LAND-CAPABILITY

CLASSIFICATION

Agriculture Handbook No. 210

SOIL CONSERVATION SERVICE

U.S. DEPARTMENT OF AGRICULTURE
Growth Through Agricultural Progress

For sale by the Superintendent of Documents, U.S. Government Printing OflSce

Washington 25, D.C. - Price 15 cents
FOREWORD

Since soil siuveys ore based on all of the characteristics of soils that influence their

use and management, interpretations are needed for each of the many uses. Among

these interpretations the grouping of soils into capability units, subclasses, and classes

is one of the most important. This grouping serves as an introduction of the soil mop

to farmers and other land users developing conservation plans.

As we have gained experience in this grouping, the definitions of the categories have

improved. It is the purpose of this publication to set forth these definitions. In using

the capability classification, the reader must continually recall that it is on interpretation.

Like other interpretations, it depends on the probable interactions between the kind

of soil and the alternative systems of management. Our management systems ore continually changing. Economic conditions change. Our knowledge grows. Land users

ore continually being offered new things, such as new machines, chemicals, and plant

varieties.

The new technology applies unevenly to the various kinds of soil. Thus the grouping

of any one kind of soil does not stay the same with changes in technology. That is,

new combinations of practices increase the productivity of some soils more than others,

so some are going up in the scale whereas others are going down, relatively. Some

of our most productive soils of today were considered poorly suited to crops a few years

ago. On the other hand, some other soils that were once regarded as good for cropping

ore now being used more productively for growing pulpwood. These facts in no way

suggest that we should not make interpretations. In fact, they become increasingly

important as technology grows. But these facts do mean that soils need to be reinterpreted and regrouped after significant changes in economic conditions and technology.

Besides the capability classification explained in this publication, other important interpretations ore made of soil surveys. Examples include groupings of sous according

to crop-yield predictions, woodland suitability, range potentiaHty, wildlife habitat, suitability for special crops, and engineering behavior. Many other kinds of special groupings are used to help meet local needs.

CHARLES E. KELLOGO

Assistant Administrator for Soil Survey

Soil Conservation Service
CONTENTS

Page

Assumptions 3

Capability classes 6

Land suited to cultivation and other uses 6

Land limited in use—generally not suited to cultivation 9

Capability subclasses 10

Capability units 12

Other kinds o£ soil groupings 12

Criteria for placing soils in capability classes 13

Arid and semiarid stony, wet, saline-sodic, and overflow soils 14

Climatic limitations 15

Wetness limitations 16

Toxic salts 16

Slope and hazard of erosion 17

Soil depth 18

Previous erosion 18

Available moisture-holding capacity 18

Glossary 18

Issued September 1961
LAND-CAPABILITY

CLASSIFICATION

By A. A. Klingebiel and P. H. Montgomery, soil scientists. Soil Conservation

Service

The standard soil-survey map shows the different kinds of soil that are

significant and their location in relation to other features of the landscape.

These maps are intended to meet the needs of users with widely different

problems and, therefore, contain considerable detail to show important

basic soil differences.

The information on the soil map must be explained in a way that has

meaning to the user. These explanations are called interpretations. Soil

maps can be interpreted by ( 1 ) the individual kinds of soil on the map, and

( 2 ) the grouping of soils that behave similarly in responses to management

and treatment. Because there are many kinds of soil, there are many individual soil interpretations. Such interpretations, however, provide the

user with all the information that can be obtained from a soil map. Many

users of soil maps want more general information than that of the individual

soil-mapping unit. Soils are grouped in different ways according to the

specific needs of the map user. The kinds of soil grouped and the variation permitted within each group differ according to the use to be made

of the grouping.

The capability classification is one of a number of interpretive groupings

made primarily for agricultural purposes. As with all interpretive groupings

the capability classification begins with the individual soil-mapping units,

which are building stones of the system (table 1). In this classification the

arable soils are grouped according to their potentialities and limitations for

sustained production of the common cultivated crops that do not require

specialized site conditioning or site treatment. Nonarable soils (soils unsuitable for longtime sustained use for cultivated crops) are grouped according to their potentialities and limitations for the production of permanent vegetation and according to their risks of soil damage if mismanaged.

The individual mapping units on soil maps show the location and extent

of the different kinds of soil. One can make the greatest number of precise

statements and predictions about the use and management of the individual

mapping units shown on the soil map. The capability grouping of soils is

designed ( 1 ) to help landowners and others use and interpret the soil maps,

(2) to introduce users to the detail of the soil map itself, and (3) to make

possible broad generalizations based on soil potentialities, limitations in use,

and management problems.

The capability classification provides three major categories of soil groupings: (1) Capability unit, (2) capability subclass, and (3) capability class.
TABLE L—Relaüonship of soil-mapping unit to capability classification

Soil-mapping unit

A soil mapping unit is a portion of

the landscape' that has similar

characteristics and qualities and

whose limits are fixed by precise

definitions. Within the cartographic limitations and considering the purpose for which the

map is made, the soil mapping

unit is the unit about which the

greatest number of precise statements and predictions can be

made.

The soil mapping units provide

the most detailed soils information. The basic mapping units

are the basis for all interpretive

groupings of soils. They furnish

the information needed for developing capability units, forest

site groupings, crop suitability

groupings, range site groupings,

engineering groupings, and

other interpretive groupings.

The most specific management

practices and estimated yields

are related to the individual

mapping unit.

Capability unit

A capabihty unit is a grouping of

one or more individual soil mapping units having similar potentials and continuing limitations or hazards. The soils in

a capability unit are sufficiently

uniform to (a) produce similar

kinds of cultivated crops and

pasture plants with similar management practices, (b) require

similar conservation treatment

and management under the

same kind and condition of

vegetative cover, (c) have comparable potential productivity.

The capability unit condenses and

simplifies soils information for

planning individual tracts of

land, field by field. Capability units with the class and

subclass furnish information

about the degree of limitation,

kind of conservation problems

and the management practices

needed.

Capability subclass

Subclasses are groups of capability units which have the

same major conservation

problem, such as—

e—Erosion and runoff.

w—Excess water.

s—Root-zone limitations.

c—Climatic limitations.

The capability subclass provides information as to the

kind of conservation problem

or limitations involved. The

class and subclass together

provide the map user information about both the degree

of limitation and kind of problem involved for broad program planning, conservation

need studies, and similar

purposes.

Capability class

Capability classes are groups of

capability subclasses or capability units that have the same

relative degree of hazard or

limitation. The risks of soil

damage or limitation in use

become progressively greater

from class I to class VIII.

The capability classes are useful

as a means of introducing the

map user to tlie more detailed

information on the soil map.

The classes show the location,

amount, and general suitabiUty of the soils for agricultural use. Only information

concerning general agricultural limitations in soil use are

obtained at the capability

class level.
The first category, capability unit, is a grouping of soils that have about the

same responses to systems of management of common cultivated crops and

pasture plants. Soils in any one capability unit are adapted to the same

kinds of common cultivated and pasture plants and require similar alternative systems of management for these crops. Longtime estimated yields

of adapted crops for individual soils within the unit under comparable

management do not vary more than about 25 percent.^

^ The second category, the subclass, is a grouping of capability units having

similar kinds of limitations and hazards. Four general kinds of limitations

or hazards are recognized: (1) Erosion hazard, (2) wetness, (3) rootingzone limitations, and (4) climate.

The third and broadest category in the capability classification places

all the soils in eight capabihty classes. The risks of soil damage or limitations in use become progressively greater from class I to class VIII. Soils

in the first four classes under good management are capable of producing

adapted plants, such as forest trees or range plants, and the common cultivated field crops ^ and pasture plants. Soils in classes V, VI, and VII are

suited to the use of adapted native plants. Some soils in classes V and VI

are also capable of producing specialized crops, such as certain fruits and

ornamentals, and even field and vegetable crops under highly intensive

management involving elaborate practices for soil and water conservation.^

Soils in class VIII do not return on-site benefits for inputs of management

for crops, grasses, or trees without major reclamation.

The grouping of soils into capability units, subclasses, and classes is done

primarily on the basis of their capability to produce common cultivated

crops and pasture plants without deterioration over a long period of time.

To express suitability of the soils for range and woodland use, the soilmapping units are grouped into range sites and woodland-suitability groups.

ASSUMPTIONS

In assigning soils to the various capability groupings a number of assumptions are made. Some understanding of these assumptions is necessary if

* Yields are significant at the capability-unit level and are one of the criteria used

in establishing capability units within a capability class. Normally, yields are estimated

under the common management that maintains the soil resource. The main periods

for such yield estimates are 10 or more years in himiid areas or under irrigation and

20 or more years in subhumid or semiarid areas. The 25 percent allowable range

is for economically feasible yields of adapted cultivated and pasture crops.

' As used here the common crops include : Com, cotton, tobacco, wheat, tame hay

and pasture, oats, barley, grain sorghimi, sugarcane, sugar beets, peanuts, soybeans,

field-grown vegetables, potatoes, sweet potatoes, field peas and beans, flax, and most

clean-cultivated fruit, nut, and ornamental plants. They do not include: Rice, cranberries, blueberries, and those fruit, nut, and ornamental plants that require little or

no cultivation.

' Soil and water conservation practices is a general expression for all practices

including but not limited to those for erosion control.

507953—61 2 3
the soils are to be grouped consistently in the capability classification and

if the groupings are to be used properly. They are:

1. A taxonomic (or natural) soil classification is based directly on soil

characteristics. The capabihty classification (unit, subclass, and class)

is an interpretive classification based on the effects of combinations of

climate and permanent soil characteristics on risks of soil damage,

limitations in use, productive capacity, and soil management requirements. Slope, soil texture, soil depth, effects of past erosion, permeability, water-holding capacity, type of clay minerals, and the many

other similar features are considered permanent soil qualities and

characteristics. Shrubs, trees, or stumps are not considered permanent

characteristics.

2. The soils within a capability class are similar only with respect to degree

of limitations in soil use for agricultural purposes or hazard to the soil

when it is so used. Each class includes many different kinds of soil, and

many of the soils within any one class require unlike management and

treatment. Valid generalizations about suitable kinds of crops or other

management needs cannot be made at the class level.

3. A favorable ratio of output to input * is one of several criteria used for

placing any soil in a class suitable for cultivated crop, grazing, or woodland use, but no further relation is assumed or implied between classes

and output-input ratios. The capability classification is not a productivity rating for specific crops. Yield estimates are developed for

specific kinds of soils and are included in sou handbooks and soil-survey

reports.

4. A moderately high level of management is assumed—one that is practical and within the ability of a majority of the farmers and ranchers.

The level of management is that commonly used by the "reasonable"

men of the community. The capability classification is not, how^ever,

a grouping of soils according to the most profitable use to be made of

the land. For example, many soils in class III or IV, defined as suitable

for several uses including cultivation, may be more profitably used for

grasses or trees than for cultivated crops.

5. Capability classes I through IV are distinguished from each other by

a summation of the degree of limitations or risks of sou damage that

affect their management requirements for longtime sustained use for

cultivated crops. Nevertheless, differences in kinds of management or

yields of perennial vegetation may be greater between some pairs of

soils within one class than between some pairs of soils from different

classes. The capability class is not determined by the kind of practices

recommended. For example, class II, III, or IV may or may not require

the same kind of practices when used for cultivated crops, and classes I

through VII may or may not require the same kind of pasture, range,

or woodland practices.

* Based on longtime economic trends for average farms and farmers using moderately

high level management. May not apply to specific farms and farmers but wiU apply

to broad areas.
6. Presence of water on the surface or excess water in the soil; lack of water

for adequate crop production; presence of stones; presence of soluble

salts or exchangeable sodium, or both; or hazard of overflow are not

considered permanent limitations to use where the removal of these

limitations is feasible.^

7. Soils considered feasible for improvement by draining, by irrigating, by

removing stones, by removing salts or exchangeable sodium, or by protecting from overflow are classified according to their continuing limitations in use, or the risks of soil damage, or both, after the improvements

have been installed. DiiTerences in initial costs of the systems installed

on individual tracts of land do not influence the classification. The

fact that certain wet soils are in classes II, III, and IV does not imply

that they should be drained. But it does indicate the degree of their

continuing limitation in use or risk of soil damage, or both, if adequately

drained. Where it is considered not feasible to improve soils by drainage, irrigation, stone removal, removal of excess salts or exchangeable

sodium, or both, or to protect them from overflow, they are classified

according to present limitations in use.

8. Soils already drained or irrigated are grouped according to the continuing soil and climatic limitations and risks that aflfect their use under

the present systems or feasible improvements in them.

9. The capability classification of the soils in an area may be changed

when major reclamation projects are installed that permanently change

the limitations in use or reduce the hazards or risks of soil or crop

damage for long periods of time. Examples include establishing major

drainage facilities, building levees or flood-retarding structures, providing water for irrigation, removing stones, or large-scale grading of

gullied land. (Minor dams, terraces, or field conservation measures

subject to change in their effectiveness in a short time are not included.)

10. Capability groupings are subject to change as new information about

the behavior and responses of the soils becomes available.

11. Distance to market, kinds of roads, size and shape of the soil areas,

locations within fields, skill or resources of individual operators, and

other characteristics of land-ownership patterns are not criteria for

capability groupings.

12. Soils with such physical limitations that common field crops can be cultivated and harvested only by hand are not placed in classes I, II, III,

and IV. Some of these soils need drainage or stone removal, or both,

before some kinds of machinery can be used. This does not imply

that mechanical equipment cannot be used on some soils in capability

classes V, VI, and VII.

13. Soils suited to cultivation are also suited to other uses such as pasture,

range, forest, and wildlife. Some not suited to cultivation are suited

to pasture, range, forest, or wildlife; others are suited only to pasture or

" Feasible as used in this context means ( 1 ) that the characteristics and qualities

of the soil are such that it is possible to remove the limitation, and (2) that over

broad areas it is within the realm of present-day economic possibility to remove the

limitation.
range and wildlife; others only to forest and wildlife; and a few suited

only to wildlife^ recreation, and water-yielding uses. Groupings of

soils for pasture, range, wildlife, or woodland may include soils from

more than one capability class. Thus, to interpret soils for these uses,

a grouping different from the capability classification is often necessary.

14. Research data, recorded observations, and experience are used as the

bases for placing soils in capability units, subclasses, and classes. In

areas where data on response of soils to management are lacking, soils

are placed in capability groups by interpretation of soil characteristics

and qualities in accord with the general principles about use and management developed for similar soils elsewhere.

CAPABILITY CLASSES

Land Suited to Cultivation and Other Uses

Class I—Soils in class I have few limitations that restrict their iise.

Soils in this class are suited to a wide range of plants and may be used

safely for cultivated crops, pasture, range, woodland, and wildlife. The

soils are nearly level ^ and erosion hazard (wind or water) is low. They are

deep, generally well drained, and easily worked. They hold water well

and are either fairly well suppHed with plant nutrients or highly responsive

to inputs of fertilizer.

The soils in class I are not subject to damaging overflow. They are productive and suited to intensive cropping. The local climate must be favorable for growing many of the common field crops.

In irrigated areas, soils may be placed in class I if the limitation of the

arid climate has been removed by relatively permanent irrigation works.

Such irrigated soils (or soils potentially useful under irrigation) are nearly

level, have deep rooting zones, have favorable permeabihty and water-holding capacity, and are easily maintained in good tilth. Some of the soils may

require initial conditioning including leveUng to the desired grade, leaching

of a slight accumulation of soluble salts, or lowering of the seasonal water

table. Where limitations due to salts, water table, overflow, or erosion are

likely to recur, the soils are regarded as subject to permanent natural limitations and are not included in class I.

Soils that are wet and have slowly permeable subsoils are not placed in

class I. Some kinds of soil in class I may be drained as an improvement

measure for increased production and ease of operation.

Soils in class I that are used for crops need ordinary management practices to maintain productivity—^both soil fertihty and soil structure. Such

practices may include the use of one or more of the following: Fertilizers

and lime, cover and green-manure crops, conservation of crop residues and

animal manures, and sequences of adapted crops.

• Some rapidly permeable soils in class I may have gentle slopes.
Class II—SoUs in class II have some limitations that reduce the choice

of plants or require moderate conservation practices.

Soils in class II require careful soil management, including conservation

practices, to prevent deterioration or to improve air and water relations

when the soils are cultivated. The limitations are few and the practices

are easy to apply. The soils may be used for cultivated crops, pasture, range,

woodland, or wildlife food and cover.

Limitations of soils in class II may include singly or in combination the

effects of (1) gentle slopes, (2) moderate susceptibility to wind or water

erosion or moderate adverse effects of past erosion, (3) less than ideal soil

depth, (4) somewhat unfavorable soil structure and workability, (5) slight

to moderate salinity or sodium easily corrected but likely to recur, (6) occasional damaging overflow, ( 7 ) wetness correctable by drainage but existing

permanently as a moderate limitation, and (8) slight climatic limitations

on soil use and management.

The soils in this class provide the farm operator less latitude in the choice

of either crops or management practices than soils in class I. They may

also require special soil-conserving cropping systems, soil conservation practices, water-control devices, or tillage methods when used for cultivated

crops. For example, deep soils of this class with gentle slopes subject to

moderate erosion when cultivated may need one of the following practices

or some combination of two or more: Terracing, stripcropping, contour

tillage, crop rotations that include grasses and legumes, vegetated waterdisposal areas, cover or green-manure crops, stubble mulching, fertilizers,

manure, and lime. The exact combinations of practices vary from place

to place, depending on the characteristics of the soil, the local climate, and

the farming system.

Class in—Soils in class III have severe limitations that reduce the

choice of plants or require special conservation practices,

or both.

Soils in class III have more restrictions than those in class II and when

used for cultivated crops the conservation practices are usually more difficult

to apply and to maintain. They may be used for cultivated crops, pasture,

woodland, range, or wildlife food and cover.

Limitations of soils in class III restrict the amount of clean cultivation;

timing of planting, tillage, and harvesting; choice of crops; or some combination of these limitations. The limitations may result from the effects

of one or more of the following: (1) Moderately steep slopes; (2) high

susceptibility to water or wind erosion or severe adverse effects of past

erosion; (3) frequent overflow accompanied by some crop damage; (4)

very slow permeability of the subsoil; (5) wetness or some continuing

waterlogging after drainage; (6) shallow depths to bedrock, hardpan,

fragipan, or claypan that limit the rooting zone and the water storage;

(7) low moisture-holding capacity; (8) low fertility not easily corrected;

(9) moderate salinity or sodium; or (10) moderate climatic limitations.

When cultivated, many of the wet, slowly permeable but nearly level
soils in class III require drainage and a cropping system that maintains

or improves the structure and tilth of the soil. To prevent puddling and

to improve permeability it is commonly necessary to supply organic material

to such soils and to avoid working them when they are wet. In some irrigated areas, part of the soils in class III have limited use because of high

water table, slow permeability, and the hazard of salt or sodic accumulation.

Each distinctive kind of soil in class III has one or more alternative combinations of use and practices required for safe use, but the number of practical

alternatives for average farmers is less than that for soils in class II.

Class rV—Soils in class IV have very severe limitations that restrict

the choice oí plants, require very careful management/ or

both.

The restrictions in use for soils in class IV are greater than those in class

III and the choice of plants is more limited. When these soils are cultivated,

more careful management is required and conservation practices are more

difficult to apply and maintain. Soils in class IV may be used for crops,

pasture, woodland, range, or wildlife food and cover.

Soils in class IV may be well suited to only two or three of the common

crops or the harvest produced may be low in relation to inputs over a

long period of time. Use for cultivated crops is limited as a result of the

effects of one or more permanent features such as (1) steep slopes, (2)

severe susceptibility to water or wind erosion, (3) severe effects of past

erosion, (4) shallow soils, (5) low moisture-holding capacity, (6) frequent

overflows accompanied by severe crop damage, (7) excessive wetness with

continuing hazard of waterlogging after drainage, (8) severe salinity or

sodium, or (9) moderately adverse climate.

Many sloping soils in class IV in humid areas are suited to occasional

but not regular cultivation. Some of the poorly drained, nearly level soils

placed in class IV are not subject to erosion but are poorly suited to intertilled crops because of the time required for the soil to dry out in the spring

and because of low productivity for cultivated crops. Some soils in class IV

are well suited to one or more of the special crops, such as fruits and ornamental trees and shrubs, but this suitability itself is not sufficient to place

a soil in class IV.

In subhumid and semiarid areas, soils in class IV may produce good

yields of adapted cultivated crops during years of above average rainfall;

low yields during years of average rainfall; and failures during years of

below average rainfall. During the low rainfall years the soil must be protected even though there can be little or no expectancy of a marketable crop.

Special treatments and practices to prevent soil blowing, conserve moisture,

and maintain soil productivity are required. Sometimes crops must be

planted or emergency tillage used for the primary purpose of maintaining

the soil during years of low rainfall. These treatments must be applied

more frequendy or more intensively than on soils in class III.
Land Limited in Use—Generally Not Suited to

Cultivation ^

Class V—Soils in class V have little or no erosion hazard but have

other limitations impractical to remove that limit their use

largely to pasture, range, woodland, or wildlife food and

cover.

Soils in class V have limitations that restrict the kind of plants that can

be grown and that prevent normal tillage of cultivated crops. They are

nearly level but some are wet, are frequently overflowed by streams, are

stony, have climatic limitations, or have some combination of these limitations. Examples of class V are (1) soils of the bottom lands subject to

frequent overflow that prevents the normal production of cultivated crops,

(2) nearly level soils with a growing season that prevents the normal production of cultivated crops, (3) level or nearly level stony or rocky soils,

and (4) ponded areas where drainage for cultivated crops is not feasible but

where soils are suitable for grasses or trees. Because of these limitations

cultivation of the common crops is not feasible but pastures can be improved

and benefits from proper management can be expected.

Class VI—Soils in class VI have severe limitations that make them

generally unsuited to cultivation and limit their use largely

to pasture or range, woodland, or wildlife food and cover.

Physical conditions of soils placed in class VI are such that it is practical

to apply range or pasture improvements, if needed, such as seeding, liming,

fertilizing, and water control with contour furrows, drainage ditches,

diversions, or water spreaders. Soils in class VI have continuing limiutions that cannot be corrected, such as (1) steep slope, (2) severe erosion

hazard, (3) efïects of past erosion, (4) stoniness, (5) shallow rooting zone,

(6) excessive wetness or overflow, (7) low-moisture capacity, (8) salinity

or sodium, or (9) severe climate. Because of one or more of these limitations these soils are not generally suited to cultivated crops. But they may

be used for pasture, range, woodland, or wildlife cover or for some combination of these.

Some soils in class VI can be safely used for the common crops provided

unusually intensive management is used. Some of the soils in this class are

also adapted to special crops such as sodded orchards, blueberries, or the

like, requiring soil conditions unlike those demanded by the common crops.

Depending upon soil features and local climate the soils may be well or

poorly suited to woodlands.

' Certain soils grouped into classes V, VI, VII, and VIII may be made fit for use

for crops with major earthmoving or other costly reclamation.

9
Class VU—Soils in class Vu hove very severe limitations that moke

them unsuited to cultivation and that restrict their use

largely to grazing, woodland/ or wildlife.

Physical conditions of soils in class VII are such that it is impractical

to apply such pasture or range improvements as seeding, liming, fertilizing,

and water control with contour furrows, ditches, diversions, or water

spreaders. Soil restrictions are more severe than those in class VI because

of one or more continuing limitations that cannot be corrected, such as

(1) very steep slopes, (2) erosion, (3) shallow soil, (4) stones, (5) wet

soil, (6) salts or sodium, ( 7) unfavorable climate, or (8) other limitations

that make them unsuited to common cultivated crops. They can be used

safely for grazing or woodland or wildlife food and cover or for some combination of these under proper management.

Depending upon the soil characteristics and local climate, soils in this

class may be well or poorly suited to woodland. They are not suited to

any of the common cultivated crops; in unusual instances, some soils in

this class may be used for special crops under unusual management practices. Some areas of class VII may need seeding or planting to protect

the soil and to prevent damage to adjoining areas.

Class Vm—Soils and landiorms in class Vm hove limitations that

preclude their use for commercial plant production and

restrict their use to recreation, wildlife, or water supply

or to esthetic purposes.

Soils and landforms in class VIII cannot be expected to return significant

on-site benefits from management for crops, grasses, or trees, although

benefits from wildlife use, watershed protection, or recreation may be

possible.

Limitations that cannot be corrected may result from the effects of one

or more of the following: (1) Erosion or erosion hazard, (2) severe climate,

(3) wet soil, (4) stones, (5) low-moisture capacity, and (6) salinity or

sodium.

Badlands, rock outcrop, sandy beaches, river wash, mine tailings, and

other nearly barren lands are included in class VIII. It may be necessary

to give protection and management for plant growth to soils and landforms

in class VIII in order to protect other more valuable soils, to control water,

or for wildlife or esthetic reasons.

CAPABILITY SUBCLASSES

Subclasses are groups of capability units within classes that have the same

kinds of dominant limitations for agricultural use as a result of soil and

climate. Some soils are subject to erosion if they are not protected, while

others are naturally wet and must be drained if crops are to be grown.

Some soils are shallow or droughty or have other soil deficiencies. Still

10
other soils occur in areas where climate limits their use. The four kinds

of limitations recognized at the subclass level are: Risks of erosion, designated by the symbol (e) ; wetness, drainage, or overflow (w) ; rooting-zone

limitations (s) ; and climatic limitations (c). The subclass provides the

map user information about both the degree and kind of limitation. Capability class I has no subclasses.

Subclass (e) erosion is made up of soils where the susceptibility to erosion is the dominant problem or hazard in their use. Erosion susceptibility

and past erosion damage are the major soil factors for placing soils in this

subclass.

Subclass (w) excess water is made up of soils where excess water is

the dominant hazard or limitation in their use. Poor soil drainage, wetness, high water table, and overflow are the criteria for determining which

soils belong in this subclass.

Subclass (s) soil limitations within the rooting zone includes, as the

name implies, soils that have such limitations as shallowness of rooting

zones, stones, low moisture-holding capacity, low fertility difficult to correct,

and salinity or sodium.

Subclass (c) climatic limitation is made up of soils where the climate

(temperature or lack of moisture) is the only major hazard or limitation

in their use.^

Limitations imposed by erosion, excess water, shallow soils, stones, low

moisture-holding capacity, salinity, or sodium can be modified or partially

overcome and take precedence over climate in determining subclasses. The

dominant kind of limitation or hazard to the use of the land determines

the assignment of capability units to the (e), (w), and (s) subclasses.

Capability units that have no limitation other than climate are assigned to

the (c) subclass.

Where two kinds of limitations that can be modified or corrected are

essentially equal, the subclasses have the following priority: e, w, s. For

example, we need to group a few soils of humid areas that have both an

erosion hazard and an excess water hazard; with them the e takes precedence

over the w. In grouping soils having both an excess water limitation and

a rooting-zone limitation the w takes precedence over the s. In grouping

soils of subhumid and semiarid areas that have both an erosion hazard and

a climatic limitation the e takes precedence over the c, and in grouping

soils with both rooting-zone limitations and climatic limitations the s takes

precedence over the c.

Where soils have two kinds of limitations, both can be indicated if needed

for local use; the dominant one is shown first. Where two kinds of problems

are shown for a soil group, the dominant one is used for summarizing data

by subclasses.

* Especially among young soils such as alluvial soils, although not limited to them,

climatic phases of soil series must be established for proper grouping into capability

units and into other interpretive groupings. Since the effects result from interactions

between soil and climate, such climatic phases are not defined the same in terms of

precipitation, temperature, and so on, for contrasting kinds of soil.

11
CAPABILITY UNITS

The capability units provide more specific and detailed information than

the subclass for application to specific fields on a farm or ranch. A capability

unit is a grouping of soils that are nearly alike in suitability for plant

growth and responses to the same kinds of soil management. That is, a

reasonably uniform set of alternatives can be presented for the soil, water,

and plant management of the soils in a capability unit, not considering

efifects of past management that do not have a more or less permanent

effect on the soil. Where sous have been so changed by management that

permanent characteristics have been altered, they are placed in different

soil series. Soils grouped into capability units respond in a similar way and

require similar management although they may have soil characteristics

that put them in different soil series.

Soils grouped into a capability unit should be sufficiently uniform in the

combinations of soil characteristics that influence their qualities to have

similar potentialities and continuing limitations or hazards. Thus the soils

in a capability unit should be sufficiently uniform to (a) produce similar

kinds of cultivated crops and pasture plants with similar management

practices, (b) require similar conservation treatment and management under

the same kind and condition of vegetative cover, and (c) have comparable

potential productivity. (Estimated average yields under similar management systems should not vary more than about 25 percent among the kinds

of soil included within the unit. )

OTHER KINDS OF SOIL GROUPINGS

Other kinds of interpretive soil groupings are necessary to meet specific

needs. Among these are groupings for range use, woodland use, special

crops, and engineering interpretation.

The range site is a grouping of soils with a potential for producing the

same kinds and amounts of native forage. The range site for rangeland

is comparable to the capability unit for cultivated land. The purpose of

such a grouping is to show the potential for range use and to provide the

basis for which the criteria for determining range condition can be established. The soils grouped into a single range site may be expected to

produce similar longtime yields and respond similarly to alternative systems

of management and to such practices as seeding, pitting, and water spreading.

Soils suitable for range but not for common cultivated crops may be

placed in capability classes V and VI if they are capable of returning inputs

from such management practices as seeding, fertilizing, or irrigating and

in class VTI if they are not. If these soils do not give economic returns

under any kind of management when used for cultivated crops, pasture,

woodland or range, they fall in class VIII.

Soil-woodland site index correlations are essential for interpreting the potential wood production of the individual sou units that are mapped.

12
Woodland-site indices are commonly developed for individual kinds of

soils. Soil-mapping units can be placed in woodland groupings according

to site indices for adapted species and other responses and limitations significant to woodland conservation. Such groupings do not necessarily parallel those for capability units or range sites; however, in some areas capability

units may be grouped into range sites and woodland-suitability groups.

Rice has soil requirements unlike those of the conmion cultivated crops

requiring well-aerated soils. Some fruits and ornamentals do not require

clean cultivation. Therefore, these crops are not given weight in the

capabiHty grouping. Instead, special groupings of the soils for each of these

crops are made in the areas where they are significant.

With a good basic table of yields and practices the soils can be placed

in any number of suitability groups. Commonly, five groups—unsuited,

fairly suited, moderately suited, well suited and very well suited—are

sufficient.

Kinds of soil shown on the soil map are also grouped according to need

for applying engineering measures including drainage, irrigation, land

leveling, land grading; determining suitability as subgrade for roads; and

constructing ponds and small dams. Such groupings may be unlike those

made for other purposes.

CRITERIA FOR PLACING SOILS IN CAPABILITY

CLASSES

Soil and climatic limitations in relation to the use, management, and

productivity of soils are the bases for differentiating capability classes.

Classes are based on both degree and number of limitations affecting kind

of use, risks of soil damage if mismanaged, needs for soil management, and

risks of crop failure. To assist in making capability groupings, specific

criteria for placing soils in units, subclasses, and classes are presented here.

Because the effects of soil characteristics and qualities vary widely with

climate, these criteria must be for broad soil areas that have similar climate.

Capability groupings are based on specific information when available—

information about the responses of the individual kinds of soil to management and the combined effect of climate and soil on the crops grown.

It comes from research findings, field trials, and experiences of farmers and

other agricultural workers. Among the more common kinds of information

obtained are soil and water losses, kinds and amounts of plants that can

be grown, weather conditions as they affect plants, and the effect of different

kinds and levels of management on plant response. This information is

studied along with laboratory data on soil profiles. Careful analysis of

this information proves useful not only in determining the capability of these

individual kinds of soil but also in making predictions about the use and

management of related kinds of soil.

Basic yield estimates of the adapted crops under alternative, defined systems of management are assembled in a table. Where data are few, the

13
estimates should be reasonable when tested against available fann records

and studies of the combinations of soil properties.

Where information on response of soils to management is lacking, the

estimates of yields and the grouping of soils into capability units, subclasses,

and classes are based on an evaluation of combinations of the following:

1. Ability of the soil to give plant response to use and management as

evidenced by organic-matter content, ease of maintaining a supply of

plant nutrients, percentage base saturation, cation-exchange capacity,

kind of clay mineral, kind of parent material, available water-holding

capacity, response to added plant nutrients, or other soil characteristics

and qualities.

2. Texture and structure of the soil to the depth that influences the environment of roots and the movement of air and water.

3. Susceptibility to erosion as influenced by kind of soil (and slope) and

the effect of erosion on use and management.

4. Continuous or periodic waterlogging in the soil caused by slow permeability of the underlying material, a high water table, or flooding.

5. Depth of soil material to layers inhibiting root penetration.

6. Salts toxic to plant growth.

7. Physical obstacles such as rocks, deep gullies, etc.

8. Climate (temperature and effective moisture).

This list is not intended to be complete. Although the soils of any area

may differ from one another in only a few dozen characteristics, none can

be taken for granted. Extreme deficiencies or excesses of trace elements,

for example, can be vital. Commonly, the underlying geological strata

are significant to water infiltration, water yield, and erosion hazard.

Any unfavorable fixed or recurring soil or landscape features may limit

the safe and productive use of the soil. One unfavorable feature in the

soil may so limit its use that extensive treatment would be required. Several

minor unfavorable features collectively may become a major problem and

thus limit the use of the soil. The combined effect of these in relation to

the use, management, and productivity of soils is the criterion for different

capability units.

Some of the criteria used to differentiate between capability classes are

discussed on the following pages. The criteria and ranges in characteristics

suggested assume that the effects of other soil characteristics and qualities

are favorable and are not limiting factors in placing soils in capability

classes.

Arid and Semiorid, Stony, Wet, Saline-Sodic, and

Overflow Soils

The capability-class designations assigned to soils subject to flooding,

poorly or imperfectly drained soils, stony soUs, dry soils needing supplemental water, and sous having excess soluble salts or exchangeable sodium

are made on the basis of continuing limitations and hazards after removal

of excess water, stones, salts, and exchangeable sodium.

When assessing the capability class of any sou the feasibility of any necessary land improvements must be considered. Feasible as used here means

14
( 1 ) that the characteristics and qualities of the soil are such that it is possible to remove the limitation, and ( 2 ) that over broad areas it is within the

realm of economic possibility to remove the limitation. The capability

designation of these areas is determined by those practices that are practical now and in the immediate future.

The following kinds of soil are classified on the basis of their present

continuing limitations and hazards: (1) Dry soils (arid and semiarid areas)

now irrigatedj (2) soils from which stones have been removed, (3) wet soils

that have been drained, (4) soils from which excess quantities of soluble salts

or exchangeable sodium have been removed, and (5) soils that have been

protected from overflow.

The following kinds of soil are classified on the basis of their continuing

limitations and hazards as if the correctable limitations had been removed

or reduced: (1) Dry soils not now irrigated but for which irrigation is

feasible and water is available, (2) stony soils for which stone removal is

feasible, (3) wet soils not now drained but for which drainage is feasible,

(4) soils that contain excess quantities of soluble salts or exchangeable

sodium feasible to remove, and (5) soils subject to overflow but for which

protection from overflow is feasible. Where desirable or helpful, the present

limitation due to wetness, stoniness, etc., may be indicated.

The following kinds of soil are classified on the basis of their present

continuing limitations and hazards if the limitations cannot feasibly be

corrected or removed: (1) Dry soils, (2) stony soils, (3) soils with excess

quantities of saline and sodic salts, (4) wet soils, or (5) soils subject to

overflow.

Climatic Limitations

Climatic limitations (temperature and moisture) affect capability. Extremely low temperatures and short growing seasons are limitations, especially in the very northern part of continental United States and at high

altitudes.

Limited natural moisture supply affects capability in subhumid, semiarid,

and arid climates. As the classification in any locality is derived in part

from observed performance of crop plants, the eñ"ects of the interaction of

climate with soil characteristics must be considered. In a subhumid climate,

for example, certain sandy soils may be classified as class VI or class VII,

whereas soils with similar water-holding capacity in a more humid climate

are classified as class III or IV. The moisture factor must be directly considered in the classification in most semiarid and arid climates. The capability of comparable soils decreases as effective rainfall decreases.

In an arid climate the moisture from rain and snow is not enough to

support crops. Arid land can be classed as suited to cultivation (class I,

II, III, or IV) only if the moisture limitation is removed by irrigation.

Wherever the moisture limitation is removed in this way, the soil is classified

according to the effects of other permanent features and hazards that limit

its use and permanence, without losing sight of the practical requirements of

irrigation farming.

15
Wetness Limitations

Water on the soil or excess water in the soil presents a hazard to or limits

its use. Such water may be a result of poor soil drainage, high water table,

overflow ( includes stream overflow, ponding, and runoff water from higher

areas), and seepage. Usually soil needing drainage has some permanent

limitation that precludes placing it in class I even after drainage.

Wet soils are classified according to their continuing soil limitations and

hazards after drainage. In determining the capability of wet areas emphasis

is placed on practices considered practical now or in the foreseeable future.

The vast areas of marshland along the seacoast or high-cost reclamation

projects not now being planned or constructed are not classified as class I,

II, or III. If reclamation projects are investigated and found to be feasible,

the soils of the area are reclassified based on the continuing limitations and

hazards after drainage. This places the classification of wet soils on a

basis similar to that of the classification of irrigated, stony, saline, or overflow

soils. Some large areas of bottom land subject to overflow are reclassified

when protected by dikes or other major reclamation work. There are

examples of these along streams where levees have been constructed. Land

already drained is classified according to the continuing limitations and

hazards that affect its use.

Needs for initial conditioning, such as for clearing of trees or swamp vegetation, are not considered in the capability classification. They may be of

great importance, however, in making some of the land-management decisions. Costs of drainage, hkewise, are not considered directly in the

capability classification, although they are important to the land manager.

Toxic Salts

Presence of soluble salts or exchangeable sodium in amounts toxic to most

plants can be a serious limiting factor in land use. Where toxic salts are

the limiting factor, the following ranges are general guides until more

specific criteria are available :

Class II—Crops slightly affected. In irrigated areas, even after salt removal, slight salinity or small amounts of sodium remains or is likely

to recur.

Class III—Crops moderately affected. In irrigated areas, even after salt

removal, moderate salinity or moderate amounts of sodium remains or is

likely to recur.

Classes IV-VI—Crops seriously affected on cultivated land. Usually only

salt-tolerant plants will grow on noncultivated land. In irrigated areas,

even after leaching, severe salinity or large amounts of sodium remains or

is likely to recur.

Class yil—Satisfactory growth of useful vegetation impossible, except

possibly for some of the most salt-tolerant forms, such as some Atriplexes

that have limited use for grazing.

16
Slope and Hazard of Erosion

Soil damage from erosion is significant in the use, management, and response of soil for the following reasons:

1. An adequate soil depth must be maintained for moderate to high crop

production. Soil depth is critical on shallow soils over nonrenewable

substrata such as hard rock. These soils tolerate less damage from erosion

than soils of similar depth wiüi a renewable substrata such as the raw

loess or soft shale that can be improved through the use of special tillage,

fertilizer, and beneficial cropping practices.

2. Soil loss influences crop yields. The reduction in yield following the

loss of each inch of surface soil varies widely for different kinds of soil.

The reduction is least on soils having litde difference in texture, consistence, and fertility between the various horizons of the soil. It is greatest where there is a marked difference between surface layers and subsoils, such as among soils with claypans. For example, com yields on

soils with dense, very slowly permeable subsoils may be reduced 3 to 4

bushels per acre per year for each inch of surface soil lost. Yield reduction is normally small on deep, moderately permeable soils having similar

textured surface and subsurface layers and no great accumulation of

organic matter in the surface soil.

3. Nutrient loss through erosion on sloping soils is important not only because of its influence on crop yield but also because of cost of replacement to maintain crop yields. The loss of plant nutrients can be high,

even with slight erosion.

4. Loss of surface soil changes the physical condition of the plow layer in

soils having finer textured layers below the surface soil. Infiltration rate

is reduced; erosion and runoff rates are increased; tilth is diflBcult to

maintain; and tillage operations and seedbed preparation are more

diflicult.

5. Loss of surface soil by water erosion, soil blowing, or land leveling may

expose highly calcareous lower strata that are difficult to make into

suitable surface soil.

6. Water-control structures are damaged by sediments due to erosion.

Maintenance of open drains and ponds becomes a problem and their

capacity is reduced as sediment accumulates.

7. Gullies form as a result of soil loss. This kind of soil damage causes

reduced yields, increased sediment damage, and physical difficulties in

farming between the gullies.

The steepness of slope, length of slope, and shape of slope (convex or

concave) all influence directly the soil and water losses from a field.

Steepness of slope is recorded on soil maps. Length and shape of slopes

are not recorded on soil maps; however, they are often characteristic of

certain kinds of soil, and their effects on use and management can be evaluated as a part of the mapping unit.

Where available, research data on tons of soil loss per acre per year

under given levels of management are used on sloping soils to differentiate

between capability classes.

17
Soil Depth

Effective depth includes the total depth of the soil profile favorable for

root development. In some soils this includes the G horizon; in a few only

the A horizon is included. Where the effect of depth is the limiting factor,

the following ranges are commonly used: Class I, 36 inches or more; class

II, 20-36 inches; class III, 10-20 inches; and class IV, less than 10 inches.

These ranges in soil depth between classes vary from one section of the

country to another depending on the climate. In arid and semiarid areas,

irrigated soils in class I are 60 or more inches in depth. Where other unfavorable factors occur in combination with depth, the capability decreases.

Previous Erosion

On some kinds of soil previous erosion reduces crop yields and the choice

of crops materially; on others the effect is not great. The effect of past

erosion limits the use of soils ( 1 ) where subsoil characteristics are unfavorable, or (2) where soil material favorable for plant growth is shallow to

bedrock or material similar to bedrock. In some soils, therefore, the degree

of erosion influences the capability grouping.

Available Moisture-Holding Capacity

Water-holding capacity is an important quaHty of soil. Soils that have

limited moisture-holding capacity are likely to be droughty and have limitations in kinds and amounts of crops that can be grown; they also present

fertility and other management problems. The ranges in water-holding

capacity for the soils in the capability classes vary to a limited degree with

the amount and distribution of effective precipitation during the growing

season. Within a capability class, the range in available moisture-holding

capacity varies from one climatic region to another.

Glossary

Alluvial soils Soils developing from transported and relatively recently

deposited material (alluvium) with little or no modification of the original

materials by soil-forming processes. (Soils with well-developed profiles that

have formed from alluvium are grouped with other soils having the same

kind of profiles, not with the allu\aal soils. )

Available nutrient in soils The part of the supply of a plant nutrient in

the soil that can be taken up by plants at rates and in amounts significant

to plant growth.

Available water in soils The part of the water in the soil that can be

taken up by plants at rates significant to their growth; usable; obtainable.

Base saturation The relative degree to which soils have metallic cations

absorbed. The proportion of the cation-exchange capacity that is saturated

with metallic cations.

Cation-exchange capacity A measure of the total amount of exchangeable cations that can be held by the soil. It is expressed in terms of milli18
equivalents per 100 grams of soil at neutrality (pH 7) or at some other

stated pH value. ( Formerly called base-exchange capacity. )

Clay mineral Naturally occurring inorganic crystalline material in soils

or other earthy deposits of clay size—particles less than 0.002 mm. in

diameter.

Deep soil Generally, a soil deeper than 40 inches to rock or other strongly

contrasting material. Also, a soil with a deep black surface layer; a soil

deeper than about 40 inches to the parent material or to other unconsolidated

rock material not modified by sou-forming processes; or a soil in which the

total depth of unconsolidated material, whether true soil or not, is 40 inches

or more.

Drainage/ soil (1) The rapidity and extent of the removal of water

from the soil by runoff and flow through the soil to underground spaces.

(2) As a condition of the soil, soil drainage refers to the frequency and

duration of periods when the soil is free of saturation. For example, in

well-drained soils, the water is removed readily, but not rapidly; in poorly

drained soils, the root zone is waterlogged for long periods and the roots of

ordinary crop plants cannot get enough oxygen; and in excessively drained

soils, the water is removed so completely that most crop plants suffer from

lack of water.

Drought A period of dryness, especially a long one. Usually considered

to be any period of soil-moisture deficiency within the plant root zone. A

period of dryness of sufficient length to deplete soil moisture to the extent

that plant growth is seriously retarded.

Erosion The wearing away of the land surface by detachment and transport of soil and rock materials through the action of moving water, wind,

or other geological agents.

Fertility, soil The quality of a soil that enables it to provide compounds,

in adequate amounts and in proper balance, for the growth of specified

plants, when other growth factors such as light, moisture, temperature, and

the physical condition of the soil are favorable.

Field capacity The amount of moisture remaining in a soil after the

free water has been allowed to drain away into drier soil material beneath;

usually expressed as a percentage of the ovendry weight of soil or other

convenient unit. It is the highest amount of moisture that the soil will hold

under conditions of free drainage after excess water has drained away

following a rain or irrigation that has wet the whole soil. For permeable

soils of medium texture, this is about 2 or 3 days after a rain or thorough

irrigation. Although generally similar for one kind of soil, values vary with

previous treatments of the soil.

First bottom The normal flood plain of a stream, subject to frequent or

occasional flooding.

Parent material The unconsolidated mass of rock material (or peat)

from which the soil profile develops.

PermeabiHty/ soil The quality of a soil horizon that enables water or air

to move through it. It can be measured quantitatively in terms of rate of

flow of water through a unit cross section in unit time under specified

temperature and hydraulic conditions. Values for saturated soils usually

19
are called hydraulic conductivity. The permeability of a soil may be

limited by the presence of one nearly impermeable horizon even though

the others are permeable.

Phase, soil The subdivision of a soil type or other classificational soil

unit having variations in characteristics not significant to the classification

of the soil in its natural landscape but significant to the use and management

of the soil. Examples of the variations recognized by phases of soÜ types

include differences in slope, stoniness, and thickness because of accelerated

erosion.

Profile (soil) A vertical section of the soil through all its horizons and

extending into the parent material.

Range (or rangelond) Land that produces primarily native forage plants

suitable for grazing by livestock, including land that has some forest trees.

Runoff The surface flow of water from an area; or the total volume of

surface flow during a specified time.

Saline soil A soil containing enough soluble salts to impair its productivity

for plants but not containing an excess of exchangeable sodium.

Señes, soil A group of soils that have soil horizons similar in their differentiating characteristics and arrangement in the soil profile, except for

the texture of the surface soil, and are formed from a particular type of

parent material. Soil series is an important category in detailed soil classification. Individual series are given proper names from place names near

the first recorded occurrence. Thus names like Houston, Cecil, Barnes,

and Miami are names of soil series that appear on soil maps and each connotes a unique combination of many soil characteristics.

Sodic soil (alkali) Soil that contains sufficient sodium to interfere with

the growth of most crop plants; soils for which the exchangeable-sodiumpercentage is 15 or more.

Soil (1) The natural medium for the growth of land plants. (2) A dynamic natural body on the surface of the earth in which plants grow, composed of mineral and organic materials and living forms. ( 3 ) The collection

of natural bodies occupying parts of the earth's surface that support plants

and that have properties due to the integrated effect of climate and living

matter acting upon parent material, as conditioned by relief, over periods

of time.

A soil is an individual three-dimensional body on the surface of the earth

unlike the adjoining bodies. (The area of individual soils ranges from less

than ^ acre to more than 300 acres.)

A kind of soil is the collection of soils that are alike in specified combinations of characteristics. Kinds of soil are given names in the system of soil

classification. The terms "the soil" and "soil" are collective terms used for

all soils, equivalent to the word "vegetation" for all plants.

Soil Characteristic A feature of a soil that can be seen and/or measured

in the field or in the laboratory on soil samples. Examples include soil

slope and stoniness as well as the texture, structure, color, and chemical

composition of soil horizons.

Soil management The preparation, manipulation, and treatment of soils

for the production of plants, including crops, grasses, and trees.

20
Soil quality An attribute of a soil that cannot be seen or measured directly

from the soil alone but which is inferred from soil characteristics and soil

behavior under defined conditions. Fertility, productivity, and erodibility

are examples of soil qualities (in contrast to soil characteristics).

Soil survey ^ A general term for the systematic examination of soils in

the field and in the laboratories, their description and classification, the mapping of kinds of soil, and the interpretation of soils according to their

adaptability for various crops, grasses, and trees, their behavior under use

or treatment for plant production or for other purposes, and their productivity under different management systems.

Structure, soil The arrangement of primary soil particles into compound

particles or clusters that are separated from adjoining aggregates and have

properties unlike those of an equal mass of unaggregated primary soil particles. The principal forms of soil structure are platy, prismatic, columnar

(prisms with rounded tops), blocky (angular or subangular), and granular.

Structureless soils are (1) single grain—each grain by itself, as in dune

sand, or (2) massive—the particles adhering together without any regular

cleavage as in many claypans and hardpans, ("Good" or "bad" tilth are

terms for the general structural condition of cultivated soils according to

particular plants or sequences of plants. )

Subsoil The B horizons of soils with distinct profiles. In soils with weak

profile development, the subsoil can be defined as the soil below the plowed

soil (or its equivalent of surface soil), in which roots normally grow. Although a common term, it cannot be defined accurately. It has been carried

over from early days when "soil" was conceived only as the plowed soil

and that under it as the "subsoil."

Surface soil The soil ordinarily moved in tillage, or its equivalent in

uncultivated soil, about 5 to 8 inches in thickness.

Texture/ soil The relative proportions of the various size groups of individual soil grains in a mass of soil. Specifically, it refers to the proportions

of sand, silt, and clay.

Type, soil A subgroup or category under the soil series based on the

texture of the surface soil. A soil type is a group of soils having horizons

similar in differentiating characteristics and arrangement in the soil profile

and developed from a particular type of parent material. The name of a

soil type consists of the name of the soil series plus the textural class name

of the upper part of the soil equivalent to the surface soil. Thus Miami

silt loam is the name of a soil type within the Miami series.

Water table The upper limit of the part of the soil or underlying rock

material that is wholly saturated with water. In some places an upper,

or perched, water table may be separated from a lower one by a dry zone.

Water-holding capacity The capacity (or ability) of soil to hold water

against gravity (see Field capacity). The water-holding capacity of sandy

soils is usually considered to be low while that of clayey soils is high. It is

often expressed in inches of water per foot depth of soil.

Waterlogged A condition of soil in which both large and small pore

spaces are filled with water. (The soil may be intermittently waterlogged

because of a fluctuating water table or waterlogged for short periods after

rain. )

21

'*^ U.S. GOVERNHrNT PRINTING OFFICE: 1961