are not less than 1 meter apart; the 

smallest boulders are not less than 3 meters apart (fig. 3-10).

Class 3.—Stones or boulders cover 3 to less than 15 percent of the

surface. The smallest stones are as little as 0.5 meter apart; the 

smallest boulders are as little as 1 meter apart (fig. 3-11).

Class 4.—Stones or boulders cover 15 to less than 50 percent of the 

surface. The smallest stones are as little as 0.3 meter apart; the

smallest boulders are as little as 0.5 meter apart. In most places 

it is possible to step from stone to stone or jump from boulder to 

boulder without touching the soil (fig. 3-12).

Class 5.—Stones or boulders appear to be nearly continuous and 

cover 50 percent or more of the surface. The smallest stones are 

less than 0.03 meter apart; the smallest boulders are less than

0.05 meter apart. Classifiable soil is among the rock fragments,

and plant growth is possible (fig. 3-13).

These limits are intended only as guides to amounts that may mark 

critical limitations for major kinds of land use. Table 3-4 is a summary

of the classes.

Table 3-4

Classes of Surface Stones and Boulders in Terms of Cover 

and Spacing

Class

Percentage 

of surface 

covered

Distance in meters 

between stones 

or boulders if the 

diameter is: Descriptive term

0.25 m* 0.6 m 1.2 m

1 0.01 to < 0.1 > 8 > 20 > 37 Stony or bouldery

2 0.1 to < 3.0 1–8 3–20 6–37 Very stony or very 

bouldery

3 3.0 to < 15 0.5–1 1–3 2–6 Extremely stony 

or extremely 

bouldery

4 15 to < 50 0.3–0.5 0.5–1 1–2 Rubbly

5 > 50 < 0.3 < 0.5 < 1 Very rubbly

* 0.38 m if the fragment is flat.
Soil Survey Manual 1

Figure 3-9

An area of bouldery soil (class 1).

Figure 3-10

An area of very bouldery soil (class 2).
144 Chapter 

Figure 3-11

An area of extremely bouldery soil (class 3).

Figure 3-12

An area of rubbly soil (class 4).
Soil Survey Manual 1

Figure 3-13

An area of very rubbly soil (class 5).

Soil Color

Most soil survey organizations, including the National Cooperative 

Soil Survey in the United States, have adopted the Munsell soil color 

system for describing soil color (using the elements of hue, value, and 

chroma). The names associated with each standard color chip (yellowish 

brown, light gray, etc.) are not strictly part of the Munsell color system. 

They were selected by the Soil Survey Staff to be used in conjunction 

with the Munsell color chips. The color chips included in the standard 

soil-color charts (a subset of all colors in the system) were selected 

so that soil scientists can describe the normal range of colors found 

in soils. These chips have enough contrast between them for different 

individuals to match a soil sample to the same color chip consistently. 

Interpolating between chips is not recommended in standard soil survey 

operations because such visual determinations cannot be repeated with 

a high level of precision. Although digital soil color meters that can 

provide precise color readings consistently are available, they are not 

widely used in field operations. Therefore, the standard procedure

adopted for soil survey work is visual comparison to the standard soilcolor charts.
146 Chapter 

Elements of Soil Color Descriptions

Elements of soil color descriptions are the color name, the Munsell 

notation, the water state (moist or dry), and the physical state. An example 

is “brown (10YR 5/3), dry, crushed and smoothed.” Physical state is 

recorded as broken, rubbed, crushed, or crushed and smoothed. The 

term “crushed” typically applies to dry samples and “rubbed” to moist 

samples. If physical state is unspecified, a broken surface is implied. The

color of the soil is normally recorded for a surface broken through a ped, 

if a ped can be broken as a unit. If ped surfaces are noticeably different in 

color from the ped interior, this should also be described. 

The color value of most soil material is lower after moistening. 

Consequently, the water state of a sample is always given. The water state 

is either “moist” or “dry.” The dry state for color determinations is air dry 

and should be made at the point where the color does not change with 

additional drying. Color in the moist state is determined on moderately 

moist or very moist soil material and should be made at the point where 

the color does not change with additional moistening. The soil should not 

be moistened to the extent that glistening takes place because the light 

reflection of water films may cause incorrect color determinations. In

a humid region, the moist state generally is standard; in an arid region, 

the dry state is standard. In detailed descriptions, colors of both dry and 

moist soil are recorded if feasible. The color for the regionally standard 

moisture state is typically described first. Both moist and dry colors are

valuable, particularly for the immediate surface and tilled horizons, in 

assessing reflectance.

A Munsell notation is obtained by comparison with a Munsell soilcolor chart. The most commonly used charts include only about one fifth

of the entire range of hues.1 They consist of about 250 different colored 

papers, or chips, systematically arranged on hue cards according to their 

Munsell notations. Figure 3-14 illustrates the arrangements of color chips

on a Munsell color card.

The Munsell color system uses three elements of color—hue, value,

and chroma. The color notation is recorded as: hue, value/chroma (e.g., 

5Y 6/3).

Hue is a measure of the chromatic composition of light that reaches 

the eye. The Munsell system is based on five principal hues: red (R),

yellow (Y), green (G), blue (B), and purple (P). Five intermediate hues 

representing midpoints between each pair of principal hues complete the 

1 The appropriate color chips, separate or mounted by hue on special cards for a looseleaf notebook, are available through several suppliers of scientific equipment.
Soil Survey Manual 1

Figure 3-14

The arrangement of color chips according to value and chroma on the Munsell soilcolor card of hue 10YR. 
148 Chapter 

10 major hue names used to describe the notation. The intermediate hues 

are yellow-red (YR), green-yellow (GY), blue-green (BG), purple-blue 

(PB), and red-purple (RP). The relationships among the 10 hues are shown 

in figure 3-15. Each ofthe 10 major huesis divided into 4 segments of equal

visual steps, which are designated by numerical values applied as prefixes

to the symbol for the hue name.2 For example, 10R marks a limit of red 

hue. Four equally spaced steps of the adjacent yellow-red (YR) hue are 

identified as 2.5YR, 5YR, 7.5YR, and 10YR, respectively. The standard

chart for soil has separate hue cards, from 10R through 5Y. In addition, 

special charts for gley colors and for very light colors are available.

Figure 3-15

A schematic diagram showing relationships among hue, value, and chroma in the 

Munsell color system (Rus, 2007).

Value indicates the degree of lightness or darkness of a color in 

relation to a neutral gray scale. On a neutral gray (achromatic) scale, 

value extends from pure black (0) to pure white (10). The value notation 

2 The notation for hue, value, and chroma is a decimal number that can be refined to

any degree. In practice, however, only the divisions on the color charts are used.
Soil Survey Manual 1

is a measure of the amount of light that reaches the eye under standard 

lighting conditions. Gray is perceived as about halfway between black 

and white and has a value notation of 5. The actual amount of light 

that reaches the eye is related logarithmically to color value. Lighter 

colors are indicated by numbers between 5 and 10; darker colors are 

indicated by numbers from 5 to 0. These values may be designated for 

either achromatic (i.e., having no hue and chroma of 0) or chromatic (i.e., 

having all three components—hue, value, and chroma) conditions. Thus, 

a card of the color chart for soil has a series of chips arranged vertically 

to show equal steps from the lightest to the darkest shades of that hue. 

Figure 3-14 shows this arrangement vertically on the card for the hue of

10YR. Note that the highest value shown on the standard color cards is 

8. Color chips with value of 9 are included on special color cards for very 

light colors.

Chroma is the relative purity or strength of the spectral color. It 

indicates the degree of saturation of neutral gray by the spectral color. 

The scales of chroma for soils extend from 0 (for neutral colors) to 8 

(for colors with the strongest expression). The color chips are arranged 

horizontally by increasing chroma from left to right on the soil-color 

chart (see fig. 3-14).

On the soil-color chart for a specific hue (e.g., 10YR), the darkest

shades of that hue are at the bottom of the card and the lightest shades are 

at the top. The weakest expression of chroma (the grayest color) is at the 

left, and the strongest expression of chroma is at the right.

At the extreme left of some cards are symbols such as N 6/. These 

colors have zero chroma and are totally achromatic (neutral). They have 

no hue and no chroma but range in value from black (N 2.5/) to white 

(N 8/). An example of a notation for a neutral (achromatic) color is N 5/ 

(gray). The color 10YR 5/1 is also called gray because the hue is hardly 

perceptible at such low chroma.

Conditions for Measuring Color

The quality and intensity of the light source affect the amount and 

quality of the light reflected. The moisture content of the sample and the

roughness of its surface affect the light reflected. The visual impression

of color from the standard color chips is accurate only under standard 

conditions of light intensity and quality. Color determination may be 

inaccurate early in the morning or late in the evening. When the sun is low 

in the sky or the atmosphere is smoky, the light reaching the sample and 

the light reflected are redder. Even though the same kind of light reaches

the color standard and the sample, the reading of sample color at these 
150 Chapter 

times is commonly one or more intervals of hue redder than at midday. 

Colors also appear different in the subdued light of a cloudy day than in 

bright sunlight. If artificial light is used, as for color determinations in

an office, the light source must be as near the white light of midday as

possible. With practice, compensation can be made for the differences. 

The intensity of incidental light is especially critical when matching soil 

to chips of low chroma and low value.

Roughness of the reflecting surface affects the amount of reflected

light, especially if the incidental light falls at an acute angle. The incidental 

light should be as near as possible at a right angle. For crushed samples, 

the surface is smoothed and the state is recorded as “dry, crushed and 

smoothed.”

Guidelines for Recording Color

Uncertainty

Under field conditions, measurements of color are reproducible by

different individuals within 2.5 units of hue (one Munsell soil-color 

chart) and 1 unit of value and chroma. Notations are made to match the 

chips included on the color charts, typically the nearest whole unit of 

value and chroma. Soil color should be recorded to the closest color chip 

provided but not interpolated between chips. For some hues, chips for 

value of 2.5 are included.

Determinations typically are not precise enough to justify interpolation between chromas of 4 and 6 or between chromas of 6 and 

8. Color should never be extrapolated beyond the highest chip. The 

soil-color charts for individual hues do not show value greater than 8. 

However, chips with higher values are included on a special “white” 

chart and should be used for soils with very light colors (e.g., those with a 

high content of calcium carbonate). Observed colors are always rounded 

to the nearest chip.

For many purposes, the differences between colors of some adjacent 

color chips have little significance. For these, color notations have been

grouped and named (see fig. 3-14).

Dominant Color

The dominant color is the one that occupies the greatest layer volume. 

It is always listed first among the colors of a multicolored layer. It is

determined using the colors on ped faces or broken peds or on a matrix 

sample in structureless horizons. If two colors occur, the dominant color 

makes up more than 50 percent of the volume. If three or more colors are 
Soil Survey Manual 1

noted, the dominant color makes up more of the layer volume than any 

other color, although it may occupy less than 50 percent. The expression 

“brown with yellowish brown and grayish brown” signifies that brown is

the dominant color and may, or may not, make up more than 50 percent 

of the layer. 

In some layers, no single color is dominant and the first color listed

is not more prevalent than others. The expression “brown and yellowish 

brown with grayish brown” indicates that brown and yellowish brown 

make up about equal amounts and are codominant. If the colors are 

described as “brown, yellowish brown, and grayish brown,” the three 

colors make up nearly equal parts of the layer.

Other Non-Matrix Colors

In addition to either a single dominant matrix color or two or more 

codominant matrix colors, other non-matrix colors may be present. 

Non-matrix colors are generally related to one of the following four 

situations: 

1. The additional colors are associated with a ped or void surface 

feature (such as clay films, silt coatings, slickensides, etc.).

2. The colors are associated with concentrations in the soil (such as 

plinthite, calcium carbonate, gypsum crystals, etc.). 

3. The colors are due to oxidation and/or reduction processes in 

wet soil (i.e., redoximorphic features, such as iron masses, iron 

depletions, and manganese nodules). 

4. The color is inherited from the parent material and is not the 

result of pedogenic processes. These colors are lithochromic or 

lithomorphic and described as mottles.

Protocols for describing redoximorphic features, surface features, 

and concentrations in the soil (including color) are presented later in this 

chapter.

Mottling

Mottling refers to repetitive color changes that cannot be associated 

with compositional properties of the soil. As described above, a color 

pattern related to a ped surface or other organizational or compositional 

feature is not mottling. In horizon description, mottle description follows 

dominant color. Mottles (and other non-matrix features) are described by 

quantity, size, contrast, color, and, if important, other attributes such as 

moisture state, shape, and location, in that order.

Quantity is indicated by three areal percentage classes of the 

observed surface:
152 Chapter 

Few ............ less than 2 percent

Common .... 2 to less than 20 percent

Many .......... 20 percent or more

The notations must clearly indicate the colors to which the terms 

for quantity apply. For example, “common grayish brown and yellowish 

brown mottles” could mean that each color makes up 2 to 20 percent 

of the horizon. By convention, the example is interpreted to mean that 

the quantity of the two colors together is between 2 and 20 percent. If 

each color makes up between 2 and 20 percent, the description should 

be “common grayish brown (10YR 5/2) and common yellowish brown 

(10YR 5/4) mottles.”

Size refers to dimensions as seen on a plane surface. If the length 

of a mottle is not more than two or three times the width, the dimension 

recorded is the greater of the two. If the mottle is long and narrow, as a 

band of color at the periphery of a ped, the dimension recorded is the 

smaller of the two and the shape and location are also described. Five 

size classes are used to describe mottles:

Fine ........................ smaller than 2 mm

Medium ................. 2 to less than 5 mm

Coarse .................... 5 to less than 20 mm

Very coarse ............ 20 to less than 76 mm

Extremely coarse 76 mm or more

Contrast refers to the degree of visual distinction that is evident 

between associated colors. The criteria for determining contrast class are 

given in table 3-5. The classes for color contrast are:

Faint.—Color is evident only on close examination. 

Distinct.—Color is readily seen but contrasts only moderately with 

the color to which it is compared. 

Prominent.—Color contrasts strongly with the color to which it is 

compared. Prominent colors are commonly the most obvious 

color feature of the section described. 

Contrast is often not a simple comparison of one color with another 

but is a visual impression of the prominence of one color against a 

background of several colors.

Mottles and other features (if significant) are described using terms

for shape, location, and boundary character. 

Shape.—These terms are the same as those used for other concentrations in the soil (i.e., cylindrical, platy, reticulate, etc.). 
Soil Survey Manual 1

Table 3-5

Color Contrast Class Terms and Their Criteria

Contrast class Difference between compared colors

Faint*

Hue Value Chroma

0; < 2 and < 1

1; < 1 and < 1

2; 0 and 0

Distinct*

0; < 2 and > 1 to < 4

or

> 2 to < 4 and < 4

1; <1 and > 1 to < 3

or

> 1 to < 3 and < 3

2; 0 and > 0 to < 2

or

0 to < 2 and < 2

Prominent*

0; > 4 or > 4

1; > 3 or > 3

2; > 2 or > 2

3

* If the compared colors have both a value < 3 and a chroma < 2, the contrast is faint, 

regardless of hue differences.

Location.—The location of the mottles relative to structure of the soil 

is described.

Boundary classes.—Terms are as follows:

Sharp: Color grades over less than 0.1 mm. Gradation is barely 

discernable or not discernible by the naked eye, but visible 

under a 10X lens. 

Clear: Color grades over more than 0.1 mm but less than 2 mm. 

Gradation can be obscure but visible to the naked eye. A 10X 

lens is not required.

Diffuse: Color grades over 2 mm or more. Gradation is easily 

discernable by the naked eye. A 10X lens is not required.
154 Chapter 

Moisture state and physical state of the dominant color are presumed 

to apply to the mottles unless the description states otherwise. For 

example, the description of a sample with a specified standard moist

broken state may be “brown (10YR 4/3), brown (10YR 5/3) dry; many

medium distinct yellowish brown (10YR 5/6) mottles, brownish yellow 

(10YR 6/6) dry.” Alternatively, the colors in the standard moisture state 

may be given together, followed by the colors in other moisture states. 

The color of mottles commonly is given only for the standard state unless 

colors in another state have special significance.

An example of a description of a sample with a nearly equal mixture 

of two colors for a moist broken standard state is “intermingled brown 

(10YR 4/3) and yellowish brown (10YR 5/6) in a medium distinct pattern;

brown (10YR 5/3) and brownish yellow (10YR 6/6) dry.” If a third color

is present, it can be added, for example, “common medium faint dark 

grayish brown (10YR 4/2) mottles, grayish brown (10YR 5/2) dry.”

If the mottles are fine and faint and cannot be compared easily

with the color standards, the Munsell notation should be omitted. Other 

abbreviated descriptions are used for specific circumstances.

Color Patterns Within the Soil

Color may be recorded separately for features that merit a distinct 

description, especially for redoximorphic features but also for peds, 

concretions, nodules, cemented bodies, filled animal burrows, etc. Color

patterns that exhibit a spatial relationship to composition changes or to 

features, such as nodules or surfaces of structural units, are useful in 

descriptions because they can infer genesis and soil behavior. Colors may 

be given for extensions of material from another soil layer. For example, 

the fine tubular color patterns that extend vertically below the A horizon

of some wet soils were determined by the environment adjacent to 

roots that once occupied the tubules. The relationship of redoximorphic 

features to locations in the horizon (such as ped faces, ped interiors, pore 

linings, etc.) provides important clues about internal patterns of wetness. 

For example, a rim of bright color around an inner zone of lighter color 

at the surface of some peds relates to water movement into and out of the 

peds and to oxidation-reduction relationships.

Ground Surface Color

The ground surface color has an important effect on heat transmission 

into the soil. Albedo (the ratio of the reflected incident short-wave solar

radiation to the total amount received) is related to soil color, especially 

value. It is an essential parameter for estimating evapotranspiration and 

for calculating water balance for hydrological models. The color value 
Soil Survey Manual 1

of the immediate ground surface may differ markedly from that of the 

surface horizon. For example, raindrop impact that removed clay-sized 

material from the surface of sand and silt particles may result in a thin 

surface crust about a millimeter thick with higher color value. Albedo for 

the fine-earth soil component of the surface cover (given the surface is

smooth) can be estimated with the equation: 

albedo = 0.069 * (value dry) - 0.114 

Specialized studies involving model inputs that include albedo use 

color information for the total ground surface, including vegetation as 

well as soil material. In some arid soils, dark rock fragments may have 

reduced the color value of the ground surface appreciably from that of the 

fine earth of the surface horizon as a whole. Furthermore, dead vegetation

may have color values that differ appreciably from those for the fine

earth of the surface horizon. Surface color influences reflectivity of light,

which influences the capacity to absorb and release radiant energy.

Soil surface colors at a given site commonly range widely due to the 

presence of more than one kind of cover. It may be necessary to estimate 

the areal proportion of the color value for each ground surface type separately (such as rock fragments, dead vegetation, or fine earth), and then

select a single color value for each important ground surface component. 

From the areal proportion of the components and their color value, a 

weighted average color value for the ground surface may be computed. 

Soil Structure

Soil structure refers to units composed of primary particles. 

Cohesion within these units is greater than the adhesion among units. 

As a consequence, the soil mass under stress tends to rupture along 

predetermined planes or zones. These planes or zones form the boundary 

of the structural units. Compositional differences of the fabric matrix 

appear to exert weak or no control over where the bounding surfaces 

occur. If compositional differences control the bounding surfaces of the 

body, then the term “concentration” is used. The term “structural unit” is 

used for any repetitive soil body that is commonly bounded by planes or 

zones of weakness that are not an apparent consequence of compositional 

differences. A structural unit that is the consequence of soil development 

is called a ped. The surfaces of peds persist through cycles of wetting 

and drying in place. Commonly, the surface of the ped and its interior 

have different composition or organization, or both, because of soil 

development. In contrast to peds, soil-forming processes exert weak or 
156 Chapter 

no control on the boundaries of earthy clods. Clods commonly form in 

the surface layer due to the rearrangement of primary particles to a denser 

configuration through plowing or other mechanical disturbance. The

same terms and criteria used to describe structured soils should be used 

to describe the shape, grade, and size of clods. Commonly, clods have a 

blocky shape and are large enough to affect tilth adversely. Although the 

descriptive terms are used for both structural units and clods, this does 

not infer that clods are the result of pedogenic processes like structural 

units are. To avoid misunderstanding, the word “clods” is substituted for 

“structure” in written descriptions (e.g., strong, coarse, angular blocky 

clods).

Some soils lack structure and are referred to as structureless. In 

structureless layers or horizons, no units are observable in place or after 

the soil has been gently disturbed, such as after tapping a spade containing 

a slice of soil against a hard surface or dropping a large fragment of the 

soil on the ground. When structureless soils are ruptured, coherent soil 

fragments or single grains, or both, result. Structureless soil material may 

be either single grain or massive. In addition to lacking structure, soil 

material of single grains is loose. On rupture, more than 50 percent of the 

mass consists of discrete mineral particles. 

Some soils have simple structure, where each unit is an entity without 

component smaller units. Others have compound structure, where large 

units are composed of smaller units separated by persistent planes of 

weakness. 

In soils that have structure, the shape, size, and grade (distinctness) 

of the units are described. Field terminology for soil structure has 

separate sets of terms designating each of the three properties that, when 

used in combination, form the names for structure. For example, “strong 

fine granular structure” is used to describe a soil that separates almost

entirely into discrete units that are loosely packed, roughly spherical, and 

mostly between 1 and 2 mm in diameter. The designation of structure 

by grade, size, and shape can be modified with other appropriate terms

to describe other characteristics, e.g., “moderate medium lenticular 

structure with peds tilted about 15 degrees from horizontal (upslope).” 

Surface characteristics of units are described separately.

Shape

Several basic shapes of structural units are recognized in soils 

(fig. 3-16). Supplemental statements about the variations in shape of

individual peds are needed in detailed descriptions of some soils. The 

following terms describe the basic shapes and related arrangements: 
Soil Survey Manual 1

Figure 3-16

Prismatic

(Subangular) (Angular)

Examples of Soil Structure Types

Blocky

Massive

(Continuous, unconsolidated mass) 

Wedge

Platy

Lenticular

Single Grain

(Loose mineral/rock grains) 

Granular

(Soil aggregates)

Columnar

Structureless Types

Examples of soil structure types.

Platy.—The units are flat and platelike. They are generally oriented

horizontally. 

Prismatic.—The individual units are bounded by flat to rounded

vertical faces. Units are distinctly longer vertically, and the 

faces are typically casts or molds of adjoining units. Vertices 

are angular or subrounded; the tops of the prisms are somewhat 

indistinct and normally flat. Figure 3-17 shows a soil profile with

prismatic structure in the subsoil.

Columnar.—The units are similar to prisms and bounded by flat or

slightly rounded vertical faces. The tops of columns, in contrast 

to those of prisms, are very distinct and normally rounded. 

Blocky.—The units are blocklike or polyhedral. They are bounded 

by flat or slightly rounded surfaces that are casts of the faces of

surrounding peds. Typically, blocky structural units are nearly 
158 Chapter 

equidimensional but grade to prisms and plates. The structure is 

described as angular blocky (fig. 3-18) if the faces intersect at

relatively sharp angles and as subangular blocky if the faces are 

a mixture of rounded and plane faces and the corners are mostly 

rounded.

Granular.—The units are approximately spherical or polyhedral. 

They are bounded by curved or very irregular faces that are not 

casts of adjoining peds. 

Wedge.—The units are approximately elliptical with interlocking 

lenses that terminate in acute angles. They are commonly 

bounded by small slickensides. 

Lenticular.—The units are overlapping lenses parallel to the soil 

surface. They are thickest in the middle and thin towards the 

edges. Lenticular structure is commonly associated with moist 

soils, texture classes high in silt or very fine sand (e.g., silt loam),

and high potential for frost action. 

Figure 3-17

Prismatic soil structure. (Photo courtesy of John Kelley)
Soil Survey Manual 1

Figure 3-18

Peds with angular blocky structure. (Photo courtesy of John Kelley)

Size

The six size classes are very fine, fine, medium, coarse, very coarse, 

and extremely coarse. The extremely coarse class is used only for 

prismatic, columnar, and wedge-shaped structures. The size limits of the 

classes differ according to the shape of the units. Table 3-6 gives the size

limit classes. The size limits refer to the smallest dimension of plates, 

lenses, prisms, and columns. For the lens-shaped structures (wedge and 

lenticular), the measurement is taken at the thickest part of the smallest 

dimension, not the tapered edges. If the units are more than twice the 

minimum size of the largest class, the actual size is given, e.g., “moderate 

very coarse prisms 100 to 150 cm across.”

Grade

Grade indicates the distinctness of units. Criteria for grade classes 

are the ease of separation into discrete units and the proportion of units 

that hold together when the soil is handled. Three classes are used: 

Weak.—The units are barely observable in place. When they are 

gently disturbed, the disturbed soil material parts into a mixture 
160 Chapter 

Table 3-6

Size Class Terms for Peds with Various Soil Structure Types

Classes

Shape of structure

Platy* and 

granular

(mm)

Prismatic, 

columnar, 

and wedge

(mm)

Blocky and 

lenticular*

(mm)

Very fine < 1 < 10 < 5

Fine 1 to < 2 10 to < 20 5 to < 10

Medium 2 to < 5 20 to < 50 10 to < 20

Coarse 5 to < 10 50 to < 100 20 to 50

Very coarse ≥ 10 100 to < 500 ≥ 50

Extremely coarse N/A ≥ 500 N/A

* In describing plates, “thin” is used instead of “fine” (i.e., very thin and thin) and

“thick” instead of “coarse” (i.e., thick and very thick).

of whole and broken units, the majority of which exhibit no 

planes of weakness. Faces that indicate persistence through 

wet-dry cycles are evident if the soil is handled carefully. 

Distinguishing structureless soils from those with weak structure 

can be difficult. Weakly expressed structural units in virtually

all soil materials have surfaces that differ in some way from the 

interiors. 

Moderate.—The units are well formed and evident in undisturbed 

soil. When disturbed, the soil material parts into a mixture of 

mostly whole units, some broken units, and material that is not in 

units. Peds part from adjoining peds to reveal nearly entire faces 

that have properties distinct from those of fractured surfaces. 

Strong.—The units are distinct in undisturbed soil. They separate 

cleanly when the soil is disturbed. When removed, the soil 

material separates mainly into whole units. Peds have distinctive 

surface properties.

The distinctness of individual structural units and the relationship 

of cohesion within units to adhesion between units determine grade 

of structure. Cohesion alone is not specified. For example, individual

structural units in a sandy loam A horizon may have strong structure yet 
Soil Survey Manual 1

be less durable than individual units in a silty clay loam B horizon of 

weak structure. The degree of disturbance required to determine structure 

grade depends largely on moisture content and percentage and kind of 

clay. Only slight disturbance may be necessary to separate the units of 

a moist sandy loam having strong granular structure, while considerable 

disturbance may be required to separate units of a moist clay loam having 

strong blocky structure.

Compound Structure

Smaller structural units can hold together to form larger units. Grade, 

size, and shape are described for both kinds of units and the relationship 

of one set to the other is indicated, e.g., “strong medium angular blocks 

within moderate coarse prisms” or “moderate coarse prismatic structure 

parting to strong medium subangular blocky.”

Extra-Structural Cracks

Cracks are macroscopic vertical planar voids that are much smaller 

in width than in length and depth. A crack represents the release of strain 

as a consequence of drying. In contrast to the relatively narrow voids 

surrounding peds in most soils, the cracks discussed here are the result of 

localized stress release, which forms planar voids that are wider than the 

repetitive planar voids normally associated with structural units. 

Importance

Cracks, especially large ones, affect water flow into and through

the soil, causing it to bypass the soil matrix (bypass flow). They exert

significant control on ponded infiltration and hydraulic conductivity,

especially if they extend to (or close to) the surface. Cracks are generally 

associated with soils that are subject to pronounced shrinking and 

swelling. They can indicate potential engineering hazards to homes, 

roads, and other structures. For taxonomic purposes, the width and depth 

of cracks as well as their temporal open-close cycles have importance. 

The areal percentage of such cracks, either on a vertical exposure or on 

the ground surface, can be measured by line-intercept methods.

Kinds of Cracks

Cracks are characterized as either crust-related or trans-horizon. 

Crust-related cracks are shallow cracks that initiate at the surface and 

are restricted to a surface crust layer. They form primarily from raindrop 

impact and soil puddling followed by drying and consolidation. 
162 Chapter 

Trans-horizon cracks commonly extend across more than one 

horizon. They may extend upward to the soil surface and downward to 

significant depth. These cracks are commonly associated with soils that

have a high content of smectitic clay minerals. They open as the soil dries 

out and close upon rewetting. Less commonly, some trans-horizon cracks 

form upon dewatering and subsequent consolidation of poorly drained 

sediments with high n value (fluid materials), e.g., upon drainage of

some soils that are classified as Hydraquents. Once formed, these cracks

do not open and close seasonally but rather remain open permanently.

Crust-related cracks.—Two kinds of crust-related cracks are recognized: reversible and irreversible.

Surface-initiated reversible crust-related cracks form as a result of 

drying from the surface downward. They close after relatively 

slight surficial wetting and have little influence on ponded

infiltration rates. These cracks tend to be very shallow (less than

about 0.5 cm) and are transient (i.e., close upon wetting). 

Surface-initiated irreversible crust-related cracks form as a result of 

the near-surface water reduction in material with an exceptionally 

high water content, commonly from frost action. These cracks 

tend to be shallow (between about 0.5 and 2 cm) and seasonally 

transient. The cracks may not close completely when rewet and 

extend through the crust. They increase ponded infiltration rates,

but only to a small degree.

Trans-horizon cracks.—Two kinds of trans-horizon cracks are recognized: reversible and irreversible.

Subsurface-initiated reversible trans-horizon cracks form as a result 

of appreciable reduction in water content from field capacity in

horizons or layers with considerable extensibility (fig. 3-19).

They close in a period of days if the horizon is brought to the 

moderately moist or wetter state. They extend upward to the soil 

surface unless there is a relatively thick overlying horizon that 

is very weakly compacted (loose or very friable) and does not 

permit the propagation of cracks. These cracks greatly influence

ponded infiltration rates, hydraulic conductivity, and evaporation.

Subsurface-initiated irreversible trans-horizon cracks are the permanent cracks described in Soil Taxonomy (as described for soil 

families). They have a similar origin to surface-initiated irreversible cracks, although quite different agencies of formation are 

involved. Rather than forming due to shrinkage of the surface 

layer upon air drying, these cracks form due to subsoil drainage 

and subsequent consolidation of some very fluid soils.
Soil Survey Manual 1

Figure 3-19

Large reversible trans-horizon cracks extend from the soil surface deep into the subsoil 

of this clayey soil, which is classified as a Vertisol.

Descriptions of Cracks 

Descriptions of cracks include:

Relative frequency.—Average number of cracks per square meter.

Depth.—Average depth of penetration.

Kind.—Reversible crust-related, irreversible crust-related, reversible 

trans-horizon, or irreversible trans-horizon.

If the cracks do not extend to the surface, this should be noted. 

Examples of crack descriptions: “On average, five reversible transhorizon cracks per square meter extend from the surface to about 50 cm” 

and “on average, five reversible trans-horizon cracks per square meter

beginning below 18 cm extend to about 50 cm.”

Internal Ped and Void Surface Features

Features formed by pedogenic processes commonly occur on or beneath ped or void surfaces. Such features include: (1) coats of various 

substances covering part or all of surfaces, (2) material concentrated on 
164 Chapter 

surfaces due to preferential removal of finer material, (3) stress formations in which thin layers at the surfaces have undergone particle reorientation or packing by stress and/or shear, and (4) material infused 

beneath surfaces (termed a “hypocoat”). All these features differ from the 

adjacent material in composition, orientation, and/or packing. Hypocoats 

commonly result from oxidation and reduction processes and are generally described as redoximorphic features (discussed below). 

Description of surface features may include kind, location, amount, 

and distinctness. Color, texture, and other characteristics may also be 

described, especially if the features contrast strongly with characteristics 

of adjacent material.

Kinds

Surface features are distinguished by differences in texture, color, 

packing, particle orientation, or reaction to selected tests. If a feature 

is distinctly different from the adjacent material but its kind cannot be 

determined, it is still described. 

Clay films.—Thin layers of oriented, translocated clay; also called 

clay skins or argillans (fig. 3-20).

Clay bridges.—Illuvial clay linking together adjacent mineral grains 

(fig. 3-21).

Figure 3-20

Shiny clay films coat the surface of this ped. (Photo courtesy of John Kelley)
Soil Survey Manual 1

Figure 3-21

Sand grains (visible as individual quartz grains) coated and bridged with illuvial clay 

(smooth yellowish color). (Photo courtesy of John Kelley)

Sand or silt coats.—Sand or silt grains adhering to a ped, void, or 

crack surface. The grains may be derived from material originally 

in a horizon from which finer particles have been removed.

These coats are also referred to as skeletans. Sand or silt coats 

may also form by translocation and deposition of sand or silt 

from upper horizons or via oxidation-reduction reactions that 

preferentially remove iron and/or manganese and, in some cases, 

clay. Coats inferred to form from oxidation-reduction reactions 

are described as redoximorphic features.

Other coats.—Coats composed of iron, aluminum, or manganese 

oxides; organic matter; salts; or carbonates. They are described 

by properties that can be observed in the field. Laboratory

analyses may be needed for verification.

Stress surfaces.—Pressure faces, also referred to as stress cutans.

These surfaces are smoothed or smeared. They form through 

rearrangement caused by shear forces. They may persist through 

successive drying and wetting cycles. Although similar in 

appearance to clay films, pressure faces can be distinguished by

sand grains that protrude slightly above the surface and are not 

coated with clay.
166 Chapter 

Slickensides.—Stress surfaces that are polished and striated (fig.

3-22). They typically have dimensions exceeding 5 cm. They

are produced when a relatively large volume of soil slides over 

another. They are common below a depth of 50 cm in swelling 

clays (clays subject to large changes in water state). Slickensides 

associated with structural surfaces resulting from pedogenesis 

are considered pedogenic in nature. Those associated with faults 

or mass soil movement are considered geogenic.

Figure 3-22

Prominent slickensides in the Bss horizon of a Vertisol.

Location

Various surface features may occur on some or all structural units, 

channels, pores, primary particles or grains, soil fragments, rock 

fragments, or pararock fragments. The kind and orientation of the surface 

on which features are observed are always described (e.g., “clay films are

on vertical but not horizontal faces of peds”). 

Amount

The percentage of the total surface area occupied by a particular 

surface feature over the extent of the horizon or layer is described. 
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Amount can be characterized using the following classes: 

Very few .......... less than 5 percent

Few .................. 5 to less than 25 percent

Common .......... 25 to less than 50 percent

Many ................ 50 to less than 90 percent

Very many 90 percent or more

These classes are also used to describe the amount of bridges 

connecting particles. This amount is based on the percentage of particles 

of a designated size that are joined to adjacent particles of similar size by 

bridges at contact points.

Distinctness

Distinctness refers to the ease and degree of certainty with which a 

surface feature can be identified. It is related to thickness, color contrast

with the adjacent material, and other properties. However, it is not itself 

a measure of any one of them (e.g., some thick coats are faint and some 

thin coats are prominent). The distinctness of some surface features 

changes markedly as water state changes. The classes of distinctness are: 

Faint.—Feature is evident only on close examination with 10X 

magnification and cannot be identified positively in all places

without greater magnification. The contrast with the adjacent

material in color, texture, and other properties is minimal.

Distinct.—Feature can be detected without magnification, although

magnification or tests may be needed for verification. The feature

contrasts enough with the adjacent material that differences in 

color, texture, or other properties are evident. 

Prominent.—Feature is conspicuous without magnification when

compared to a surface broken through the soil. Color, texture, or 

other property or a combination of properties contrasts sharply 

with properties of the adjacent material, or the feature is thick 

enough to be conspicuous.

The typical order of description is: amount, distinctness, color, 

texture, kind, and location. For example: “few distinct grayish brown 

(10YR 5/2) clay films on vertical faces of peds” or “many distinct brown

(10YR 4/3) clay bridges between mineral grains.” Only properties that

add to the understanding of the soil are listed. If texture of the surface 

feature is obvious, as in most stress surfaces, it is not described. Kind 

and location are needed for all features identified. Volume, if important,

is estimated separately.
168 Chapter 

Concentrations

Concentrations are identifiable bodies within the soil that form

and accumulate due to pedogenesis. Pedogenic processes responsible 

for concentration development in the soil include chemical dissolution 

and precipitation, oxidation/reduction, and accrual due to physical or 

biological processes. Some concentrations are thin and sheet-like, some 

are nearly equidimensional, and others have irregular shapes. They may 

contrast sharply with the surrounding material in strength, composition, 

or internal organization, or their differences with the surrounding material 

may be slight. Rock and pararock fragments or inherited minerals (such 

as pockets of mica flakes) are not considered concentrations.

Kinds

Masses are non-cemented concentrations that commonly cannot be 

removed from the soil as a discrete unit. Masses may consist of, but are 

not limited to, calcium carbonate (fig. 3-23), fine crystals of gypsum or

soluble salts, or iron and manganese oxides. In most cases, masses form 

in place.

Plinthite consists of reddish, iron-enriched bodies that have a low 

content of organic matter. In contrast to most other masses, plinthite 

bodies are coherent enough to be separated readily from the surrounding 

soil.

Plinthite commonly occurs within and above reticulately mottled 

horizons (fig. 3-24). It has higher penetration resistance than adjacent

brown or gray bodies or than red bodies that do not harden. Soil layers 

that contain plinthite rarely become dry in their natural setting. Plinthite 

bodies are commonly about 5 to 20 mm across their smallest dimension. 

They are firm or very firm when moist, hard or very hard when air dry, 

and moderately cemented on repetitive wetting and drying, especially 

when exposed to sunlight (e.g., in road banks and gully walls). 

Upon repeated wet-dry cycles, plinthite may irreversibly harden 

and convert to indurated ironstone. Plinthite bodies commonly occur 

as discrete nodules or plates. The plates are oriented horizontally. 

The nodules occur above, and the plates within, the upper part of the 

reticulately mottled horizon. The plates generally have a uniformly 

reddish color and have sharp boundaries with the surrounding brown or 

gray material. The part of the iron-rich body that is not plinthite normally 

stains the fingers if rubbed while wet, but the plinthite center does 

not. Plinthite has a harsh, dry feel when rubbed, even if wet. Horizons 
Soil Survey Manual 1

Figure 3-23

Masses of secondary calcium carbonate (white bodies below a depth of about 60 cm) in 

the calcic horizon of an Aridisol.

containing plinthite are more difficult to penetrate with an auger than

adjacent horizons at the same water state and with the same clay content. 

Plinthite generally becomes less cemented after prolonged submergence 

in water. An air-dried sample can be dispersed by normal procedures for 

particle-size distribution.

Ironstone is an iron-oxide concentration that is at least weakly 

cemented. Ironstone nodules commonly occur in layers above plinthite. 

They are thought to be plinthite that has cemented irreversibly because 

of repeated wet-dry cycles. Commonly, the center of iron-rich bodies 

cement upon repeated wetting and drying while the periphery does not. 
170 Chapter 

Figure 3-24

A soil in which a reticulately mottled zone with plinthite (darkest red colors) is below a 

depth of about 2 feet.

Nodules and concretions are cemented bodies of various shapes that 

can be removed from the soil intact and do not slake in water. They do 

not have a crystal structure discernable by field observation (10X lens).

Concretions differ from nodules by internal organization. They possess 

a crude internal symmetry organized around a point, a line, or a plane. 

The internal structure typically takes the form of concentric layers that 

are visible to the naked eye. A coat or a thin outer layer of an otherwise 

undifferentiated body does not indicate a concretion. Nodules, by 

contrast, lack evident internal organization.

Crystals are macro-crystalline forms of relatively soluble salts that 

form in place. They may occur singly or in clusters (fig. 3-25). Gypsum,
Soil Survey Manual 1

Figure 3-25

A cluster of gympsum crystals (selenite) in an Aridisol.

calcite, halite, and other salt crystals are common in arid and semiarid 

soils. Crystals composition should be denoted if known.

Finely disseminated materials are small precipitates dispersed 

throughout the matrix. The material is not observable with the naked 

eye but can be detected by chemical reactions (e.g., effervescence of 

calcium carbonate with dilute HCl). An example is calcium carbonate 

that has accrued due to dust fall and its subsequent dissolution and reprecipitation throughout the matrix of a horizon.

Biological concentrations are discreet bodies accumulated by biological process. Examples include fecal pellets and wormcasts.

Inherited minerals consist of distinct, observable mineral particles 

in the soil that formed from geologic rather than pedogenic processes. 

Mica flakes and glauconite pellets are examples.

Describing Concentrations Within the Soil

Concentrations within the soil may have several important attributes, 

including number or amount, size, shape, consistence, color, composition, 

kind, and location. Not all attributes are necessarily described. The order 

as listed above is convenient for describing them, e.g., “many, fine,

irregular, hard, light gray (10YR 7/1) carbonate nodules distributed 
172 Chapter 

uniformly through the horizon.” Descriptions for kind have already 

been discussed in this section, and those for consistence and color are 

discussed in other parts of this chapter.

Amount or quantity of concentrations refers to the relative volume 

of a horizon or other specified unit occupied by the bodies. The classes

are the same as those used for quantity of redoximorphic features and 

mottles:

Few ............ less than 2 percent

Common .... 2 to less than 20 percent

Many .......... 20 percent or more

Size may be measured directly or designated by a class. The 

dimension to which size-class limits apply depends on the shape of the 

body described. If the body is nearly uniform, the shortest dimension is 

measured, such as the effective diameter of a cylinder or the thickness 

of a plate. For irregular bodies, the longest dimension is measured (if 

needed, more measurements can be given for clarification). The classes

are the same as those used for mottles and redoximorphic features:

Fine ........................ less than 2 mm

Medium ................. 2 to less than 5 mm

Coarse .................... 5 to less than 20 mm

Very coarse ............ 20 to less than 76 mm

Extremely coarse ... 76 mm or more

Shape of concentrations varies according to kinds of concentrations 

and commonly within a concentration. (Shape terms are generally not 

used to describe crystals, however, because the crystal type itself implies 

its shape.) The terms for concentration shape are: 

Cubic.—Roughly equidimensional, blocklike structures.

Cylindrical.—Cylindrical or tubular shape; one dimension is greater 

than the other two. 

Dendritic.—Branched, elongated, tubular forms.

Irregular.—Concentrations characterized by nonrepeating spacing or 

shape, but not elongated (as a dendritic form).

Lenticular.—Roughly disk-shaped forms, thickest in the middle and 

thinning toward the edges.

Pendular.—Coatings or nodules formed on the undersides of rock 

fragments.

Platy.—Shaped like a plate; one dimension is much smaller than the 

other two. 

Reticulate.—Crudely interlocking structures (common with some 

plinthite concentrations).
Soil Survey Manual 1

Rosettelike.—Interlocking blade-like structures forming a petal-like 

structure.

Spherical.—Approximately equidimensional and well rounded. 

Threadlike.—Thin, elongated filament-like structures (but not dendritic).

Composition of bodies (calcium carbonate, iron-manganese, 

gypsum, etc.) is described if known and if important for understanding 

their nature or the nature of the soil in which they occur. Some of the 

physical attributes of the interior of a feature are implied by the name. 

Other features, such as enclosed mineral grains, patterns of voids, or 

similarity to the surrounding soil, may be important.

A distinction is made between bodies composed dominantly of a 

single substance and those composed of earthy material impregnated by 

various substances. For many bodies, the chemical composition cannot 

be determined with certainty in the field. If the substance dominates the

body, then the body is described as a substance body (durinode nodules, 

carbonate concretions, salt crystals, etc.). If the substance impregnates 

other material, the body is described as a body of substance accumulation 

(carbonate masses, gypsum masses, plinthite, etc.).

Carbonates and iron commonly dominate or impregnate nodular 

or concretionary bodies. Discrete nodules of clay occur in some soils; 

argillaceous impregnations are less common. Materials dominated by 

manganese are rare; manganese is conspicuous in some nodules that 

have a high content of iron, called “iron-manganese nodules.” Crystals 

are commonly calcite, gypsum, and other salts (such as sodium chloride) 

and less commonly barite, selenite, or satin spar. Some concentrations 

have biological sources, such as fecal pellets and wormcasts.

Pedogenic Carbonates 

Carbonates that have translocated within the soil and subsequently 

precipitated in place from the soil solution are considered to be pedogenic. 

They are not simply inherited from the parent material. They are the same 

as “identifiable secondary carbonates” discussed in Soil Taxonomy.

Forms of Carbonate Accumulation

The term “forms” refers to the outward expression of bodies of 

pedogenic carbonate accumulations. Carbonate itself exists as crystals, 

predominantly calcite (CaCO3

), in the size range of fine silt to coarse

clay (approximately 10 to 1 μm). These crystals precipitated on the 
174 Chapter 

surfaces of rocks, sand, and silt particles or in association with roots and 

microorganisms. With time, carbonate crystals accumulate within the 

soil fabric and are visible as: 

Filaments.—Threadlike concentrations of carbonate typically < 1 

mm in diameter and a few centimeters long.

Root casts.—Branching (and commonly tubular) forms of carbonate 

accumulation (carbonate pseudomorphs of roots). 

Bands.—Sheet-like deposits of carbonate typically ranging from 

about 1 to several millimeters thick. They form along the bedding 

planes of finely stratified parent material and are separated by

soil with little or no macroscopic carbonate.

Joint fillings.—Vertical bands of carbonate in the fracture planes of 

large prisms in soil. Joint fillings, in profile, range from less than

1 to a few centimeters in width.

Coatings.—Deposits of carbonate on the surfaces of rock fragments 

and sand grains. They may be continuous or discontinuous 

and have a rupture resistance ranging from non-cemented to 

extremely weakly cemented. 

Pendants.—Deposits of laminar carbonate coatings on rock that are 

very weakly cemented to indurated. They are more common 

on the bottom of rocks than on the top. They commonly have 

stalactite-like protrusions radiating perpendicularly away from 

the rock fragment.

Masses.—Bodies of carbonate accumulation of various shapes that 

are non-cemented or extremely weakly cemented and cannot be 

removed as discrete units from soil. 

Nodules.—Rounded bodies of carbonate accumulation that are very 

weakly cemented to indurated and can be removed as discrete 

units from soil.

Concretions.—Rounded bodies of carbonate accumulation that 

are very weakly cemented to indurated. They have spherically 

concentric layers surrounding a nucleus.

Cylindroids.—Cylindrical bodies of carbonate accumulations that 

are very weakly cemented to indurated. Many are cicada casts 

impregnated with calcium carbonate while others developed in 

soil material filling former root channels or small krotovinas.

Cylindroids are typically less than 2.5 cm thick. They are 

typically vertical but may also be diagonal or horizontal.

Beds.—Carbonate accumulations along bedding planes of parent 

material that are similar to bands but differ in size (ranging 

from a few centimeters to a meter or more thick). They range 
Soil Survey Manual 1

from non-cemented to indurated, occur below the main zone 

of pedogenic horizons, and preserve the original sedimentary 

structure. 

Plugged horizons.—Pedogenic carbonate accumulations that occur 

at the soil-horizon-landscape scale, which is larger than the soilprofile scale, at which filaments, nodules, and other carbonate

forms occur. They are characterized by laterally continuous 

pedogenic carbonate that has engulfed soil particles, filled most

or all pores, and obliterated the original sedimentary structure. 

Most plugged horizons are strongly cemented to indurated, 

although some are non-cemented.

Laminar horizons.—Smooth, strongly cemented to indurated deposits of carbonate that develop on top of plugged horizons (or 

shallow bedrock). They have a fabric that contains much more 

carbonate than the underlying plugged horizon and essentially 

no allogenic skeletal grains.

Laminae.—Thin (less than 1 mm to a few millimeters) individual 

layers of carbonate comprising the laminar horizon. They 

typically parallel one another, but one set may truncate another 

set at various angles.

Pisoliths.—Subangular to spheroidal carbonate masses (2 to more 

than 100 mm in diameter) that form within highly developed 

petrocalcic horizons. They are characterized by concentric 

banding and an internal structure of disrupted laminae or by 

disrupted concentric banding that may or may not have detrital 

material at the core. 

Ooliths.—Spheroidal carbonate masses (less than 2 mm in diameter) 

that form within highly developed petrocalcic horizons. They 

have an internal structure of laminae that may or may not have 

detrital material at the core.

Stages of Carbonate Accumulation

Pedogenic carbonate that forms in soils in arid and semiarid climates 

is closely linked to age (i.e., progressively older geomorphic surfaces 

have soils with commensurately greater amounts of carbonate). Gile et 

al. (1966) described carbonate accumulation through four successive 

morphogenic stages with a distinction between gravelly and nongravelly 

soils (fig. 3-26). Soils that form in gravelly parent materials progress 

from pebble coatings (Stage I) to interpebble fillings (Stage II), a plugged 

horizon (Stage III), and eventually a laminar horizon atop the plugged 

horizon (Stage IV). Soils that form in nongravelly parent materials
176 Chapter 

progress from filaments (Stage I) to nodules (Stage II), a plugged

horizon (Stage III), and eventually a laminar horizon atop the plugged 

horizon (Stage IV). Bachman and Machette (1977) and Machette (1985) 

recognized two additional stages. Stage V is characterized by laminae 

less than 1 cm thick and may contain pisoliths as well as vertical faces 

and fractures coated with laminated carbonate. Stage VI is characterized 

by multiple generations of recemented laminae, breccia, and pisoliths. 

The time required to reach a certain morphogenetic stage depends on soil 

particle size and quantity of carbonate inputs. Gravelly soils pass through 

the stages more quickly than nongravelly soils because they have less 

surface area and less total pore space. Pedogenic carbonate that forms 

above the phreatic zone by means of capillary rise does not necessarily 

proceed through the morphogenetic stages. 

Figure 3-26

Schematic diagram of diagnostic carbonate morphology for the four main stages of 

carbonate accumulation in two morphogenetic sequences (Gile et al., 1966).
Soil Survey Manual 1

Redoximorphic Features

Redoximorphic features (RMF) include color patterns, mineral 

concentrations, and a reduced (in respect to iron) soil state that form 

via coupled oxidation/reduction reactions involving iron and manganese 

under anaerobic soil conditions. Saturation or near saturation limits 

oxygen diffusion in soil. Microbial activity consumes existing oxygen, 

and an anaerobic condition results. Under anaerobic conditions, certain 

microbes utilize chemical species other than oxygen as the terminal 

electron acceptor for carbon metabolism. Such microbially mediated 

redox processes follow a sequence based on electrochemical or redox 

potential, which is a measure of anaerobic intensity. The metabolic 

energy gain for an organism is the energy difference between reduced 

carbon and the specific electron acceptor. The oxidant with the highest

redox potential is utilized first, followed by the oxidant with the next

highest potential. The favorability order for electron acceptors in soil is:

O2 > NO3

2- > Mn (VI) > Fe (III) > SO42- > CO2

 > H+

Iron (Fe), and to a lesser extent manganese (Mn), serve as soil 

color pigments. Thus, oxidation and reduction of these species results 

in color variations and/or concentrations indicative of soil wetness. 

Reduced iron, Fe(II), and manganese ions are mobile in soil compared 

to their oxidized state. They are subject to leaching or migration from 

higher concentration (an anaerobic zone, such as a ped interior) to 

lower concentration (aerobic zone, such as a ped surface). Upon oxygen 

exposure (higher redox potential), reduced species become oxidized and 

immobile and form iron and manganese concentrations, which commonly 

are redder than the adjacent matrix. Areas that lose iron or manganese are 

pigment depleted (redox depletions) and have a grayer or lighter color 

due to the clean, exposed mineral surface. Depletions of chroma 2 or less 

are key morphological indicators of seasonal or periodic saturation. It is 

important to note that depletions form via iron or manganese reduction 

while concentrations form via iron or manganese oxidation. 

Chemical and physical soil properties influence oxidation-reduction

reactions. Thus, RMF formation may not occur in saturated soils in 

certain settings. For example, higher pH decreases the electron-accepting 

tendency of a chemical species so that RMF formation is less likely in 

alkaline soils. Similarly, because cold temperature reduces microbial 

activity, RMF may not form during winter months even though the soil 

is saturated. Moreover, oxidation/reduction reactions may not produce 

visually observable RMF, such as in red soil parent materials or horizons 

in which organic matter controls soil color.
178 Chapter 

Once formed, iron-oxide concentrations are stable in an oxidized soil 

and depletions remain pigment-free. Thus, some RMF may be relict in 

that they formed under an anaerobic condition that no longer exists. For 

example, a stream terrace that currently lacks internal wetness due to 

stream downcutting may retain RMF formed when a water table existed 

at a higher position in the soil. If proven relict, RMF can be described as 

such. Redoximorphic features in anthropologically drained soils are not 

considered relict because original conditions can be restored. 

Describing Redoximorphic Features

Because RMF have a strong relationship to soil wetness, they 

are typically described separately from other color variations or 

concentrations. Mottles (color variations not due to iron loss or accrual, 

such as variegated weathered rock) are described with soil color. The 

characteristics routinely described for RMF include quantity, size, 

contrast, color, kind, and location. If important, the moisture state, shape, 

hardness, and boundary can also be described. Guidelines for describing 

color and the associated moisture state are the same as those for recording 

color (see previous section). Terms for describing hardness of cemented 

redox concentrations are given in the section “Rupture Resistance” below 

(cementation classes). Terms for describing quantity, size, contrast, color, 

and location of RMF are the same as those for describing concentrations 

within the soil (see section above). Kinds of RMF are discussed below.

Redox Concentrations

Redox concentrations are localized accretions of Fe and/or Mn, 

which may occur as cemented nodules or amorphous phases, that result 

in enhanced pigmentation and/or a cemented precipitate. Generally, 

concentrations form through iron oxidation from Fe(II) to Fe(III), 

which yields a color redder than that of the adjacent matrix (fig. 3-27).

In strongly reducing conditions, however, ferrous iron [Fe(II)] may 

accrete, especially in the presence of S, forming a concentration that is 

black or blue gray in color. Redox concentrations have the following 

forms:

Masses.—Non-cemented bodies or localized regions of enhanced 

pigmentation due to Fe and/or Mn accretion. Masses that occur 

as coatings or thin matrix impregnations along pores (such as 

root channels) are referred to as pore linings.

Nodules or concretions.—Cemented bodies of iron-manganese 

oxides that formed through successive wet-dry cycles. (See the 

discussion of concentrations above for more detail).
Soil Survey Manual 1

Figure 3-27

Redoximorphic features that consist of a redox concentration, as an iron mass (reddish 

area along ped surface) and an iron depletion (light-colored area surrounding the root 

channel in ped interior). (Photo courtesy of John Kelley)

Redox Depletions

Redox depletions are localized zones of decreased pigmentation due 

to a loss of iron or manganese, with or without clay loss. The pigment 

loss produces a color grayer, lighter, or less red than that of the adjacent 

matrix (fig. 3-27). The pigment loss reveals the underlying mineral color.

Redox depletions have a hue that is yellower, greener, or bluer than that 

of the adjacent matrix and/or a higher value and/or a lower chroma. 

Redox depletions include, but are not limited to, what were previously 

called “low chroma mottles” (chroma < 2), which are key indicators 

of seasonal or periodic soil saturation. Redox depletions occur in the 

following forms:

Iron depletions.—Localized zones that have lost iron and/or manganese pigment due to oxidation or reduction reactions under 

anaerobic conditions but that have a clay content similar to that 

of the adjacent matrix. 
180 Chapter 

Clay depletions.—Localized zones that have lost iron, manganese, 

and clay. These features are commonly referred to assilt coatings 

or skeletans. Silt coatings may form by eluvial processes rather 

than from oxidation and reduction. Soil features of inferred 

eluvial origin (for example, albic materials, silt coatings, 

and skeletans) are not considered or described as a redox 

depletion.

Reduced matrix.—A soil horizon, layer, or zone that is reduced in 

respect to iron. It has an in situ matrix chroma < 2 and/or a 

hue of 5GY, 5G, or 5BG that reflects the presence of Fe(II).

The color of a soil sample becomes visibly redder or brighter 

(oxidizes) when exposed to air. The color change typically occurs 

within 30 minutes. A 0.2% solution of alpha,alpha-dipyridyl

dissolved in 1N ammonium acetate (NH4OAc) pH 7 can verify 

the presence of Fe+2 in the field (Childs, 1981).

Consistence

Soil consistence in the general sense refers to the soil material’s 

degree of cohesion and adhesion or resistance to deformation or rupture. 

As described here, consistence includes: (1) resistance of soil material 

to rupture, (2) resistance to penetration, (3) plasticity, toughness, and

stickiness of puddled soil material, and (4) the manner in which the soil 

material behaves when subject to compression. Although several tests are 

described, only those which may be useful should be applied. In addition 

to descriptions of soil consistence, this section discusses the classes for 

excavation difficulty, which is reflective of the consistence properties of

the soil.

Consistence is not synonymous with consistency. Originally, consistency was used in soil engineering to define degree of resistance to

penetration by thumb or thumbnail (test designation D 2488, ASTM, 

2011). The engineering term was generalized for use in soil survey 

and called “consistence.” The set of tests to determine consistency, 

however, is different from those for consistence. Consistence is highly 

dependent on the soil water state and the description has little meaning 

unless the water state class is specified or is implied by the test. For

rupture resistance, separate class terms are provided for tests on dry soil 

(moderately dry or very dry soil water states) and moist soil (slightly 

dry through satiated soil moisture states). To determine cementation 
Soil Survey Manual 1

class, the sample is air dried, then submerged in water for at least 1 hour, 

and checked for slaking. Stickiness, plasticity, and toughness tests are 

performed on puddled soil material. Stickiness determinations are made 

at the highest moisture content, when the sample is most sticky. Tests 

for manner of failure are best performed on moderately moist or wetter 

samples. Penetration resistance depends strongly on moisture state, 

which should always be noted.

The class definitions for rupture resistance and toughness include

both qualitative descriptions as well as quantitative limits for the stress or 

force applied to the sample. Since the perception of the relative amount 

of force required to cause a sample to fail varies by individual, one 

should learn what the various classes of applied force feel like personally. 

The tactile sense of the class limits may be learned by applying force 

to top-loading scales and sensing the pressure through the tips of the 

fingers or through the ball of the foot. Postal scales may be used for the

resistance range testable with the fingers. A bathroom scale may be used

for higher rupture resistance. To calculate force in newtons, multiply kg 

by 10, or pounds by 4.54. One joule is equal to the energy delivered by 

dropping a 1-kg weight from a height of 10 cm. (The 3 joules class limit

is approximately equivalent to dropping a 2-pound weight from a height 

of 1 foot). An easy to develop training tool for calibrating one’s fingers to

estimate applied force is to record the force required to compress springs 

with varying degrees of resistance using scales as described above. A set 

of small springs that approximate the class limits for rupture resistance 

classes can then be used as “known samples” for estimating rupture 

resistance and toughness classes. For determinations on the natural fabric, 

variability among specimens is likely to be large. Multiple measurements 

may be necessary. Recording median values helps reduce the influence of

the extremes measured.

Rupture Resistance for Blocklike Specimens

Table 3-7 shows the classes of resistance to rupture and the means

of determination for specimens that are blocklike. Different class sets are 

provided for moderately dry and very dry soil material and for slightly dry 

and wetter soil material. Unless otherwise specified, the soil water state

is assumed to be that indicated for the horizon or layer when described. 

Cementation is an exception. To test for cementation, the specimen is air 

dried and then submerged in water for at least 1 hour. Cemented materials 

will resist slaking. Cementation class placements do not pertain to the 

soil material at the field water state.
182 Chapter 

The blocklike specimen should be 25 to 30 mm on edge. Direction

of stress relative to the in-place axis of the specimen is not defined unless

otherwise indicated. The specimen is compressed between the extended 

thumb and forefinger, between both hands, or between the foot and a

nonresilient flat surface. If the specimen resists rupture by compression, a

weight is dropped onto it from increasingly greater heights until rupture. 

Failure is at the initial detection of deformation or rupture. Stress applied 

in the hand should be for a period of 1 second. 

Specimens of standard size and shape are not always available. 

While large blocks can be trimmed to the standard size, smaller blocks 

cannot. Blocks of specimens that are smaller than 25 to 30 mm on edge

may still be tested, but the observed force required for rupture needs to 

be adjusted to approximate what would be expected if the block were 

the standard size. The force withstood may be assumed to decrease as 

the reciprocal of the dimension along which the stress is applied. For 

example, if a moist block specimen with a length of 10 mm along 

the direction the force is applied ruptures at 4 N stress, the following 

equation can be used to adjust the observed stress at failure to that for a 

standard 28-mm block:

Adjusted stress value =

[(28 mm/actual cube length mm)2

 x observed stress (N) at failure] 

For the 10-mm specimen in the example above, [(28/10)2

 x 4 N] 

equals an adjusted stress of about 31 N. According to table 3-7, the

rupture resistance class is firm.

Soil structure complicates the evaluation of rupture resistance. If 

a specimen of standard size can be obtained, rupture resistance of the 

standard specimen is reported. Other individual constituent structural 

units can also be described. Typically, the constituent structural units 

must exceed about 5 mm in the direction of applied stress. Expression 

must exceed weak for the rupture resistance of the individual structural 

units to be evaluated. 

If structure size and expression are such that a specimen cannot be 

obtained, then the soil material overall is loose. Structural unit resistance 

to rupture may be determined if the size is large enough (exceeds about 

5 mm in the direction of applied stress) for a test to be performed. Soils 

with moderate or strong structure and structural units that are less than 

5 mm in the direction of applied stress are considered very friable or 

loose.
Soil Survey Manual 1

Table 3-7

Rupture Resistance Classes for Blocklike Specimens

Classes Test description

Moderately 

dry and 

very dry

Slightly 

dry and 

wetter

Air dried, 

submerged Operation Stress applied*

Loose Loose Not 

applicable

Specimen not 

obtainable

Soft Very 

friable

NoncementedFails under very 

slight force 

applied slowly 

between thumb 

and forefinger

< 8 N

Slightly 

hard 

Friable Extremely 

weakly 

cemented 

Fails under slight 

force applied 

slowly between 

thumb and 

forefinger

8 to 

 < 20 N

Moderately 

hard

Firm Very weakly 

cemented

Fails under 

moderate force 

applied slowly 

between thumb 

and forefinger

20 to 

 < 40 N

Hard Very firm Weakly 

cemented

Fails under strong 

force (maximum 

of about 80 N) 

applied slowly 

between thumb 

and forefinger

40 to 

 < 80 N

Very hard Extremely 

firm

Moderately 

cemented

Cannot be failed 

between thumb 

and forefinger

but can be 

between both 

hands or by 

placing on a 

nonresilient 

surface and 

applying gentle 

force underfoot

80 to 

 < 160 N
184 Chapter 

Table 3-7.—continued

Classes Test description

Moderately 

dry and 

very dry

Slightly 

dry and 

wetter

Air dried, 

submerged Operation Stress applied*

Extremely 

hard 

Slightly 

rigid

Strongly 

cemented

Cannot be failed 

in hands but can 

be underfoot by 

full body weight 

(about 800 N) 

applied slowly

160 to 

 < 800 N

Rigid Rigid Very 

strongly 

cemented

Cannot be failed 

underfoot by full 

body weight but 

can be by blow 

of < 3 J

800 N to

< 3 J

Very rigid Very rigid Indurated Cannot be failed 

by blow of < 3 J

> 3 J

* Both force (newtons; N) and energy (joules; J) are employed. The number of 

newtons is 10 times the kilograms of force. One joule is the energy delivered by 

dropping a 1 kg weight 10 cm.

Rupture Resistance for Plate-Shaped Specimens

The following procedure is used to determine rupture resistance 

for plate-shaped specimens, such as surface crusts, peds with platy 

or lenticular structure, and similar plate-shaped specimens for which 

the length and width are several times more than the thickness. The 

specimen should be 10 to 15 mm on edge. It should be about 5 mm thick, 

or the thickness of occurrence if less than 5 mm. For surface crusts, the 

thickness includes the crust proper and the soil material adhering beneath 

it. For some crusts with closely spaced cracks, however, the specimens 

may be too small to make the test applicable. The specimen is grasped 

on edge between extended thumb and forefinger. Force is applied along

the longest of the two principal dimensions. Table 3-8 lists the classes

and their criteria. Compression to failure should be about 1 second in 

duration. 

For calibration of finger force applied with the quantitative class

limits, a scale may be used to both rupture the specimens directly and 

develop the finger tactile sense. Force is applied with the forefinger

through a bar 5 mm across on the scale to create a similar bearing area to 
Soil Survey Manual 1

that of the plate-shaped specimen. The specimen is compressed between 

the thumb and forefinger of one hand while simultaneously the same felt

pressure is exerted on the scale with the forefinger of the other hand. The

scale is read at the failure of the specimen. 

For specimens that cannot be broken between thumb and forefinger,

the resistance to rupture may be evaluated using a small penetrometer. 

The specimen is formed by orienting the two larger surfaces parallel to 

one another and then creating a flat surface. It is placed with one of the

larger faces downward on a nonresilient surface. Force is applied through 

the 6-mm-diameter penetrometer tip until rupture occurs.

Table 3-8

Rupture Resistance Classes Applied to Crushing PlateShaped Specimens

Classes Force

(newtons)

Fragile < 3 N

Extremely weak Not removable

Very weak Removable; < 1 N

Weak 1 to < 3 N

Medial 3 to < 20 N

Moderate 3 to < 8 N

Moderately strong 8 to < 20 N

Resistive ≥ 20 N

Strong 20 to < 40 N

Very strong 40 to < 80 N

Extremely strong ≥ 80 N

Plasticity

Plasticity is the degree to which puddled soil material is permanently 

deformed without rupturing by force applied continuously in any direction. Table 3-9 lists the classes and their criteria. Plasticity is determined

on material smaller than 2 mm.

The determination is made using thoroughly puddled soil material 

at a water content where maximum plasticity is expressed. This water 
186 Chapter 

content is above the plastic limit but less than the water content at which 

maximum stickiness is expressed. The water content is adjusted by 

adding or removing water during hand manipulation. The plastic limit 

used in engineering classifications, which is closely related, indicates the

water content at which a roll that consists of < 0.4 mm material, is 3

mm in diameter, and was formed at a higher water content breaks apart 

(method D 4318 in ASTM, 2011).

Table 3-9

Plasticity Classes

Classes Criteria

Nonplastic A roll 4 cm long and 6 mm thick that can support 

its own weight if held on end cannot be formed. 

Slightly plastic A roll 4 cm long and 6 mm thick can be formed 

and can support its own weight if held on end. A 

roll 4 mm thick cannot support its own weight. 

Moderately 

plastic 

A roll 4 cm long and 4 mm thick can be formed 

and can support its own weight, but a roll 2 mm 

thick cannot support its own weight. 

Very plastic A roll 4 cm long and 2 mm thick can be formed 

and can support its own weight. 

Toughness

Toughness is related to plasticity. Table 3-10 lists the classes and

their criteria. The classes are based on the relative force necessary to 

form, with the fingers, a roll 3 mm in diameter of < 2 mm soil material at

a water content near the plastic limit (test D 2488 in ASTM, 2011).

Table 3-10

Toughness Classes

Classes Criteria

Low The specimen diameter at or near the plastic limit 

can be reduced to 3 mm by exertion of < 8 N.

Medium The specimen diameter at or near the plastic limit 

requires 8 to < 20 N to be reduced to 3 mm.

High The specimen diameter at or near the plastic limit 

requires > 20 N to be reduced to 3 mm.
Soil Survey Manual 1

Stickiness

Stickiness refers to the capacity of a soil to adhere to other objects. 

Table 3-11 lists the classes and their criteria. The determination is made

on puddled < 2 mm soil material at the water content at which the material 

is most sticky. The sample is crushed in the hand, water is applied, and 

manipulation continues between thumb and forefinger until maximum

stickiness is reached.

Table 3-11

Stickiness Classes

Classes Criteria

Nonsticky After release of pressure, practically no soil material 

adheres to thumb or forefinger.

Slightly sticky After release of pressure, soil material adheres 

perceptibly to both digits. As the digits are 

separated, the material tends to come off one or 

the other digit rather cleanly. The material does not 

stretch appreciably on separation of the digits.

Moderately 

sticky 

After release of pressure, soil material adheres to 

both digits and tends to stretch slightly rather than 

to pull completely free from either digit.

Very sticky After release of pressure, soil material adheres so 

strongly to both digits that it stretches decidedly 

when the digits are separated. Soil material remains 

on both digits.

Manner of Failure

The manner in which specimens fail under increasing force ranges 

widely and typically is highly dependent on water state. The categories 

of manner of failure are brittleness, fluidity, and smeariness (see 

table 3-12). To evaluate brittleness or smeariness, a roughly cubical

specimen 25–30 mm on edge is pressed between extended forefinger and

thumb. To evaluate fluidity, a handful of soil material is squeezed in the

hand (fig. 3-28). Some soil materials are brittle even wet, some can be

compressed markedly without cracks appearing, others behave like liquids 

if wet, and others smear if subjected to shear stress until failure. Soil in 

the slightly moist or dry states, if coherent, is nearly always brittle and 

commonly does not exhibit smeariness; consequently, manner of failure 
188 Chapter 

is generally only useful for moderately moist or wetter soil material. 

Smeariness is a property most commonly associated with soils having 

andic soil properties(i.e.,soils classified asAndisols and some Spodosols).

Table 3-12

Manner of Failure Classes

Classes Operation Test result

Brittleness

Brittle Gradually increasing 

compressive 

pressure is applied 

to a 25–30 mm

specimen held 

between extended 

thumb and 

forefinger.

Specimen retains its size and shape 

(no deformation) until it ruptures 

abruptly into subunits or fragments.

SemideformableSame as above Deformation occurs prior to rupture. 

Cracks develop and specimen 

ruptures before it is compressed to 

half its original thickness.

Deformable* Same as above Specimen can be compressed to half its 

original thickness without rupture. 

Radial cracks may appear and extend 

inward less than half the radius 

distance under normal compression.

Fluidity

Nonfluid A handful of soil 

material is 

squeezed in the 

hand.

No material flows through the fingers

after full compression.

Slightly 

fluid*

Same as above After full compression, some material 

flows through the fingers but most

remains in the palm of the hand.

Moderately 

fluid*

Same as above After full pressure, most material flows

through the fingers; a small residue

remains in the palm of the hand.

Very fluid* Same as above Under very gentle pressure, most 

material flows through the fingers

like a slightly viscous fluid and very

little or no residue remains.
Soil Survey Manual 1

Table 3-12.—continued

Classes Operation Test result

Smeariness

Non-smeary Gradually increasing 

pressure is applied 

to a 25–30 mm

specimen held 

between extended 

thumb and 

forefinger in such

a manner that 

some shear force 

is exerted on the 

specimen.

At failure, the specimen does not 

change suddenly to a fluid, the

fingers do not skid, and no smearing

occurs.

Weakly 

smeary

Same as above At failure, the specimen changes 

suddenly to fluid, the fingers skid,

and the soil smears. Afterward, little 

or no free water remains on the 

fingers.

Moderately 

smeary

Same as above At failure, the specimen changes 

suddenly to fluid, the fingers skid,

and the soil smears. Afterward, some 

free water can be seen on the fingers.

Strongly 

smeary

Same as above At failure, the specimen changes 

suddenly to fluid, the fingers skid,

and the soil smears and is very 

slippery. Afterward, free water is 

easily seen on the fingers.

* The approximate equivalent n values (Pons and Zonneveld, 1965) are as follows: 

Deformable < 0.7 

Slightly fluid 0.7–1 

Moderately fluid 1–2 

Very fluid 

Penetration Resistance

Penetration resistance is the capacity of the soil in its confined

state (in situ) to resist penetration by a rigid object. Shape and size of 

the penetrating object must be defined. Penetration resistance depends

strongly on the water state, which should be specified.
190 Chapter 

Figure 3-28

A field test on a soil with a moderately fluid manner of failure class. (Photo courtesy of 

John Kelley)

The classes in table 3-13 pertain to the pressure required to push the

flat end of a cylindrical rod with a diameter of 6.4 mm a distance of 6.4

mm into the soil in about 1 second (Bradford, 1986). Three generalized 

classes and seven more narrowly defined classes are used. Orientation

of the axis of insertion should be specified. A correction should be made

for the weight of the penetrometer if the axis of insertion is vertical and 

the resistance is small. If rock fragments are present, the lower values 

measured are typically more descriptive of the fine-earth fabric.

The pocket penetrometer, shown in Bradford (1986), is the standard 

instrument. Penetrometers with the same 6.4-mm-diameter flat-end tip

and a dial reading device are available. The resistance can be read with 

less variability using the dial device. The scale on the barrel of the pocket 

penetrometers should be converted to units of force. The supplied scale 

on such instruments commonly is based on a regression between penetration resistance and unconfined, compressive strength measurements

and has no application in the context used here. Penetration resistance 

is expressed in units of pressure. The preferred unit is the megapascal 

(MPa). For the 6.4-mm-diameter tip, the measured force in kilograms is 
Soil Survey Manual 1

Table 3-13

Penetration Resistance Classes

Classes Penetration resistance

(MPa)

Small < 0.1

Extremely low < 0.01

Very low 0.01 to < 0.1

Intermediate 0.1 to < 2

Low 0.1 to < 1

Moderate 1 to < 2

Large > 2

High 2 to < 4

Very high 4 to < 8

Extremely high > 8

multiplied by 0.31 to obtain the pressure in megapascals. To extend the

range of the instrument, weaker and stronger springs may be substituted. 

Values in megapascals obtained with any diameter of flat-end rod are

used to determine the class (see table 3-13).

In addition to the flat-end tip, cone-shaped tips may be mounted 

on the penetrometers with flat ends as well as other penetrometers.

Two 30-degree cone penetrometer tips are specified by the American

Society of Agricultural Engineers (Ayers and Perumpral,1982). One 

has a base area of 1.3 cm2 and the other of 3.2 cm2. The tips should 

be inserted where the base of the cone is flush with the soil surface.

Insertion times of 2 seconds and 4 seconds should be used for the 

smaller and larger cones, respectively. A relationship between the cone 

tips and the specified rod with a flat end must be established before cone

measurements. Table 3-13 can be modified to use the corresponding

cone measurements.

Determination of penetration resistance while the soil layer is at or 

near its maximum water content is useful in evaluation of root limitations. 

The relationship between penetration resistance and root growth has been 

the subject of numerous studies—Blanchar et al. (1978), Campbell et al. 

(1974), Taylor et al. (1966), and Taylor and Ratliff (1969). These studies 

suggest the following generalities (which can be modified for particular
192 Chapter 

plants and soils): (i) if the soil material is wet or very moist and there 

are no closely spaced vertical structural planes, the limit of 2 MPa (6.4-

mm flat-end rod) indicates strong root restriction for several important

annual crops (this is the basis for the penetration resistance criterion in 

the criteria for physical root restriction); (ii) if MPa is between 2 and 1, 

root restriction may be assumed to decrease roughly linearly; (iii) if MPa 

is below 1, root restriction may be assumed to be small.

Excavation Difficulty

Table 3-14 givesthe classes of excavation difficulty and their criteria.

The classes can be used to describe both non-cemented and cemented or 

indurated horizons, layers, or pedons for a one-time observation or over 

time. In most cases, excavation difficulty is related to and controlled by

a water state.

Table 3-14

Excavation Difficulty Classes

Classes Criteria

Low Material can be excavated with a spade using arm-applied 

pressure only. Neither application of impact energy 

nor application of pressure with the foot to a spade is 

necessary.

Moderate Arm-applied pressure to a spade is insufficient.

Excavation can be accomplished quite easily by 

application of impact energy with a spade or by foot 

pressure on a spade.

High Excavation with a spade can be accomplished, but with 

difficulty. Excavation is easily possible with a full

length pick using an over-the-head swing.

Very high Excavation with a full length pick using an over-the-head 

swing is moderately to markedly difficult. Excavation is

possible in a reasonable period of time with a backhoe 

mounted on a 40 to 60 kW (50 to 80 hp) tractor.

Extremely

high

Excavation is nearly impossible with a full length pick 

using an over-the-head arm swing. Excavation cannot 

be accomplished in a reasonable time period with 

a backhoe mounted on a 40 to 60 kW (50 to 80 hp) 

tractor.
Soil Survey Manual 1

Roots

Quantity, size, and location of roots in each layer are recorded. 

Features of the roots—length, flattening, nodulation, and lesions—

and their relationships to special soil attributes or to structure may be 

recorded as notes.

Quantity of Roots

Quantity of roots is described in terms of numbers of each size per 

unit area. The observed value is used to assign a class. The classes for 

quantity of roots pertain to an area in a horizontal plane unless otherwise 

stated. However, most soil profiles are described from a vertical plane

and the number of roots observed per unit area may differ depending 

on the orientation. Therefore, a horizontal cross-section should be used 

when practical to determine quantity of roots. The required unit area 

for observation changes according to root size: 1 cm2 for very fine and

fine roots, 1 dm2 for medium and coarse roots, and 1 m2 for very coarse 

roots. 

Ideally, class limits correspond to a root abundance level where 

there are sufficient roots to exploit much of the soil water that is

present in the withdrawal range of the plant over the growing season. 

This can be difficult because species differ in the efficiency of their

roots. Soybeans and cotton are several fold more efficient than

grasses, and there are undoubtedly other differences among specific

groups. The quantity classes have been formulated so that the 

few-common separation is about where the annual grasses have 

insufficient numbers of roots for seasonally complete exploitation.

The few class can be subdivided if useful. The moderately few-very 

few separation is where soybeans and cotton would have insufficient

numbers.

The quantity classes are:

Few ........................ less than 1 per unit area

 Very few .............. less than 0.2 per unit area

 Moderately few ... 0.2 to less than 1 per unit area

Common ................ 1 to less than 5 per unit area

Many ...................... 5 or more per unit area

Size Classes of Roots

Roots are described in terms of a specified diameter size. The size

classes are:
194 Chapter 

Very fine ................ less than 1 mm

Fine ........................ 1 to less than 2 mm

Medium ................. 2 to less than 5 mm

Coarse .................... 5 less than 10 mm

Very coarse 10 mm or larger

Location of Roots

The location of roots within a layer may be described in relation 

to other features of the layer. Relationships to layer boundaries, 

animal traces, pores, and other features are described as appropriate. 

The description may indicate, for example, whether roots are inside 

structural units or only along parting planes between structural units. 

A convenient order is quantity, size, location. The description “many 

very fine and common fine roots” implies that roots are uniformly

distributed, since location is not given. Examples of descriptions with 

locational information are: “common very fine and common fine roots

concentrated along vertical faces of structural units” and “common very 

fine roots inside peds, many medium roots between structural units.”

In some soils, the pattern or root growth may not correspond to 

soil horizons or layers. A summary statement of root development by 

increments of 15 cm or 30 cm (or some other convenient thickness)

can be helpful. In other soils, root distribution may be summarized by 

grouping layers. For example, in a soil having a strongly developed clayey 

illuvial horizon and a horizon sequence of Ap-A-E1-E2-Bt1-Bt2, root 

development might have one pattern throughout the A horizons, another 

pattern in the E horizons, and yet another pattern in the B horizons. In 

this case, root distribution can be described for the A, E, and B horizons, 

each horizon treated as a whole.

For annual plants, the time of the root observation may be indicated. 

Root traces (channels left by roots that have died) and the dead roots 

themselves can be clues to soil properties that change with time. The 

rate of root decay depends on the species, root size, and the soil moisture 

and temperature regimes. Local experience can determine the time after 

maturity or harvest that the root distribution is affected by decay. Root 

traces in deep layers may persist for years. Many of these traces have 

organic coatings or linings. If they occur below the normal rooting depth 

of annual crops, they were left by deeper-rooted plants, perhaps native 

perennials. The presence of dead roots below the current rooting depth 

may indicate a change in the soil water regime. The roots may have 

grown normally for a few years, then died when the soils were saturated 

for a long period.
Soil Survey Manual 1

In addition to recording the rooting depths at the time of observation, 

generalizations about the rooting depth may be useful. These generalizations should emphasize very fine and fine roots, if present, because

roots of these sizes are active in absorption of water and nutrients. The 

generalizations may be for a few plants or plant communities that are of 

particular importance. For observation of annual plants, the generalization should assume plant maturity.

Pores

Pore space is a general term for voids in the soil material. The term 

includes matrix, non-matrix, and interstructural pore space. 

Kinds of Pores

Matrix pores (also called interstitial pores) are formed by the 

agencies that control the packing of the primary soil particles. In fine and

medium textured soils these pores are typically smaller than non-matrix 

pores. Additionally, their aggregate volume and size change markedly 

with water state for soil horizons or layers with high extensibility. In 

coarse textured soils, the interstitial pore size is controlled dominantly 

by the primary particle packing and remains fairly stable, although pores 

may become filled with finer material over time.

Non-matrix pores are relatively large voids that occur not only when 

the soil is dry but also when it is moderately moist or wetter. The voids 

are not bounded by the planes that delimit structural units. Interstructural 

pores are delimited by structural units. The interstructural porosity may 

be inferred from the structure description. Commonly, interstructural 

pores are at least crudely planar.

Non-matrix pores may be formed by roots, animals, compressed 

air, and other agents. The size distribution of these pores typically is 

not associated with the particle-size distribution and the related matrix 

pore-size distribution. For water movement at low suction and conditions 

of satiation, the non-matrix and interstructural porosity have particular 

importance. 

Non-matrix pores are described by quantity, size, shape, and vertical 

continuity—generally in that order. Quantity classes pertain to numbers 

per unit area—1 cm2 for very fine and fine pores, 1 dm2

 for medium and 

coarse, and 1 m2

 for very coarse.
196 Chapter 

Quantity Classes of Pores

The pore quantity classes are:

Few ............ less than 1 per unit area

Common .... 1 to less than 5 per unit area

Many .......... 5 or more per unit area

Size Classes of Pores

Pores are described relative to a specified diameter size. The five

pore size classes are:

Very fine ................ less than 1 mm

Fine ........................ 1 to less than 2 mm

Medium ................. 2 to less than 5 mm

Coarse .................... 5 to less than 10 mm

Very coarse ............ 10 mm or more

Shape Classes of Pores

Common non-matrix pore shapes include:

Vesicular.—Small, approximately spherical or elliptical. These cavities 

are caused by entrapped air bubbles, most commonly occurring 

in or below mineral or biological crusts or desert pavement, 

especially in arid soils. As the size and/or number of near-surface 

vesicular pores increases, infiltration is drastically reduced and

surface runoff increases. A horizon dominated by vesicular pores 

is identified as a vesicular master horizon (capital letter V).

Tubular.—Approximately cylindrical and elongated, as in worm 

channels. 

Dendritic tubular.—Like tubular, but branching as in root channels.

Irregular.—Nonconnected. These cavities or chambers are commonly 

called “vughs.”

Continuity Classes of Pores

Vertical continuity involves assessment of the average vertical 

distance through which the minimum pore diameter exceeds 0.5 mm 

when the soil layer is moderately moist or wetter. Three classes are used: 

Low ............ less than 1 cm

Moderate .... 1 to less than 10 cm

High ........... 10 cm or more
Soil Survey Manual 1

Additionally, the designation “continuous” is used if the non-matrix 

pores extend through the thickness of the horizon or layer. Vertical 

continuity has extreme importance in assessing the capacity of the soil 

layer to transmit free water vertically.

Special aspects are noted, such as orientation in an unusual direction, 

concentration in one part of a layer, or conditions where tubular pores are 

plugged with clay at both ends. Some examples of descriptions of pores 

are “many fine tubular pores,” “few fine tubular pores and many medium

tubular pores with moderate vertical continuity,” and “many medium 

vesicular pores in a horizontal band about 1 cm wide at the bottom of 

the horizon.”

Animals

The mixing, changing, and moving of soil material by animals is a 

major factor affecting properties of some soils. The features resulting 

from animal activity reflect mainly mixing or transport of material from

one part of the soil to another or to the surface. The original material may 

be substantially modified physically or chemically.

The features that animals produce on the land surface may be 

described. Termite mounds, ant hills, heaps of excavated earth beside 

burrows, the openings of burrows, paths, feeding grounds, earthworm 

castings, other castings, and other traces on the surface are easily 

observed and described. Simple measurements and estimates (such as 

the number of structures per unit area, proportionate area occupied, and 

volume of above-ground structures) give quantitative values that can be 

used to calculate the extent of activity and even the number of organisms.

The marks of animals below the ground surface are more difficult to

observe and measure. Observations are confined mainly to places where

pits are dug. The volume of soil generally studied is limiting. For the 

marks of many animals, the normal pedon for soil characterization is 

large enough to provide a valid estimate. For some animals, however, the 

size of the marks is too large for the usual pedon. 

Krotovinas are irregular tubular streaks in a layer that consists of 

material transported from another layer. They are caused by the filling

of tunnels made by burrowing animals in one layer with material from 

outside the layer. In a profile, they appear as rounded or elliptical

volumes of various sizes. They may have a light color in dark layers 

or a dark color in light layers, and their other qualities of texture and 

structure may be unlike those of the soil around them.
198 Chapter 

Description of Animal-Related Features

The features produced by animals in the soil are described in terms 

of amount, location, size, shape, and arrangement and also in terms of 

the color, texture, composition, and other properties of the component 

material. There are no special conventions for descriptions. Common 

words should be used in conjunction with appropriate special terms 

for the soil properties and morphological features that are described 

elsewhere in this manual. 

Selected Chemical Properties

This section discusses selected chemical properties that are important 

for describing and identifying soils. Included in the discussion are 

reaction, carbonates, manganese oxides, salinity, sodicity, sulfates, and 

sulfides.

Reaction

The numerical designation of reaction is expressed as pH. With this 

notation, pH 7 is neutral. Values lower than 7 indicate acidity; higher 

values indicate alkalinity. Soils as a whole range in pH from slightly 

less than 2.0 to slightly more than 11.0. Individual soils have a much 

narrower pH range within these overall limits. Reaction varies seasonally 

and is affected by such factors as moisture, temperature, plant growth, 

and microbial activity. A significant change in pH also occurs when

some naturally wet soils that contain sulfides are drained. In these cases,

sulfuric acid forms and pH may decrease to below 2.0.

The standard field pH measurement is performed with a 1:1 soil:water

mixture so that comparisons of pH readings are on an equivalent basis. A 

more dilute sample (for example, a 1:5 soil:water mixture) generally has 

a higher pH, and less dilute samples generally have lower pH. 

While the standard for measuring pH in the field is a 1:1 soil:water

mixture, other methods of measuring pH are also used in soil survey for 

specific purposes, especially those required for some taxonomic criteria

in Soil Taxonomy. They include:

0.01 M CaCl2

.—This method has the advantage of dampening 

seasonal variation in pH. It is used for some taxonomic familylevel criteria.

1N KCl.—This method is used to infer aluminum saturation levels in 

some great groups of Oxisols (e.g., Acrudox). If the criteria are 

met, aluminum toxicity may be a concern.
Soil Survey Manual 1

1M NaF.—This method is used to infer the presence of short-range 

order minerals. It is used in the criteria for the isotic mineralogy 

class.

Various methods can be used to measure pH in the field. Pocket

pH meters are a popular tool. Care must be taken to clean the sensor 

tip between readings and to periodically calibrate the meter to standard 

samples of known pH. It is also important to ensure calibration reagents 

are fresh. Other common measurement methods include the use of 

indicator solution dyes, colorimetric kits, and paper pH indicator strips. 

Proper storage of these materials—out of direct sunlight and not exposed 

to extreme temperatures—is important. Over time, the test materials 

become less reliable. It is important to record the method of pH reading.

Reaction Class Terms

Reaction class terms are commonly used to communicate information 

about soil pH. The terms are given in table 3-15.

Table 3-15

Reaction Class Terms and Their Ranges in pH

Class term pH range

Ultra acid < 3.5

Extremely acid 3.5–4.4

Very strongly acid 4.5–5.0

Strongly acid 5.1–5.5

Moderately acid 5.6–6.0

Slightly acid 6.1–6.5

Neutral 6.6–7.3

Slightly alkaline 7.4–7.8

Moderately alkaline 7.9–8.4

Strongly alkaline 8.5–9.0

Very strongly alkaline > 9.0

Carbonates of Divalent Cations

A solution of cold, 1-normal hydrochloric acid (1N HCl) is used 

to test for the presence of free carbonates in the field. The proper

concentration is made by combining 1 part concentrated HCl (37%) with

11 parts distilled water. Add acid to water, not water to acid. Care must 
200 Chapter 

be taken when handling concentrated HCl as it can cause severe skin 

burns. The application of a few drops of HCl to a sample containing 

carbonates results in the evolution of CO2

 gas, which forms bubbles 

(effervescence). The amount and expression of effervescence is affected 

by size distribution of the carbonates and their mineralogy as well as the 

overall amount of carbonates present. Consequently, effervescence is a 

qualitative test and cannot be used to estimate the quantitative amount of 

carbonate. 

Effervescence Class Terms

The five classes of effervescence and their criteria are shown in table

3-16.

Table 3-16

Effervescence Class Terms

Effervescence class Criteria

Noneffervescent No bubbles form

Very slightly effervescent Few bubbles form

Slightly effervescent Numerous bubbles form

Strongly effervescent Bubbles form a low foam

Violently effervescent Bubbles quickly form a thick foam

An example of the use of effervescence class in a description is 

“strongly effervescent with 1N HCl.” While calcium carbonate reliably 

effervesces when treated with cold dilute hydrochloric acid, effervescence 

is not always easily observable for some sandy soils. Dolomite reacts 

to cold dilute acid slightly or not at all and may be overlooked. It can 

be detected by heating the sample, using more concentrated acid, and 

grinding the sample. The effervescence of powdered dolomite with cold 

dilute acid is slow (a few minutes) and frothy.

Calcium Carbonate Equivalent

Calcium carbonate equivalent refers to the amounts of CaCO3

 in soil. 

Other names include soil carbonate, soil inorganic carbon, pedogenic 

carbonate, caliche, nari, tosca, croute calcaire, kankar, and soil lime 

(Monger et al., 2015). The most common mineral form is calcite. 

Aquantitative field test for measuring the soil carbonate uses a simple

volume calcimeter (see Soil Survey Staff, 2009, for details). For this 

procedure, a small sample (commonly 0.33 g) is placed in a syringe. A
Soil Survey Manual 2

10 percent HCl solution is placed in a second syringe which is connected 

to the first syringe by a rubber tube. The HCl solution is injected into

the syringe containing the soil sample and the evolution of CO2 gas is 

recorded and adjusted to compensate for temperature and elevation. Soil 

carbonate is recorded to the nearest whole number.

Manganese Oxides

Hydrogen peroxide (H2O2, 3-4% solution, commonly available in

pharmacies) can be used to test for the presence of manganese oxides 

(MnO2

, a kind of redoximorphic concentration). The effervescence 

classes used for carbonates (table 3-16) are also used for describing the

presence of manganese oxides. The effervescence class and reagent are 

recorded, e.g., “violently effervescent in 3% H2O2.”

It should be noted that organic matter also reacts to hydrogen 

peroxide. This reaction is typically slow, while MnO2

 reacts quickly. 

Hydrogen peroxide is also used for a color change test to detect the 

presence of reduced monosulfides (e.g., FeS) in subaqueous soils.

Salinity and Sodicity

Accurate determinations of salinity and sodicity in the field require

special equipment and are not necessarily part of each pedon investigation. 

Reasonable estimates of salinity and sodicity can be made if field criteria

are correlated to more precise laboratory measurement.

Salinity

The electrical conductivity (EC) of a saturation paste extract is the 

standard method for measuring salinity and is denoted as ECe. Electrical 

conductivity is related to the amount of salts that are more soluble than 

gypsum in the soil. A small amount (up to 2 dS/m) of dissolved gypsum 

may also contribute to the EC.

The preparation of a saturation paste extract is most commonly performed in the laboratory rather than in the field. Pocket electrical conductivity meters can be used in the field for measuring electrical conductivity

of soil:water solutions of various ratios (e.g., 1:1, 1:2, 1:5, etc.). The EC 

values recorded reflect the concentration of the mixture. Lower readings

are associated with higher amounts of water relative to soil. Electrical 

conductivity measured this way should be denoted with the soil:water 

ratio (e.g., EC1:1, EC1:5). There is no universal correction factor to equate 

these results to the standard saturation paste extract method performed 

in the laboratory. General correlations between field-measured EC using

a soil:water solution and laboratory-measured ECe for the same samples 
202 Chapter 

may be possible for soils in a localized geographic area having similar 

salt chemistry and other properties and similar environmental conditions. 

Additional samples having only field-measured EC may then be estimated as to their expected ECe values for classification purposes and assignment to salinity classes. Care must be taken to not extend the relationship 

beyond the area for which it was established.

The 1:5 soil:water mixture (by volume) is commonly used as a field

test for measuring EC of subaqueous soil samples. These results are 

recorded as described above (e.g., EC1:5 10.5 dS/m). The salinity classes 

shown in table 3-17 are not applicable to these measurements.

The standard international unit of measure for EC is decisiemens 

per meter (dS/m) corrected to a temperature of 25 °C. (Millimhos per 

centimeter [mmhos/cm] are equivalent to dS/m, but this notation is 

not preferred.) Measured electrical conductivity is reported in soil 

descriptions. Table 3-17 shows the classes of salinity used if the electrical

conductivity (ECe) has not been determined but salinity is inferred.

Table 3-17

Salinity Class Terms

Salinity class

Electrical 

conductivity (ECe)

dS/m (mmhos/cm)

Nonsaline < 2

Very slightly saline 2 to < 4

Slightly saline 4 to < 8

Moderately saline 8 to < 16

Strongly saline 

Field measurements of electrical conductivity can be made using 

other methods, such as electromagnetic induction or salinity probes (see 

chapter 6). Again, these measures are useful for the area where they are 

made but the observed values are not equivalent to EC measured with a 

saturation paste extract or with various soil:water ratios.

Sodicity

The sodium adsorption ratio (SAR) is the standard measure of the 

sodicity of a soil. It is a measure of the equilibrium between sodium ions 

in the soil solution and the exchangeable sodium ions adsorbed on the 

soil cation-exchange complex. The sodium adsorption ratio is calculated 
Soil Survey Manual 2

from the concentrations (in milliequivalents per liter) of sodium, calcium, 

and magnesium in the saturation extract:

 Na+

 SAR= Ca++ + Mg++

 √ 2

Formerly, the primary measure of sodicity was the exchangeable 

sodium percentage, which equals exchangeable sodium (meq/100 g soil) 

divided by the cation-exchange capacity (meq/100 g soil) times 100. The 

test for exchangeable sodium percentage, however, has proved unreliable 

in soils containing soluble sodium silicate minerals or large amounts of 

sodium chloride.

Sodium is toxic to some crops and affects soil physical properties, 

mainly saturated hydraulic conductivity. A sodic condition has little effect 

on hydraulic conductivity in highly saline soils. A soil that is both saline 

and sodic may, when artificially drained, drain freely at first. After some

of the salt has been removed, however, further leaching of salt becomes 

difficult or impossible. The sodium adsorption ratio (SAR) typically

decreases as a soil is leached because the amount of change depends in 

part on the composition of the water used for leaching.

The following procedure can be used to predict whether the soil will 

be sodic after leaching. If the initial SAR is greater than 10 and the initial 

electrical conductivity is more than 20 dS/m, the SAR is determined on 

a sample after first leaching it with the intended irrigation water. For

soils with an electrical conductivity of more than 20 dS/m, the SAR 

is determined after first leaching the sample with distilled water to an

electrical conductivity of about 4 dS/m.

No classes for sodium levels in the soil are provided. Laboratory 

analysis is required to document the SAR of individual horizons. Soils 

that have high sodium levels, but are not otherwise saline, commonly 

have pH of 9.0 or greater. Soils that are both saline and sodic tend to have 

pH values of less than 8.5. In addition, some sodic soils, especially in 

slightly depressional areas, may be black at the surface due to dispersion 

of clay and organic matter and poor drainage. In some sodic soils, natric 

horizons exhibit columnar structure.

Sulfates

Gypsum (hydrous calcium sulfate: CaSO4 • 2H2O) can be inherited 

from the parent material or can precipitate from supersaturated solutions 

in the soil or substratum. It can alleviate the negative effects of sodium on 

soil structure, allowing the use of irrigation water that has a relatively high 

amount of sodium. Soils that contain large amounts of gypsum can settle 
204 Chapter 

unevenly after irrigation, and frequent releveling may be required. Soil 

subsidence due to gypsum dissolution, especially from concentrated flow

of rainwater from roofs or paved areas, can be a serious hazard to roads 

and buildings. Gypsum is soluble in water. The electrical conductivity of 

a distilled water solution with gypsum is about 2 dS/m. In the absence 

of other salts, salinity is not a hazard except for such sensitive plants 

as strawberries and some ornamentals. Gypsum and other sulfates may 

cause damage to concrete.

Gypsum is commonly tabular or fibrous and tends to accumulate as

clusters of crystals or as coats on peds. Some is cemented. Gypsum can 

typically be identified by its form and lack of effervescence with acid.

Gypsum in parent material may not be readily identifiable. If determined,

the amount of gypsum is given in the description. If not, the amount may 

be estimated. Semiquantitative field methods for determining amounts of

gypsum are available.

Some soils contain large amounts of sodium sulfate, which looks 

like gypsum in hand specimens of soil. Sodium sulfate is in the form 

of thenardite (Na2

SO4) at temperatures above 32.4 °C and in the form

of mirabilite (Na2SO4 • 10H2O) at lower temperatures. The increase in 

volume and decrease in solubility as thenardite changes to mirabilite can 

cause extreme salt heaving. In saline-sodium soils, sodium sulfate is a 

common water-soluble salt.

Sulfides

Sulfides, mainly iron sulfide, occur in some tidal marsh soils and in

some sedimentary rocks, such as those associated with coal or shale. In 

marsh soils, soil layers with significant sulfide content are commonly

permanently saturated and are neutral in color (N) or have hue of 5Y, 

5GY, 5BG, or 5G; value of 2, 3, or 4; and chroma of 1 or less (Fanning

et. al., 2002; IUSS Working Group WRB, 2015). When these materials 

are exposed (e.g., when marsh soils are drained or sulfide-bearing rock

is excavated), oxidation commonly produces sulfuric acid. Sulfuric acid 

is toxic to plants and animals in the soil and to fish and other aquatic

organisms in nearby waters. The solutions produced are extremely acid 

and are highly corrosive to exposed metal and concrete. Soils and rock 

that may have potential sulfur acidity (especially material dredged from 

coastal water areas and applied on the land) should be tested for the 

presence of sulfide salts.

A few soils with appreciable amounts of sulfides contain enough

carbonates to neutralize all or part of the acidity when the sulfides are

oxidized. In these soils, the total amounts of both calcium carbonate and 
Soil Survey Manual 2

sulfides are needed to determine if effective neutralization can occur

naturally.

No reliable field methods are available for determining the amount 

of sulfides in marshes. A simple test to confirm the presence of sulfides

(but not the amount) uses 30% hydrogen peroxide (H2O2) to determine 

if rapid oxidation causes a significant decrease in pH as compared to a

similar sample treated with water (Soil Survey Staff, 2009). 

Marsh soils may give off a sulfurous odor. This odor is not a reliable 

indicator of the presence of significant amounts of oxidizable sulfides;

however, odor can be a reliable indicator that sulfides are present. The

sulfurous odor (“rotten egg smell”), if detected, should be noted in the 

soil description. Qualitative class terms for odor intensity are:

Slight.—Odor is faint, only detected close to nose.

Moderate.—Odor is readily noticeable, even at arm’s length from 

the nose.

Strong.—Odor is intense and readily noticed as soon as sample is 

exposed to the air.

 Drained or excavated marsh soils that contain large amounts of 

sulfides commonly have yellow efflorescences of the mineral jarosite on 

the exteriors of clods (fig. 3-29).

Two simple laboratory tests are commonly used to detect excess 

oxidizable sulfides (Soil Survey Staff, 2009). In one test, pH is measured

before and after the soil is incubated for several weeks at field capacity.

A large drop in pH, or a pH of 3.5 or less after drying, indicates excessive

amounts of sulfides. In the other test, the sample is treated with 30- to

36-percent hydrogen peroxide and heated to complete oxidation and to

drive off the excess peroxide. Then, pH is measured. If the decrease in 

pH is large, sulfides are probably present. Use of an electronic meter

rather than colorimetric methods to measure pH is preferred because of 

the possible oxidation of indicator dyes. Special dyes suitable for this test 

are available. If the qualitative tests for oxidizable sulfides are positive,

laboratory determinations of sulfur content are required for precise 

interpretations and recommendations regarding use and management.

Soil Water

This section discusses “water regimes”—schemes for the description 

of the state of the soil water at a particular time and for the change in soil 

water state over time. Soil water state is evaluated from water suction, 

quantity of water, whether the soil water is liquid or frozen, and the 
206 Chapter 

Figure 3-29

Jarosite concentrations (yellowish color) that formed due to oxidation in this drained 

marsh soil containing sulfides.

occurrence of free water within the soil and on the land surface. The 

complexity and detail of water regime statements can range widely.

Inundation Classes

Free water may occur above the soil. Inundation is the condition 

when the soil area is covered by liquid free water. Flooding is temporary 

inundation by flowing water. If the water is standing, as in a closed

depression, the term ponding is used. Flooding and ponding are temporal 

conditions. In most cases, soils are not described while inundated 

(exceptions include subaqueous soils and some soils that are subject 

to ponding of very long duration). To the extent possible, estimates 

for inundation should include frequency, duration, and months of 

occurrence. Depth of inundation is also commonly recorded. Table 3-18

shows the classes for frequency and duration of inundation. The rare and 

very rare frequency classes may be combined. The very frequent class 

takes precedence over frequent if both definitions are met. Very frequent

flooding includes tidal inundation. Frequency of flooding should reflect

the current conditions. A soil that would be frequently flooded in its

natural state, but is now protected by a dam or levee, should be assigned 

the class that reflects the level of protection provided.
Soil Survey Manual 2

Table 3-18

Frequency and Duration of Inundation Classes (Flooding or 

Ponding)

Class Criteria

Frequency

None No reasonable possibility of inundation; one chance out 

of 500 in any year or less than 1 time in 500 years.

Very rare Inundation is very unlikely but is possible under 

extremely unusual weather conditions; less than 1 

percent chance in any year or less than 1 time in 100 

years but more than 1 time in 500 years.

Rare Inundation is unlikely but is possible under unusual 

weather conditions; 1 to 5 percent chance in any year 

or nearly 1 to 5 times in 100 years.

Occasional Inundation is expected infrequently under usual weather 

conditions; more than 5 to 50 percent chance in any 

year or 6 to 50 times in 100 years.

Frequent Inundation is likely to occur often under usual weather 

conditions; more than 50 percent chance in any year 

(i.e., more than 50 times in 100 years) but 50 percent 

or less chance in all months in any year.

Very 

frequent

Inundation is likely to occur very often under usual 

weather conditions; more than a 50 percent chance in 

all months of any year.

Duration

Extremely 

brief

0.1 hour to less than 4 hours (flooding only)

Very brief 4 hours to less than 48 hours

Brief 2 days to less than 7 days

Long 7 days to less than 30 days

Very long 30 or more days

Internal Soil Water State

This section discussions the occurrence of water within the soil, 

classes used to describe the soil water state at the time the soil is described, 

and methods for evaluating soil water in the field.
208 Chapter 

Classes 

In describing classes of soil water state for individual layers or 

horizons, only matrix suction is considered in the definition of the

classes.3 Osmotic potential is not considered. For water contents of 

medium and fine textured soil materials at suctions of less than about 200

kPa, the reference laboratory water retention is for the natural soil fabric. 

Class limits are expressed both in terms of suction and water content. To 

make field and field office evaluation more practicable, water content

refers to gravimetric rather than volumetric quantities. The classes apply 

to mineral and organic soil material. The frozen condition is indicated 

separately by the symbol “f.” This symbol indicates the presence of ice; 

some of the water may not be frozen. If the soil is frozen, the water 

content or suction pertains to what it would be if not frozen.

Three classes and eight subclasses for water state are defined in table

3-19. Classes and subclasses may be combined as desired. The desired

specificity and characteristics of the water desorption curve determine

whether classes or subclasses should be used. Coarse soil material has 

little water below the 1500 kPa retention, and so subdivisions of dry 

generally are not useful. 

Dry is separated from moist at 1500 kPa suction. Wet is separated 

from moist at the condition where water films are readily apparent. The

water suction at the moist-wet boundary is assumed to be about 0.5 

kPa for coarse soil materials and 1 kPa for other materials. The formal 

definition of coarse soil material is given later.

Three subclasses of dry are defined—very dry, moderately dry, and 

slightly dry. Very dry cannot be readily distinguished from air dry in the 

field. The water content extends from ovendry to 0.35 times the water

retention at 1500 kPa. The upper limit is roughly 150 percent of the airdry water content. The limit between moderately dry and slightly dry is a 

water content 0.8 times the retention at 1500 kPa.

The moist class is subdivided into slightly moist, moderately moist,

and very moist. Depending on the kind of soil material, laboratory 

retention at 5 or 10 kPa suction (method 4B, Soil Survey Staff, 2014a) 

determines the upper water retention (UWR). A suction of 5 kPa is used 

for coarse soil material; a suction of 10 kPa is used for other material.

To be considered coarse, the soil material that is strongly influenced

by volcanic ejecta must be nonmedial and have few or no vesicular pores 

in the mineral particles. If not strongly influenced by volcanic ejecta, it

must meet the sandy or sandy-skeletal family particle-size criteria and also 

3 The primary unit for suction is the pascal (Pa). The kilopascal (kPa), equal to 1000 

pascals, is commonly employed. One kPa = 1000 Pa = .01 bar = 10 cm of H2

O. 
Soil Survey Manual 2

Table 3-19

Water State Classes

Class Criteriaa

Dry (D) > 1500 kPa suction 

Very dry (DV) < 0.35 x 1500 kPa retention

Moderately dry (DM) 0.35 to < 0.8 x 1500 kPa retention

Slightly dry (DS) 0.8 to 1.0 x 1500 kPa retention

Moist (M) ≤ 1500 kPa to > 1.0 or 0.5 kPab

Slightly moist (MS) 1500 kPa suction to MWRc

Moderately moist (MM) MWR to UWRc

Very moist (MV) UWR to > 1.0 or 0.5 kPab

 suction

Wet (W) ≤ 1.0 kPa or 0.5 kPab

Nonsatiatedd

 (WN) > 0.0 to ≤ 1.0 kPa or ≤ 0.5 kPab

Satiatede

 (WA) ≤ 0.0 kPa

aCriteria use both suction and gravimetric water contents as defined by suction. b 0.5 kPa only if coarse soil material (see text). 

c UWR indicates upper water retention, which is the laboratory water retention at 5 

kPa for coarse soil material and 10 kPa for other material (see text). MWR indicates 

midpoint water retention, which is halfway between the upper water retention and the 

retention at 1500 kPa. d Peds glisten; no free water present.

e Peds glisten; free water present.

be coarser than loamy fine sand, have less than 2 percent organic carbon,

and have less than 5 percent water at 1500 kPa suction. Furthermore, the 

computed total porosity of the < 2 mm fabric must exceed 35 percent.4

Very moist has an upper limit at the moist-wet boundary and a lower 

limit at the upper water retention. Similarly, moderately moist has an 

4 Total porosity = 100 - (100 x Db/Dp), where Db is the bulk density of the < 2 mm 

material at or near field capacity and Dp is the particle density. The particle density

may be computed from the following: 

Dp = 100/[[(1.7xOC)/Dp1] + [(1.6xFe)/Dp2] + [[100 - [(1/7xOC) + (1.6xFe)]]/Dp3]]

where OC is the organic carbon percentage and Fe is the extractable iron by method 

6C2 (Soil Survey Staff, 2014a) or an equivalent method. The particle density of the 

organic matter (Dp1) is assumed to be 1.4 Mg/m3

, that of the minerals from which the 

extractable iron originates (Dp2) to be 4.2 Mg/m3

, and that of the material exclusive 

of the organic matter and the minerals contributing to the extractable Fe (Dp3) to be

2.65 Mg/m3.
210 Chapter 

upper limit at the upper water retention and a lower limit at the midpoint 

in gravimetric water content between retention at 1500 kPa and the 

upper water retention. This lower limit is referred to as the midpoint 

water retention (MWR). Slightly moist extends from the midpoint water 

retention to the 1500 kPa retention.

The wet class has nonsatiated and satiated subclasses distinguished 

on the basis of absence or presence of free water. Miller and Bresler 

(1977) defined satiation as the condition in which free water first appears

through saturation. The nonsatiated wet state may be applicable at zero 

suction to horizons with low or very low saturated hydraulic conductivity. 

These horizons may not exhibit free water. Horizons may have parts that 

are satiated wet and other parts that are nonsatiated wet because of low 

matrix saturated hydraulic conductivity and the absence of conducting 

macroscopic pores. Free water develops positive pressure with depth 

below the top of a wet satiated zone.

A class for saturation (that is, zero air-filled porosity) is not

provided because the term suggests that all of the pore space is filled

with water. This condition typically cannot be evaluated in the field.

Furthermore, if saturation is used for the concept of satiation, then a term 

is not available to describe known saturation. There is an implication 

of saturation if the soil material is satiated wet and coarse textured 

or otherwise has properties indicative of high or very high saturated 

hydraulic conductivity throughout the mass. A satiated condition does 

not necessarily indicate reducing conditions. Air may be present in the 

water and/or the microbiological activity may be low. The presence of 

reducing conditions may be inferred from soil color in some cases. A 

test may be performed for ferrous iron in solution. The results of the 

test for ferrous iron should be reported separately from the water state 

description.

Evaluation

Wet is indicated by the occurrence of prominent water films on

surfaces of sand grains and structural units that cause the soil material to 

glisten. If free water is absent, the term nonsatiated wet is used. If free 

water is present, the term satiated wet is used. In the field, the position of

the uppermost boundary in the soil profile meeting the satiated wet class

is the top of the water in an unlined bore hole after equilibrium has been 

reached. Lined bore holes or piezometers, installed to several depths 

across the zone of free water occurrence, are needed to determine the 

thickness of a perched zone of free water. Piezometers are tubes placed to 

a designated depth that are open at both ends. They may have a perforated 

zone at the bottom but do not permit water entry along most of their 
Soil Survey Manual 2

length. For the purpose of simply obtaining information about the depth 

of free water and the location and thickness of the free water zone, the 

installation of bore holes or piezometers is not required. This information 

can be obtained in the course of observing soils during regular soil survey 

field operations. Additional information about piezometers can be found

in Installing Monitoring Wells in Soils (Sprecher, 2008).

Ideally, evaluation within the moist and dry classes should be based 

on field instrumentation. However, this instrumentation is typically not

available and approximations must be made. Gravimetric water content 

measurements may be used. To make the conversion from measured 

water content to suction, information is needed on the gravimetric water 

retention at different suctions. The water retention at 1500 kPa may be 

estimated from the field clay percentage evaluation if dispersion of clay

is relatively complete for the soils concerned. Commonly, the 1500 kPa 

retention is roughly 0.4 times the clay percentage. This relationship can 

be refined considerably as the soil material composition and organization

are increasingly specified. Another rule of thumb is that the water content

at air dryness is about 10 percent of the clay percentage, assuming 

complete dispersion of clay. Model-based curves that relate gravimetric 

water content and suction are available for many soils (Baumer, 1986). 

These curves may be used to determine upper water retention and the 

midpoint water retention and to place the soil material in a water state 

class based on gravimetric water contents. Furthermore, in many cases 

they can be used as the basis for estimating water retention at 10 kPa 

from measurements at 33 kPa. Figure 3-30 shows a model-based curve

for a medium textured horizon and the relationship of water state class 

limits to water contents determined from the desorption curve. The figure

includes the results of a set of tests designed to provide local criteria for 

field and field office evaluation of water state. These tests are discussed

later in this chapter.

Commonly, information on gravimetric water content is not available. 

Visual and tactile observations must suffice for the placement. Separation

between moist and wet and the distinction between the two subclasses of 

wet may be made visually, based on water-film expression and presence

of free water. Similarly, the separation between very dry and moderately 

dry can be made by visual or tactile comparison of the soil material at the 

field water content and after air drying. The change on air drying should

be very small if the soil material initially is in the very dry class.

Criteria are more difficult to formulate forsoil material that is between

the moist-wet and the moderately dry-very dry separations. Four tests 

useful for mineral soils are the color value, ball, rod, and ribbon tests. 

The three tests that involve tactile examination are performed on soil 
212 Chapter 

Figure 3-30

Model-based curve for a medium textured horizon and the relationship of water state 

class limits to water contents determined from the desorption curve.

material that has been manipulated and mixed. This manipulation and 

mixing must be thorough enough to break down aggregates and provide 

consistent, repeatable results. The change may be particularly large for 

dense soil. In the field, this limitation should be kept in mind.

Color value test.—The crushed color value of the soil for an 

unspecified water state is compared to the color value when the soil

is at air dryness and when it is moderately moist or very moist. This 

test generally is useful only if the full range of color value from air 

dry to moderately moist exceeds one unit of color value. The change 

in color value and its interpretation depend upon the water desorption 

characteristics of the soil material. For example, as the water retention at 

1500 kPa increases, the difference between the minimum color value in 

the dry state and the color value in the very moist state tends to decrease.

Ball test.—A quantity of soil is squeezed firmly in the palm of

the hand to form a ball about 3 to 4 cm in diameter. This is done in

about five squeezes. The sphere should be near the maximum density

that can be obtained by squeezing. Different people will prepare the ball 

differently; however, an individual should learn to perform the procedure 

consistently.

In one approach, the ball is dropped from progressively increasing 

heights onto a nonresilient surface. The height in centimeters at which 

rupture occurs is recorded. Typically heights above 100 cm are not 
Soil Survey Manual 2

measured. Additionally, the manner of rupture is recorded. If the ball 

flattens and does not rupture, the term “deforms” is used. If the ball

breaks into about five or fewer units, the term “pieces” is used. Finally, if

the ball breaks into five or more units, the term “crumbles” is used.

Another approach uses penetration resistance. A penetrometer 

is inserted in the ball the same way it is done for soil in place. This 

alternative is only applicable for medium and fine textured soil materials

at higher water contents because these materials are relatively plastic and 

not subject to cracking.

Rod test.—The soil material is rolled between the thumb and 

forefinger or on a surface to form a rod 3 mm or less in diameter. A rod

2 cm or more in length must be able to remain intact while being held 

vertically from one end. If the maximum length that can be formed is 2 

to 5 cm, the rod is weak. If the maximum length equals or exceeds 5 cm, 

the rod is strong.

The rod test has close similarities to the plastic limit test (ASTM, 

2011). Plastic limit values exceed the 1500 kPa retention at moderate 

clay contents and approach, but are not commonly lower than, the 1500 

kPa retention at high clay contents. If a strong rod can be formed, the 

water content typically exceeds the 1500 kPa retention. The same is 

generally true for a weak rod. An adjustment is necessary if 2 to 0.5 mm 

material is present because the plastic limit is measured on material that 

passes a number 40 sieve (0.43 mm in diameter).

Ribbon test.—The soil material is smeared out between thumb and 

forefinger to form a flattened body about 2 mm thick. A ribbon must be

at least 2 cm in length. If the maximum length possible is 2 to 4 cm, the 

ribbon is weak. If the maximum length possible is 4 cm or more, the 

ribbon is strong.

General relationships of the tests to water state, with the exception 

of the relationship of the rod test to 1500 kPa retention, have not been 

formulated. Locally applicable field criteria can be formulated using

groupings of soils based on composition. 

Natural Drainage Classes

Natural drainage class refers to the frequency and duration of wet 

periods under conditions similar to those under which the soil developed. 

Alteration of the water regime by humans, either through drainage or 

irrigation, is not a consideration unless the alterations have significantly

changed the morphology of the soil. Descriptions of these classes follow.

Excessively drained.—Water is removed very rapidly. Internal 

free water occurrence commonly is very rare or very deep. The soils 
214 Chapter 

are commonly coarse textured and have very high saturated hydraulic 

conductivity or are very shallow. 

Somewhat excessively drained.—Water is removed from the soil 

rapidly. Internal free water occurrence commonly is very rare or very 

deep. The soils are commonly coarse textured and have high saturated 

hydraulic conductivity or are very shallow. 

Well drained.—Water is removed from the soil readily but not 

rapidly. Internal free water occurrence commonly is deep or very deep; 

annual duration is not specified. Water is available to plants throughout

most of the growing season in humid regions. Wetness does not inhibit 

root growth for significant periods during most growing seasons. The

soils are mainly free of, or are deep or very deep to, redoximorphic 

features related to wetness. 

Moderately well drained.—Water is removed from the soil somewhat 

slowly during some periods of the year. Internal free water occurrence 

is commonly moderately deep and transitory through permanent. The 

soils are wet for only a short time within the rooting depth during the 

growing season but long enough that most mesophytic crops are affected. 

They commonly have a moderately low or lower saturated hydraulic 

conductivity in a layer within the upper 1 meter, periodically receive 

high rainfall, or both. 

Somewhat poorly drained.—Water is removed slowly so that the 

soil is wet at a shallow depth for significant periods during the growing

season. Internal free water occurrence is commonly shallow to moderately 

deep and transitory to permanent. Wetness markedly restricts the growth 

of mesophytic crops, unless artificial drainage is provided. The soils

commonly have one or more of the following characteristics: low or very 

low saturated hydraulic conductivity, a high water table, additional water 

from seepage, or nearly continuous rainfall.

Poorly drained.—Water is removed so slowly that the soil is wet 

at shallow depths periodically during the growing season or remains 

wet for long periods. Internal free water occurrence is shallow or very 

shallow and common or persistent. Free water is commonly at or near 

the surface long enough during the growing season that most mesophytic 

crops cannot be grown, unless the soil is artificially drained. The soil,

however, is not continuously wet directly below plow depth. Free water 

at shallow depth is common. The water table is commonly the result 

of low or very low saturated hydraulic conductivity, nearly continuous 

rainfall, or a combination of these. 

Very poorly drained.—Water is removed from the soil so slowly that 

free water remains at or very near the surface during much of the growing 

season. Internal free water occurrence is very shallow and persistent 
Soil Survey Manual 2

or permanent. Unless the soil is artificially drained, most mesophytic

crops cannot be grown. The soils are commonly level or depressed and 

frequently ponded. In areas where rainfall is high or nearly continuous, 

slope gradients may be greater.

Subaqueous.—Free water is above the soil surface. Internal free 

water occurrence is permanent, and there is a positive water potential 

at the soil surface for more than 21 hours of each day. The soils have a 

peraquic soil moisture regime.

Internal Free Water Occurrence

Table 3-20 gives the classes and criteria used to describe free water

regimes in soils. The term “free water occurrence” is used instead of 

“satiated wet” in order to facilitate discussion of interpretations. These 

Table 3-20

Classes of Internal Free Water

Classes Criteria

Thickness if perched

Extremely thin < 10 cm

Very thin 10 cm to < 30 cm

Thin 30 cm to < 100 cm

Thick > 100 cm

Depth

Very shallow < 25 cm

Shallow 25 cm to < 50 cm

Moderately deep 50 cm to < 100 cm

Deep 100 cm to < 150 cm

Very deep > 150 cm

Cumulative annual pattern

Absent Not observed

Very transitory Present < 1 month

Transitory Present 1 to 3 months

Common Present 4 to 6 months

Persistent Present 7 to 12 months

Permanent Present continuously
216 Chapter 

classes indicate thickness if water is perched, depth to the upper boundary, 

and the aggregate time of occurrence in the calendar year. The free water 

need only occur in some parts of the horizon or layer to be recognized. 

If not designated as perched, the zone of free water is assumed to occur 

in all horizons or layers from its upper boundary to below a depth of 2 

meters or to the depth of observation. Artesian effects also may be noted.

Water State Annual Pattern

The water state annual pattern is a description of field soil water

over the year as applied to horizons and layers or to standard depth 

zones. Table 3-21 gives an example using the classes of internal water

states. Months when the layer is frozen are indicated by the symbol “F.” 

Typically, the use of the soil is indicated and the time interval is at least 

monthly. More general records may be constructed based on less-specific

soil uses and on soil concepts at a higher categorical level. Records may 

be constructed for classes of relative precipitation: wet—the wettest 2 

years in 10; dry—the driest 2 years in 10; and average—the conditions 6 

years in 10. Unless otherwise indicated, the class placement for relative 

precipitation is based on the more critical part of the growing season 

for the vegetation specified in the use. The frequency and duration of

inundation for each month also can be given.

Table 3-21

Example of a Water State Annual Pattern

[Months when the layer is frozen are indicated by an “F.” Other 

symbols are defined in table 3-19.]

Depth 

cm Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Average 6 years in 10

Fine, smectitic, mesic Typic Argiudoll a

0–25 MM MM MM MM MM MM MS DS DS MS MM MM

F F F

25–50 MM MM MM MM MM MM MM MS MS MS MS MM

F F F

50–100 MS MS MM MM MM MM MM MS MS MS MS MS

100–150 MM MM MM MM MM MM MM MM MM MM MM MM

150–200 MM MM MM MM MM MM MM MM MM MM MM MM
Soil Survey Manual 2

Table 3-21.—continued

Depth 

cm Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Fine-loamy, mixed, superactive, thermic Typic Haploxeralf b

0–30 MM MM MS MS DS DS D1c D1 D1 D1 D1 MS

30–70 MM MM MM MM MS DS D1 D1 D1 DS MS MM

70–100 MV MV MM MM MM MM MS D1 D1 D1 D1 MS

120–170 MM MM MM MS MS MS MS D1 D1 D1 DS MS

Driest 2 years in 10

Fine, smectitic, mesic Typic Argiudoll a

0–25 MM MM MM MM MM MS DS DS DS MS MS MM

F F F

25–50 MS MS MS MS MS MS MS MS MS MS MS MS

F F F

50–100 MS MS MS MM MM MS MS MS MS MS MS MS

100–150 MM MM MM MM MM MM MM MM MM MM MM MM

150–200 MM MM MM MM MM MM MM MM MM MM MM MM

Fine-loamy, mixed, superactive, thermic Typic Haploxeralf b

0–30 MS MM MS MS DS DS D1 D1 D1 D1 D1 DS

30–70 MM MM MM MM MS DS D1 D1 D1 D1 MS MS

70–100 MS MM MM MM MM MM MS D1 D1 D1 D1 DS

120–170 MS MM MS MS MS MS MS D1 D1 D1 D1 D1

Wettest 2 years in 10

Fine, smectitic, mesic Typic Argiudoll a

0–25 MM MM MV MV MV MM MM MM MM MM MM MM

F F F

25–50 MM MM MV MV MM MM MM MM MM MM MM MM

F F F

50–100 MM MM MM MM MM MM MM MM MM MM MM MM

100–150 MM MM MM MM MM MM MM MM MM MM MM MM

150–200 MM MM MM MM MM MM MM MM MM MM MM MM

Fine-loamy, mixed, superactive, thermic Typic Haploxeralf b

0-30 MM MM MM MS DS DS D1 D1 D1 DS MS MM

30-70 MV MV MM MM MS DS D1 D1 D1 DS MM MV

70-100 MV MV MM MM MM MM MS D1 D1 D1 MS MM

120-170 MM MM MS MS MS MS MS D1 D1 D1 DS MS
218 Chapter 

a Otoe County, Nebraska (USDA-NRCS, 2009). Aksarben silty clay loam, 2 to 6 

percent slopes. Corn (Zea mays) following corn. Assume: contoured, terraced, over 

20 percent residue cover. Disk twice in April. Field cultivate once. Plant May 1–15. 

Cultivate once or twice. Harvest November 1–15. Cattle graze after harvest. Based on 

a discussion with H.E. Sautter, soil scientist (retired), Syracuse, NE. Monthly water 

states based on long-term field mapping experience and water balance computations. b San Diego Area, California (USDA-SCS, 1973). Fallbrook sandy loam, 5 to 9 percent

slopes, eroded. Mean annual precipitation at Escondido is 344 mm, and potential

evaporation at Thornwaite is 840 mm. Study area has slightly greater slope than the 

upper limit of the map unit. Vegetation is annual range, fair condition. Generalizations 

were made originally for the 1983 National Soil Survey Conference based on field

measurements in 1966 by Nettleton et al. (1969), as interpreted by R.A. Dierking, 

soil correlator, Portland, OR. At the time, moderately dry and very dry were not 

distinguished. c D1 = very dry and moderately dry water states.

Water Movement

Water movement concerns rates of flow into and within the soil and

the related amount of water that runs off and does not enter the soil. 

Saturated hydraulic conductivity, infiltration rate, and surface runoff are

part of the evaluation (see chapter 2 for a discussion about runoff).

Saturated Hydraulic Conductivity

Saturated hydraulic conductivity is a quantitative measure of a 

saturated soil’s ability to transmit water when subjected to a hydraulic 

gradient. It can be thought of as the ease with which pores of a saturated 

soil permit water movement. Water movement in soil is controlled by two 

factors: 1) the resistance of the soil matrix to water flow, and 2) the forces

acting on each element or unit of soil water. Darcy’s law, the fundamental 

equation describing water movement in soil, relates the flow rate to these

two factors. Mathematically, the general statement of Darcy’s law for 

vertical, saturated flow is:

Q/At = -Ksat dH/dz

The flow rate Q/At is what soil physicists call the flux density, i.e., the

quantity of water Q moving past an area A, perpendicular to the direction 

of flow, in a time t. The vertical saturated hydraulic conductivity Ksat is 

the reciprocal, or inverse, of the resistance of the soil matrix to water 

flow. The hydraulic gradient dH/dz is the driving force causing water 

to move in soil, the net result of all forces acting on the soil water. Rate 

of water movement is the product of the hydraulic conductivity and the 

hydraulic gradient.
Soil Survey Manual 2

A distinction is made between saturated and unsaturated hydraulic 

conductivity. Saturated flow occurs when the soil water pressure is positive, i.e., when the soil matric potential is zero (satiated wet condition). 

In most soils this condition occurs when about 95 percent of the total 

pore space is filled with water. The remaining 5 percent is filled with

entrapped air. If the soil remains saturated for several months or longer, 

the percent of the total pore space filled with water may approach 100.

Saturated hydraulic conductivity cannot be used to describe water movement under unsaturated conditions.

The vertical saturated hydraulic conductivity Ksat is the proportionality constant relating soil water flow rate (flux density) to the

hydraulic gradient and is a measure of the ease of water movement in 

soil. Ksat is the reciprocal of the resistance of soil to water movement. 

As resistance increases, hydraulic conductivity decreases. Resistance 

to water movement in saturated soil is primarily a function of the 

arrangement and size distribution of pores. Large, continuous pores 

have a lower resistance to flow (and thus higher conductivity) than

small or discontinuous pores. Soils with high clay content generally 

have lower hydraulic conductivities than sandy soils because the pore 

size distribution in sandy soil favors large pores, even though sandy soils 

typically have higher bulk densities and lower total porosities (total pore 

space) than clayey soils. As illustrated by Poiseuille’s law, the resistance 

to flow in a tube varies as the square of the radius. Thus, as a soil pore or

channel doubles in size, its resistance to flow is reduced by a factor of 4,

i.e., hydraulic conductivity increases fourfold.

Saturated hydraulic conductivity can be expressed by different forms 

of mathematical equations. When the flux and gradient are expressed on

a mass basis, the resulting dimensions for Ksat are (mass x time)/volume 

and the SI units (International System of Units) are kg s m-3 (kilogram 

seconds per cubic meter). When they are expressed on a volume 

basis, the dimensions are (volume x time)/mass and the SI units are 

m3

 s kg-1 (cubic meter seconds per kilogram). If one expresses the flux on

a volume basis and the gradient on a weight basis, then the dimensions 

of Ksat are length/time and the SI units are m s-1 (meters per second). 

This last mathematical form has the simplest units, but it only applies 

under unique conditions in the field. Care must be taken to interpret this

correctly and not conclude that hydraulic conductivity is literally the rate 

of water movement through the soil. Saturated hydraulic conductivity is 

not a rate of water movement; it is a measure of a saturated soil’s ability 

to transmit water under a hydraulic gradient. Or, in general terms, it is 

the ease with which pores of a saturated soil permit water to move. Low 
220 Chapter 

values indicate restricted movement, and higher values indicate relative 

ease of movement.

Data on saturated hydraulic conductivity are valuable in overall 

planning for irrigation, drainage, erosion control, and flood control. Ksat

can be used to predict flow rate under specified hydraulic gradients and

boundary conditions. It is an important component in solute transfer and 

drainage models. Surface ponding and runoff are regulated to a great extent by saturated hydraulic conductivity. Ksat can also be used for estimating transport coefficients of nonaqueous fluids (e.g., air and organic liquids). In addition, because saturated hydraulic conductivity is a powerful 

indicator of pore geometry, it can be used as an index for soil structure. 

Saturated hydraulic conductivity is one of the most variable soil 

properties. This variability is determined by total porosity, pore-size 

distribution, and tortuosity of flow paths, all of which are highly affected

by land use and management. Different crop management systems on the 

same soil type may cause 100-fold differences in Ksat of surface horizons. 

Coefficients of variability in excess of 100 percent for saturated

hydraulic conductivity are common. Measured Ksat values may vary 

dramatically with the method used for measurement. Laboratorydetermined values rarely agree with field measurements; the differences

can be on the order of 100-fold or more. Field methods generally are 

considered more reliable than laboratory methods, but this may be an 

illusion due to differences in sample volumes and method. The volume 

of the sample being tested relates to the possibility of a sample including 

unusually large pores due to animal burrows, root channels, desiccation 

cracks, etc. For smaller volumes, this has the character of a “hit or miss” 

proposition, and the result can be high variability within relatively small 

areas. For larger sampling volumes, the chance of observing similar 

Ksat values from multiple readings within a study area is higher and the 

variability among samples is lower. The smallest volume in which the 

lowest variability can be attained is called the “representative elementary 

volume” (REV) (Bear, 1972). The REV for Ksat measurements is 

currently unknown. It likely varies by soil type. Because the field is the

best setting for approximating a representative elementary volume, field

measurements are emphasized. 

Because of the highly variable nature of soil hydraulic conductivity, 

a single measured value is an unreliable indicator of the hydraulic 

conductivity of a soil. An average of several values provides a reliable 

estimate, which can be used to place the soil in a particular saturated 

hydraulic conductivity class. Log averages (geometric means)5

 should be 

5 meanKs=(Ks1 xKs2 xKs3 x...xKsx

)

1/x
Soil Survey Manual 2

used rather than arithmetic averages because hydraulic conductivity is a 

property with log-normal distribution. The antilog of the average of the 

logarithms of individual conductivity values is the log average, or geometric 

mean, and should be used to place a soil into the appropriate hydraulic 

conductivity class. Log averages are lower than arithmetic averages.

Classes of saturated hydraulic conductivity.—In this manual, 

saturated hydraulic conductivity classes are defined in terms of vertical,

saturated hydraulic conductivity. Table 3-22 identifies the vertical, saturated hydraulic conductivity classes used in the National Cooperative Soil 

Survey. The saturated hydraulic conductivity classes in this manual have 

a wider range of values than the classes that were previously used by the 

NCSS, as published in the previous edition of the Soil Survey Manual

(Soil Survey Staff, 1951) and the Guide for Interpreting Engineering Uses 

of Soils (USDA-SCS, 1971). The dimensions of hydraulic conductivity 

vary depending on whether the hydraulic gradient and flux density have

mass, weight, or volume bases. Values can be converted from one basis 

to another with the appropriate conversion factor. Typically, the hydraulic 

gradient is given on a weight basis, the flux density is given on a volume

basis, and the dimensions of Ksat are length per time. The correct SI units 

are therefore meters per second.6

 Micrometers per second are also acceptable SI units and, due to fewer decimal places, are more convenient (table 

3-22). Table 3-23 gives the equivalent class limits in other commonly

used units. Converting to equivalent units is useful when presenting the 

data to members of the public who may not be familiar with SI units.

Table 3-22

Classes of Saturated Hydraulic Conductivity 

Class Ksat (μm/s)

Very high ≥ 100

High 10 to < 100

Moderately high 1 to < 10

Moderately low 0.1 to < 1

Low 0.01 to < 0.1

Very low < 0.01

6 The Soil Science Society of America prefers that all quantities be expressed on a mass 

basis. This results in Ksat units of kg s m-3. Other acceptable units are m3 s kg-1, where 

all quantities are expressed on a volume basis, and m s-1, where hydraulic gradient is 

expressed on a weight basis and flux density on a volume basis.
222 Chapter 

Saturated hydraulic conductivity does not describe the capacity of 

soils in their natural setting to dispose of water internally. A soil placed 

in a very high class may contain free water because there are restricting 

layers below the soil or because the soil is in a depression where water 

from surrounding areas accumulates faster than it can pass through the 

soil. The water may actually move very slowly despite a high Ksat.

Table 3-23

Saturated Hydraulic Conductivity Class Limits in Equivalent 

Units

μm/s m/s cm/day in/hr cm/hr kg s m-3 m3

 s kg-3

100 10-4 864 14.17 36.0 1.02x10-2 1.02x10-8

10 10-5 86.4 1.417 3.60 1.02x10-3 1.02x10-9

1 10-6 8.64 0.1417 0.360 1.02x10-4 1.02x10-10

0.1 10-7 0.864 0.01417 0.0360 1.02x10-5 1.02x10-11

0.01 10-8 0.0864 0.001417 0.00360 1.02x10-6 1.02x10-12

Guidelines for Ksat class placement.—Measured values of Ksat are 

available from the literature or from researchers working on the same 

or similar soils. If measured values are available, their geometric means 

should be used for class placement.

Saturated hydraulic conductivity is a fairly easy, inexpensive, and 

straightforward measurement. If measured values are unavailable, a 

project to make measurements should be considered. Field methods 

are the most reliable. Standard methods for measurement of Ksat are 

described in Agronomy Monograph No. 9 (Klute and Dirksen, 1986; 

Amoozegar and Warrick, 1986) and in SSIR 38 (Bouma et al., 1982).

Researchers have attempted to estimate Ksat based on various soil 

properties. These estimation methods typically use one or more of the 

following soil physical properties: surface area, texture, structure, bulk 

density, and micromorphology. The success of the individual methods 

varies, and no single method works well for all soils. In some cases, a 

method works well only in a localized area. In other cases, measurement 

of the predictor variables is more difficult than measurement of hydraulic

conductivity. Generally, adjustments must be made for soil properties 

that affect the integrity and continuity of macropores when the soil 

is moderately moist or wet. These properties include high sodium 

concentrations; certain clay mineralogies; grade, size, and shape of soil 

structure; and the presence of coarse fragments, fragipans, cemented 

layers, and other miscellaneous features.
Soil Survey Manual 2

The method presented here is very general (Rawls and Brakensiek, 

1983). It was developed from a statistical analysis of several thousand

measurements on a variety of soils. It is intended for a wide application 

and must be used locally with caution. This method does not account for 

the circumstances mentioned in the previous paragraph. Commonly, the 

results must be adjusted based on experience and local conditions.

Figures 3-31 and 3-32 used together provide a method for approximating Ksat class based on soil texture and bulk density. Each figure

consists of three textural triangles. Based on the texture and bulk 

density of a particular soil horizon, the bulk density class is estimated 

by determining which triangle in figure 3-31 the horizon belongs to. The

chosen bulk density class determines which triangle in figure 3-32 is

used to estimate Ksat class.

For a particular soil texture with either measured or estimated bulk 

density, interpolating between the iso-bulk density lines in figure 3-31

yields a bulk density class. The triangle in the figure that provides the

value closest to the measured or estimated bulk density for that particular 

textural class determines which triangle in figure 3-32 should be used.

For example, in figure 3-31, a clay loam with both 35 percent sand and

clay and a bulk density of 1.20 g/cc plots between the iso-bulk density 

lines of 1.06 and 1.32 of the textural triangle marked “low” and thus is

in the low bulk density class. A clay loam with both 35 percent sand and

clay and a bulk density of 1.40 g/cc plots between the iso-bulk density 

lines of 1.32 and 1.48 on the textural triangle marked “medium” and thus

has a medium bulk density class. For soils having medium or average 

bulk densities, the center triangle is used. The triangles above and 

below the center triangle are for soils with high and low bulk densities, 

respectively. The textural triangle in figure 3-32 that corresponds with

the bulk density class determined from figure 3-31 is selected, and the

clay and sand percentages are plotted to determine Ksat class placement. 

The Ksat class is “moderately high” for the clay loam in the low bulk 

density class and “moderately low” for the clay loam in the medium bulk 

density class. A numerical value of Ksat can be estimated by interpolating 

between the iso-Ksat lines. However, the values should be used with 

caution. They should be used only to compare classes of soils and not as 

an indication of the Ksat of a particular site. If site values are needed, it is 

best to make several measurements at the site.

The Ksat values determined using the above procedure may need to 

be adjusted based on other known soil properties. Currently, there are 

no guidelines for adjusting the estimated Ksat. The soil scientist must 

use best judgment based on experience and the observed behavior of the 

particular soil.
224 Chapter 

Figure 3-31

Bulk density classes: low, medium, and high (Rawls and Brakensiek, 1983).
Soil Survey Manual 2

Figure 3-32

Saturated hydraulic conductivity classes based on bulk density and texture relationships.
226 Chapter 

Saturated hydraulic conductivity can be given for the soil as a whole, 

for a particular horizon, or for a combination of horizons. The horizon 

with the lowest value determines the saturated hydraulic conductivity 

class assigned to the whole soil. If an appreciable thickness of soil above 

or below the horizon with the lowest value has significantly higher

conductivity, then estimates for both parts are typically given (i.e., high 

over very low).

Infiltration

Infiltration is the process of downward water entry into the soil. It is

typically sensitive to near surface conditions as well as to the antecedent 

water state. Hence, it is subject to significant change with soil use and

management and over time. As a result, assigning infiltration values

to soil map units for most soil survey projects (unless they are large 

scale, high-intensity surveys) is generally not practical. The following 

discussion describing infiltration is provided for background information.

Infiltration rate classes are not provided. Field measured values can be

recorded as part of the site description for pedons.

Infiltration stages.—Three stages of infiltration may be recognized:

preponded (before ponding occurs), transient ponded (ponding is 

transient), and steady ponded (a constant ponded condition). Preponded 

infiltration pertains to downward entry of water into the soil under 

conditions where free water is not present on the land surface. At this 

stage, the rate of water addition determines the rate of water entry. If 

rainfall intensity increases twofold, then the infiltration increases

twofold. In addition, surface-connected macropores are not involved in 

transporting water downward (water is only moving through the matrix). 

No runoff occurs during the preponded stage.

As water addition continues, the point may be reached where free 

water occurs on the ground surface. This condition is called ponding. The 

term in this context is less restrictive than its use in inundation. The free 

water may be restricted to depressions and be absent from the majority of 

the ground surface. Once ponding has taken place, the infiltration is controlled by soil characteristics rather than by the rate of water addition. As 

a result, surface-connected macropores and subsurface-initiated cracks 

are involved in transporting water downward.

Infiltration under conditions where free wateris present on the ground

surface is referred to as ponded infiltration. In the initial stage of ponded

infiltration, the rate of water entry typically decreases appreciably with

time because of the deeper wetting of the soil, which results in a reduced 

suction gradient and the closing of cracks and other surface-connected 

macropores. Transient ponded infiltration is the stage at which the 
Soil Survey Manual 2

ponded infiltration decreases markedly with time. After long, continued

wetting under ponded conditions, the rate of infiltration becomes steady.

This stage is referred to as steady ponded infiltration. Surface-connected 

cracks, if reversible, close. The suction gradient is small, and the driving 

force is reduced to near that of the gravitational gradient. If there is no 

ice and no zones of free water within moderate depths and if surface 

or near surface features (e.g., a crust) do not control infiltration, the

minimum saturated hydraulic conductivity within a depth of 1⁄2 to 1 

meter is a useful predictor of steady ponded infiltration rate.

Minimum annual steady ponded infiltration.—The steady ponded infiltration rate when the soil is in the wettest state that regularly

occurs while not frozen is called the minimum annual steady ponded 

infiltration rate. It can be estimated using the equation for the GreenAmpt infiltration model (see below). The estimated rate is subject to

reduction if free water is present at shallow depths. The minimum 

annual steady ponded infiltration rate has application for prediction

of runoff at the wettest times of the year when the runoff potential is 

typically highest.

Green-Ampt infiltration model.—The Green-Ampt model is 

one model used to compute infiltration rate. The model assumes that

infiltrating water uniformly wets to a depth and stops abruptly at a front.

This front moves downward as infiltration proceeds. The soil above the

wetting front is in the satiated wet condition throughout the wetted zone.

The equation (Rawls and Brackensick, 1983) describing infiltration

is:

f=Ka 1 + MxS

 ( F )

Ka is the hydraulic conductivity for satiated, but not necessarily 

saturated, conditions; M is the porosity at a particular water state that 

has the potential to be filled with water; S is the effective suction at 

the wetting front; and F is the cumulative infiltration. The hydraulic

conductivity at satiation is somewhat lower than the saturated value 

because of the presence of entrapped air. The available porosity (M) 

changes for surficial horizons according to bulk density and for all

horizons according to the water state. It is, therefore, sensitive to soil use 

that may affect both bulk density of surficial horizons and the antecedent

water state. The value of the effective suction at the wetting front (S) is 

determined largely by texture and is a tabulated quantity. The cumulative 

infiltration (F) increases with time asinfiltration proceeds.Aconsequence

of the increase in the cumulative infiltration is that the infiltration rate (f)

decreases with time. As the cumulative infiltration becomes large and the
228 Chapter 

depth of wetting considerable, the infiltration rate approaches the value

of the hydraulic conductivity for the satiated condition.

Soil Temperature

Soil temperature, like soil moisture, is an important component 

of the overall soil climate. It exerts a strong influence on biological

activities. It also influences the rates of chemical and physical

processes within the soil. As a result, the chemical properties of the 

soil, including organic matter content, mineralogy, and fertility levels, 

are significantly impacted by soil temperature. When the soil is frozen,

biological activities and chemical processes essentially stop. Physical 

processes that are associated with ice formation are active if unfrozen 

zones are associated with freezing zones. Below a soil temperature of 

about 5 °C (referred to as “biologic zero” in Soil Taxonomy), growth 

of roots of most plants is negligible. However, in areas where soils 

have permanently frozen layers near the surface, even large roots of 

adapted plants are present immediately above the frozen layer in late 

summer. Most plants grow best within a restricted range of soil and air 

temperatures. Knowledge of soil and air temperatures is essential in 

understanding soil-plant relationships. Temperature, like the soil water 

state, changes with time. It generally differs from layer to layer at any 

given time.

Characteristics of Soil Temperature

Heat is both absorbed at and lost from the surface of the soil. 

Temperature at the surface can change in daily cycles. The soil transmits 

heat downward when the temperature near the surface is higher than the 

temperature below. It transmits heat upward when the temperature is 

warmer within the soil than at the surface. Soil temperatures at various 

depths within the soil follow cycles. The cycles deeper in the soil lag 

behind those near the surface. The daily cycles decrease in amplitude 

as depth increases and are scarcely measurable below 50 cm in most 

soils. Seasonal cycles are evident to much greater depths if seasonal air 

temperature differences are pronounced. The temperature at a depth of 

10 m is nearly constant in most soils and is about the same as the mean 

annual temperature of the soil above.

Soil temperature varies at a given site from layer to layer according 

to the time of the year; yet, if the average annual temperatures at different 

depths in the same pedon are compared, they typically do not differ. Mean 
Soil Survey Manual 2

annual soil temperature is one of several useful values that describe the 

temperature regime of a soil.

The seasonal fluctuation of soil temperature is a characteristic of

a soil. Soil temperature fluctuates little seasonally near the equator; it

fluctuates widely according to season in the middle and high latitudes.

Mean seasonal temperatures can be used to characterize soil temperature. 

As soil depth increases, the magnitude of the differences in seasonal soil 

temperature decreases and the seasonal cycles exhibit a delay compared 

to temperatures at shallower depths.

For soils that freeze in winter, soil temperature is influenced by the

release of heat when water changes from liquid to solid. This release is 

about 80 calories per gram of water. Heat must be dissipated before the 

water in soil freezes. The rate of thaw of frozen soils is slower, because 

heat is required to warm the soil in order to melt the ice. In areas of heavy 

snowfall, the snow provides an insulating blanket and soils do not freeze 

as deeply or do not freeze at all.

Many factors influence soil temperature. They include amount,

intensity, and distribution of precipitation; daily and monthly fluctuations

in air temperature; insolation; kinds, amounts, and persistence of 

vegetation; duration of moisture states and snow cover; kinds of organic 

deposits; surface soil color; aspect and gradient of slope; elevation; and 

ground water. All of these factors may be described in a soil survey.

Estimating Soil Temperature

Soil temperature can be monitored over time through the use of 

automated digital temperature recorders. The recorders are commonly 

buried in water-tight containers in the soil. They automatically record and 

store temperature readings at preprogrammed intervals throughout the 

day (five readings per day is sufficient). Sensors at the ends of wire leads

extending from the buried container are commonly placed about 1 meter 

above the ground (for air temperature) and at a depth of 50 cm in the soil. 

Additional sensors can be placed at other depths if desired. At the end 

of the study period (generally 1 year), the recording device is retrieved 

and the data are downloaded to a computer for analysis. From these data 

one can calculate mean annual soil temperature as well as mean annual 

summer and winter temperatures. The relationship between average soil 

temperature and average air temperature can also be determined for the 

site. Plots of diurnal, seasonal, and annual temperature variation can be 

prepared to illustrate the variation of soil temperature through time.

Estimates of soil temperature can be made without 1 or more complete 

years of collected data. Mean annual soil temperature in temperate, 
230 Chapter 

humid, continental climates can be approximated by adding 1 °C to 

the mean annual air temperature reported by standard meteorological 

stations at locations near the soil under study. The mean annual soil 

temperature at a given place can be estimated more reliably by a single 

reading at a depth of 10 m. If water in wells is at depths between 10 and 

20 m, the temperature of the water typically gives a close estimate of 

mean annual soil temperature. Mean annual soil temperature can also be 

estimated from the average of four readings at about 50 cm or greater 

depth, equally spaced throughout the year.

The mean soil temperature for summer can be estimated by averaging 

three measurements taken at a constant depth between 50 cm and 1 m on 

the 15th day of each of the three months of the season. Similar methods 

may be used to estimate soil temperature for other seasons. These methods 

give values slightly different from the actual soil temperature, due to 

factors such as vegetation (particularly density of canopy), ground water, 

snow, aspect, rain, unusual weather conditions, and other factors. Tests 

for nearly level, freely drained soils, both grass-covered and cultivated, 

produce comparable values. Over the usual period of a soil survey, 

systematic studies can be made to establish temperature relationships in 

the survey area.

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