Chapter

8

By Soil Science Division Staff. Revised by Robert Dobos, Cathy 

Seybold, Joseph Chiaretti, Susan Southard, and Maxine Levin, 

USDA-NRCS.

Interpretations:

The Impact of Soil Properties 

on Land Use

Introduction

This chapter explains the concepts and principles used in the 

interpretation of soil property data to evaluate or predict 

suitabilities, limitations, or potentials of soils for a variety of uses. 

Soil survey information answers a wide range of soil-related questions, 

such as which crops will grow where and what are the best locations 

for infrastructure. Soil information can be used alone or as one layer 

of information in integrated systems that also consider other natural 

resources, demographics, climate, and ecological and environmental 

factors in decision making. 

In the United States, soil survey data and soil interpretive information 

from the official Soil Survey Geographic Database (SSURGO) are a major 

part of a growing number of geographic information systems (GIS) and 

models. These systems and models are used in regional planning, erosion 

prediction, estimating crop yields, timber and energy management, urban 

planning, public health considerations, and determining a soil’s ability 

to perform certain ecosystem services (such as carbon storage) that can 

affect global climates. Historically, soil survey interpretations primarily 

have been used to provide the public with soil interpretive predictions 

specific to a land use. Soil interpretation in the U.S. aims to quantify 

the soil function parameters expounded by the Food and Agriculture 

Organization of the United Nations. Ecosystem services performed 

by soil include provision of construction materials, filtering of water, 

providing habitat for organisms, sequestering carbon, flood mitigation, 

anchoring human infrastructure, supporting the growth of crops, and 

being a reservoir of genetic resources.
434 Chapter 

For the National Cooperative Soil Survey (NCSS) program, 

interpretive information is available in a public database and displayed in 

Web Soil Survey (WSS) (Soil Survey Staff, 2016). The baseline data and 

criteria are revised and refined continuously. The interpretive information 

is kept up to date by yearly refreshes. (The appendices provide examples 

of soil interpretations available through WSS, including thematic maps 

of soil properties and suitability ratings as well as tabular reports.) Soil 

interpretation reflects the capacity of the soil to support various uses and 

management practices. The level of data collection needed to execute the 

current interpretations program of the NCSS is outlined in relevant parts 

of the National Soil Survey Handbook (USDA-NRCS, 2016).

Generally, preparation of interpretations involves the following 

steps: (1) assembling information about soils and their landscapes, (2) 

deriving inferences, rules, and models for predicting the impact of soil 

properties on soil behavior under specific land uses, and (3) integrating 

these predictions into generalizations for each map unit component.

Soil interpretations provide numerical and descriptive information 

pertaining to a wide range of soil interpretive predictions. This 

information can be expressed as classes, indexes, or values with different 

units of measure. For example, particle-size data can be inferred from 

soil separates of sand, silt, and clay; USDA texture classes; or Unified 

soil classes. Generally, soil interpretations are made for specified uses 

and are reported in the form of limitations, suitabilities, or potentials. For 

limitations, soil properties that limit land use or establish the severity of 

limitation are typically indicated. For suitabilities, soil properties that 

determine a soil’s suitable characteristics may be given. In addition, soil 

interpretations, either as limitations or suitabilities, may be incorporated 

into potential ratings along with other resource data and interpretive 

information. The interpretive results can be presented in tables or in maps 

that depict the spatial extent at scales appropriate for a specific application.

The predicted practicality of alternative management options can 

be derived from soil interpretations. For any particular land use, soil 

responses to management alternatives can be predicted, the kinds of 

management needed can be identified, and the benefit-to-cost relationship 

for the management selected can be evaluated.

Considerations for Developing Soil Interpretations

An interpretation, such as limitations for septic tank absorption 

fields, provides information for a specific purpose and rarely is adaptable 

without modification to other purposes. Application of interpretations 

for a specific land area has an inherent constraint related to the scale 
Soil Survey Manual 4

of mapping and the composition variability within a map unit. This 

constraint is related to how soil surveys are made and the spatial 

relationship of the area of interest to the map unit delineations. These 

concerns are particularly significant for land areas for which large 

capital expenditures are contemplated (e.g., homesites). These areas 

are typically small relative to the size of map unit delineations and may 

occur on a dissimilar minor component that has interpretations that differ 

from those of the major components of the unit. These concerns are even 

greater for multi-taxa units. See chapter 4 for a complete discussion of 

map units, map unit components, and mapping scale.

Inherent soil property spatial variability defines the resolution of soil 

interpretations and the precision of soil behavior predictions for specific 

areas. Soil survey interpretations are rarely suitable for such onsite 

evaluations as homesites without further evaluations at the specific site. 

Soil interpretations do provide information on the likelihood that an area 

is suitable for a particular land use and so are valuable for screening areas 

for a planned use. This likelihood may be expressed as a suitability or a 

limitation. 

Specific soil behavior predictions are commonly presented as the 

degree of limitation imposed by one or more soil properties. Limitations 

posed by a particular soil property must be considered along with those 

of other soil properties to determine which property poses the most 

serious limitation. A high shrink-swell potential, for example, may be the 

only limiting soil property for building houses with basements for some 

soils. However, other soils that have a high shrink-swell potential may 

also have bedrock at shallow depths, and shallow depth to bedrock may 

represent a greater limitation than shrink-swell. Relatedly, some soils that 

have a low shrink-swell potential, which is favorable for homesites, may 

have limitations because of wetness, flooding, slope, etc. The degree of 

limitation imposed by a soil property on a land use may be thought of in 

terms of the added cost to perform the land use relative to a less limiting 

soil. If necessary, any limitation may be overcome, but the additional 

expense of installation, maintenance, and decreased performance may 

be prohibitive.

Other soil behavior predictions are presented in terms of how 

suitable a soil is for a particular land use. Historically, soils have been 

rated for their suitability as a material, such as topsoil or a source of sand. 

Soil productivity indices for crops and plants are also typically reported 

in terms of suitability. The underlying principle is that the soil will be 

used as it exists with no measures to overcome whatever makes the 

soil less suitable for a function. The major disadvantage of a suitability 

interpretation is that all of the soil and site properties that might impact 
436 Chapter 

the land use must be identified and evaluated. If a property that does not 

exist in the database is identified as being important, it must be derived 

or included in some manner in the rating process. Omission of a soil 

property that is not suitable will cause invalid positive ratings. 

Certain considerations that determine economic value of land are not 

part of soil interpretations but are an integral part of determining soil 

potentials for a given land use. For example, local groups consider the 

location of a land area in relation to roads, markets, and other services 

when developing soil potential ratings based on costs to maintain the soil 

resource versus benefits derived. 

Interpretations are sensitive to changes in technology and land uses. 

Crop yields generally have increased over time, and new practices may 

reduce limitations for nonagricultural uses. For example, the introduction 

of reinforced concrete slab-on-ground house construction has markedly 

reduced the limitation of shrink-swell for small building construction. 

Additionally, new uses of land or changes in technology will require new 

prediction models for soil interpretations. 

Soil properties can also be interpreted in terms of the favorability of a 

soil for the growth of certain fungi, bacteria, and other organisms that are 

either unwanted (such as a disease-causing organism) or economically 

desirable. While the land is not necessarily managed for a particular 

organism, prediction of the presence or absence of the organism can be 

useful. Also, soil properties can be used to assess the propensity of a soil 

to retain or transmit certain chemicals or energy (heat and cold). This 

propensity is not a limitation or a suitability, because it does not indicate 

a hazard or desirability, but rather a tendency. 

Finally, interpretations based on properties of the soil in place are 

only applicable if characteristics of the land area are similar to what they 

were when soil mapping was done. New interpretations may be required 

if the soil and site properties have been affected by physical movement, 

compaction, or bulking of soil material or changes in patterns of water 

states by irrigation, drainage, or alteration of runoff by construction. 

Interpretive Models

Interpretations are models that predict soil behavior based on soil 

physical and chemical attributes. The spectra of soil, site, and climatic 

properties that are available are addressed later in the chapter. The 

generalizations of soil behavior are based largely on a known or obtainable 

set of soil and site properties that are maintained in a database or predicted 

for each soil component. These soil properties or characteristics can be 
Soil Survey Manual 4

used to predict other attributes of soil, such as potential for frost heave 

or concrete corrosion. In addition, documented experiences with soils 

having certain sets of properties are used to generalize or predict soil 

behavior for many land uses. These generalizations are commonly 

formalized in interpretive models for computer-generated ratings.

Interpretive models may be based on knowledge of how soils 

perform under different uses or based on research data and/or inferences. 

These models may contain a narrow set of inferences for specific uses or 

applications (e.g., limitation of the soil for trench-type sanitary landfill), 

or they may have a highly integrated set of inferences about complex 

practices that are based on a large number of considerations, only some 

of which are interpretive soil properties (such as the land capability 

classification system; Klingebiel and Montgomery, 1961). Like other 

processes in a soil survey, the process of developing interpretations for a 

specific land use follows a scientific method. The soil scientist or group 

preparing the criteria reviews the literature, interviews experts, makes 

observations of soil performance under the specific use, develops a set of 

criteria using basic soil properties, tests the criteria, and finally adopts the 

system. The process rarely becomes static; as new technologies become 

available, the criteria must be reevaluated.

Developing a Soil Interpretation

One of the first tasks in developing an interpretation is to create a 

criteria table of the soil, site, and climatic attributes that are thought 

to impact the land use. Table 8-1 provides an example. It contains 

a comprehensive set of criteria for interpreting soils for septic tank 

absorption fields. Some of the included criteria may not be applicable 

in some places (e.g., areas of permafrost). Using this example, the soil 

scientist or group developing an interpretation first determines a list of 

soil properties that are known, or thought to be, important for septic tank 

absorption fields. Depth to water table, saturated hydraulic conductivity, 

depth to bedrock, depth to cemented pan, depth to permafrost, slope, 

flooding, ponding, fragments > 75 mm, and susceptibility to downslope 

movement or subsidence are considered important properties. After 

determining the list of soil properties, the soil scientist or group develops 

limits for each property and each class. This iterative phase is commonly 

the most difficult. The initial set of criteria is tested in different areas 

of the country under a wide variety of soil conditions. Results of the 

tests may require adjustments to the criteria and retesting. Once the 

limits are set, they may be arrayed in the table according to degree of 

severity or importance. Soil interpretations are models for predicting 
438 Chapter 

Table 8-1

Interpretive Soil Properties and Limitation Classes for Septic 

Tank Absorption Fields

Interpretive 

soil property

Limitation class

Limiting 

feature Not 

limited

Somewhat 

limited Very limited

Total subsidence 

(cm)

--- --- > 60 Subsidence

Flooding None Rare Very frequent, 

frequent, 

occasional

Flooding

Bedrock depth (m) > 1.8 1–1.8 < 1 Too shallow

Cemented pan 

depth (m)

> 1.8 1–1.8 < 1 Too shallow

Free water 

occurrence (m)

> 1.8 1–1.8 < 1 Depth to 

saturation

Saturated hydraulic 

conductivity 

(μm/s)—

Minimum 0.6 

to 1.5 m a/

10–40 4–10 < 4 Slow water 

movement

Maximum 0.6 

to 1 m a/

> 40 Poor filter

Slope (pct) < 8 8–15 > 15 Too steep

Fragments > 75 

mm b/

< 25 25–50 > 50 Large stones

Downslope 

movement

c/ Landslides

Permafrost d/ Permafrost

a/ 0.6 to 1.5 m pertains to the water transmission rate; 0.6 to 1 m pertains to filtration 

capacity. b/ Weighted average to 1 m.

c/ Rate “severe” if occurs.

d/ Rate “severe” if occurs above a variable critical depth (see discussion of the 

interpretive soil property).

how soils respond under a specific use. They use a set of rules or criteria 

that are based on the basic soil properties, modeled properties, or classes 

of properties. In some cases, it may be necessary to model a subset or 

intermediate interpretation to evaluate such properties as potential frost 
Soil Survey Manual 4

action, corrosivity, or potential for mass movement.

Interpretations are mostly developed in response to user needs; thus, 

the development process must include input from users and professionals 

in other disciplines. User feedback is crucial in the iterative process of 

refining a specific interpretation.

The “interpretive soil property” is the attribute to be provided to the 

model, generally by extraction from the database. However, the criteria in 

the table can be applied to individual soils without the use of a computer, 

depending on the circumstances. The “limitation classes” are determined 

by the team of experts in collaboration with the projected users of 

the interpretation. The magnitudes of the soil attributes at the critical 

thresholds of impact and the presence or absence of some condition are 

also established by the team of experts. The “limiting feature” is the 

reason that particular soil attribute limits the land use.

Table 8-2 illustrates how criteria are applied locally to a component 

of Aksarben soils. Tables 8-1 and 8-2 illustrate the process of developing 

an interpretation. Note that in table 8-2, only those soil properties that 

are applicable to the local area are required, so the number of properties 

evaluated is less than the number included in table 8-1. 

Table 8-2

Values of Applicable Interpretive Properties for Septic 

Systems for an Aksarben Component

Property

Limitation Class

Not Values

limited

Somewhat 

limited

Very 

limited

Flooding X None

Bedrock depth X > 1.8 m

Free water occurrence X > 1.8 m

Saturated hydraulic 

conductivity—

 Min. 0.6 to 1.5 m X 2 μm/s

 Max. 0.6 to 1 m X 6 μm/s

Slope X 8 percent

Fragments > 75 mm X 0 percent

In the example above, flooding, soil depth, depth to free water, and 

rock fragment content are not limiting. The slope, at 8 percent, presents 

some limitation. The maximum saturated hydraulic conductivity in 
440 Chapter 

the depth range of 0.6 to 1.0 m (i.e., 6 micrometers per second) is not 

limiting. However, the minimum saturated hydraulic conductivity in the 

depth range of 0.6 to 1.5 m (i.e., 2 micrometers per second) is a severe 

limitation as it causes slow water movement.

Testing and Reevaluation

The interpretive model is under continuous scrutiny through user 

feedback, ranging from local homeowners’ associations and units of 

government to national environmental agencies and organizations. Soil 

scientists continue testing of interpretations through observations and 

discussions with local user groups during the soil survey process.

Current U.S. Interpretive System

This section describes how soil interpretations are developed and 

managed in the National Cooperative Soil Survey (NCSS). The commonly 

used system of placing the soil into an interpretive limitation or suitability 

class is discussed briefly, then a newer and more sophisticated system is 

explained. The newer system uses fuzzy system concepts to more fully 

express the degree of membership of a soil in a particular interpretive class.

Overview of the Interpretations System

Historically, soil interpretation results were expressed as limitation 

or suitability classes. Limitation style interpretations typically placed 

soils into three interpretive classes, such as “slight,” “moderate,” or 

“severe,” and reported which soil properties or features were restrictive 

to the land use. An example would be a “severe” rating for dwellings with 

basements for soils with a high shrink-swell potential. Suitability style

interpretations placed soils into “good,” “fair,” or “poor” interpretive 

classes and reported the soil properties or features that make the soil 

least suitable for the use or management practice. An example would 

be a “good” rating for potential sand source. Actually, the class names 

for interpretive results may take any form that suits the needs of the 

user. Some users prefer a positive statement with a listing of limiting 

properties. Many U.S. soil surveys were made with interpretations 

expressed this way.

Fuzzy System Concepts

The current methodology for developing and processing interpretive 

information allows not only class names to be reported but also numeric 
Soil Survey Manual 4

ratings that indicate the degree of limitation or suitability of a soil for 

a land use or management practice. These index numbers are based on 

fuzzy system concepts (Cox and O’Hagan, 1998) that describe a soil’s 

membership in the set of soils that are either limiting or suitable for 

the specified use. Using this technology, soil map units and map unit 

components can be described as full members, partial members, or nonmembers of a defined interpretive group. This membership is presented 

as a numeric index ranging from 0 to 1, where the higher the index 

number the more fully a soil is a member of the set and thus the greater 

the degree of limitation or suitability for a specific use. 

A team of subject matter experts evaluates the impact of each soil 

property on the specific land use and sets the interpretive thresholds. For 

a limitation style interpretation, an attribute such as slope gradient may 

have a level that is not limiting and the associated index is 0, meaning 

it is absolutely false that this soil is a member of the set of soils limited 

by slope gradient. As slope increases, a level is reached where the soil 

cannot be successfully used for a particular land use and the associated 

index is 1, meaning it is absolutely true that this soil is in the set of soils 

limited by slope gradient. This relationship is depicted by a curve called 

an evaluation or a membership function (see figure 8-1).

Figure 8-1

Low Value High Value

5 12

Membership Graph Membership Value

SLOPE PERCENT

5.000 6.000 7.000 8.000 9.000 10.000 11.000 12.000 13.000

0.000

0.200

0.400

0.600

0.800

1.000

Data

Membership function for slope percent for a limitation style interpretation where a 

membership value of 1 denotes limiting and lower values denote less limiting (i.e., 

more gentle slopes).

In the example given in figure 8-1, when a soil has a slope of 12 

percent or greater, it is absolutely true that this soil is limited for the 
442 Chapter 

land use. When the slope is 5 percent or less, it is absolutely false that 

this soil is limited. Slopes between 5 and 12 are given numerical ratings 

that indicate the degree of partial membership in the set of soils that are 

limited due to slope. The character of the curve is also determined by the 

team of experts.

The overall automated system has three parts: (1) an attribute that is 

extracted from the database, (2) an evaluation of the membership value 

of the attribute, and (3) a reason or descriptive term assigned to the 

membership value (referred to as a “rule”). A set of these is associated 

with each soil, site, or climatic attribute. A particular depth range can 

be specified for horizon data, and items such as seasonal wetness, 

flooding, and ponding can be parsed by month. If needed, existing data 

can be used to model a piece of data that is not captured in soil survey. 

The piece of soil, site, or climate data extracted from the database 

undergoes an evaluation in which the estimated data is rated against a 

curve like that in figure 8-1. These curves have three basic forms: more 

is better, less is better, or a mid-range concentration is better for an 

intended use (fig. 8-2). Carbon sequestration or maximizing crop yields 

are examples of intended uses.

From the evaluation, the rating for a particular property is sent 

to the corresponding child rule where a rating reason is attached to 

the membership value. Rating reasons are phrases that describe the 

nature of the limiting factor, such as “too steep,” “floods,” “too wet,” 

or “too expansive.” Since normally more than one rating makes up an 

interpretation, the rules are referred to as “child rules” in the U.S. system. 

The membership values produced by the set of child rules that make up 

an interpretive model (parent rule) are combined using fuzzy math to 

produce an overall membership value from 0 to 1 (index number). The 

final membership value and its associated verbal limitation or suitability 

rating are assigned in the parent rule. 

Figure 8-3 is a diagram of a simplified parent rule for dwellings with 

basements. The “or” operator dictates that according to the rules of fuzzy 

math for a limitation style interpretation, the highest membership value 

from the set of child rules will be returned as the overall rating (index 

number) for a particular component. The rectangles represent the child 

rules for the restrictive features. The “and” operator, which returns the 

lowest of the child rule membership values, is typically used for suitability 

style interpretations where the least suitable attribute defines how well a 

soil may function for a land use. Other operators include “mean,” “sum,” 

and “product.” The operator used in an interpretive model depends on 

what makes most sense for the system being modeled.
Soil Survey Manual 4

Figure 8-2

Membership Value

20.000 40.000

0.000

0.200

0.400

0.600

0.800

1.000

ORGANIC CARBON IN KG/M2 TO 30CM

Membership Graph

60.000 80.000 100.000

Membership Value

8.000 10.000

0.000

0.200

0.400

0.600

0.800

1.000

WTD_AVG EC 0-30cm

Membership Graph

12.000 14.000 16.000

Membership Value

4.000 5.000

0.000

0.200

0.400

0.600

0.800

1.000

WTD_AVG PH 0-30cm

Membership Graph

6.000 7.000 8.000 9.000

Graphs representing the three basic suitability styles. Top.—More is better. In this 

case, more organic carbon content (kg/square meter) in the upper 30 cm of the soil is 

better. Middle.—Less is better. In this case, less electrical conductivity (ds/m) in the 

upper 30 cm of the soil is better. Bottom.—Mid-range is better. In this case, a midrange average pH in the upper 30 cm of the soil is better.

Limitation Ratings

Soils may be rated according to limitations for soil uses. Limitation 

ratings typically are based on hazards, risks, or obstructions presented by 

properties or characteristics of undisturbed soil. The rating consists of a 
444 Chapter 

Figure 8-3

Dwellings with Basements

Depth to water table:

100 to 200 cm

Depth to bedrock:

100 to 200 cm

Flooding frequency:

None to frequent

Slope:

8 to 15 percent

SELECT slope SELECT flooding

frequency

SELECT depth

to bedrock

SELECT depth

to water table

Slope (%) Flooding frequency Depth to bedrock (cm) Depth to water table (cm)

OR

0 15 none 0 0

0 0 0 0

1 1 1 1

frequent 100 200 100 200

Diagram of a hypothetical parent rule for Dwellings with Basements (a limitation 

style interpretation).

combination of descriptive terms and membership values that define a 

soil’s membership in the set of soils that have limiting features.

Not limited.—Soils in this interpretive class are not members of the 

set of soils that have limitations. They are assigned an index number of 0. 

These soils give satisfactory performance with little or no modification. 

Modifications or operations dictated by the use are simple and relatively 

inexpensive. With normal maintenance, performance should be satisfactory for a period of time generally considered acceptable for the use. 

Somewhat limited.—Soils in this interpretive class are partial 

members of the set of soils that have limitations. The membership value 

is more than 0 but less than 1.0. In this case, the greater the membership 

value the greater the soil’s membership in the set of soils that have 

limiting features or characteristics. For example, two soils (A and B) 

have partial membership in the set of soils that are limited and have slope 

as a restrictive feature. Soil A has a membership index of 0.13 while 

soil B has a membership index of 0.87. Although both soils have slope 

as a restrictive feature, soil A is less restricted than soil B. Soils that 

are partial members of the set of soils that are limited for a specific use 

do not involve exceptional risk or cost for the specified use. However, 

they do have certain undesirable properties or features. Modification of 

the soil itself, special design, or maintenance is required for satisfactory 
Soil Survey Manual 4

performance over an acceptable period of time. The needed measures 

typically increase the cost of establishing or maintaining the use, but the 

added cost is generally not prohibitive. 

Very limited.—Soils in this interpretive class are members of the 

set of soils that are limited for the specified use or management practice. 

They have an index number of 1.0. These soils, if not appreciably 

modified, have a high risk for the use. Special design, a significant 

increase in construction cost, or an appreciably higher maintenance cost 

is required for satisfactory performance over an acceptable period of time. 

A limitation that requires removal and replacement of the soil would be 

rated “very limited.” The rating does not imply that the soil cannot be 

adapted to a particular use, but rather that the cost of overcoming the 

limitation would be high. 

Not rated.—Not rated is a special interpretive class used only when 

data essential for producing a rating is missing.

Suitability Ratings

Soils may be rated according to the degree of suitability for specific 

uses. Suitability ratings are based on soil characteristics that influence 

the ease of using or adapting a soil for a specific use. Suitability ratings 

also use a combination of descriptive terms (classes) and indexed 

scoring functions to define a soil’s membership in the set of soils that 

have features or properties that support the intended use or management 

of the soil. Suitability ratings differ from limitation ratings in that the 

interpretive model design reports soil features that support the intended 

application rather than restrictive soil features.

Good.—Soils in this interpretive class are members of the set of soils 

that have characteristics that sustain the intended use or management 

practice. They are assigned an index number of 1.0. Satisfactory 

performance and low maintenance cost can be expected. 

Fair.—Soils in this interpretive class are partial members of the set 

of soils that have characteristics that sustain the intended use. The index 

number is more than 0 and less than 1.0. In this case, the greater the index 

value the greater the soil’s membership in the set of soils suitable for the 

use or management practice and the better the soil characteristics. For 

example, two soils (A and B) have partial membership in the set of soils 

that are suitable as a source of sand. Soil A has a membership index of 

0.27 while soil B has a membership index of 0.78. Although both soils 

are partial members of the set of soils that are a “fair” source of sand, 

soil B is better suited. Soils that are partial members of the set of soils 

that are suitable for a specific use require additional cost because they 
446 Chapter 

have certain undesirable properties or features. That cost is generally 

proportional to the membership index. 

Poor.—Soils in this interpretive class are not members of the set of 

soils that are suitable for the specified use or management practice. They 

have an index number of 0. These soils have one or more properties 

that are unfavorable for the specified use. For example, a soil that does 

not contain sand is rated as a poor source of sand. Unlike other soil 

limitations, there are no means or treatment for correcting the lack of 

sand in a soil. In this respect, unfavorable suitabilities generally do not 

have remedial solutions. Suitability ratings may also be supplemented 

with the restrictive features that affect soil performance for a specific 

use. These restrictive features may be a list of soil properties that are 

important for a specific use and be listed with each class for which 

they apply. Examples are “fair—water table at depths of 25 to 50 cm” 

and “poor—bedrock at depths of less than 50 cm.” Listing suitabilities 

with restrictive features in this manner gives the user more complete 

information by identifying other properties or features that may need 

treatment for the given use.

Most interpretations designed for general widespread use (such as 

those used within a large geographic region or a nation) have narrowly 

defined objectives that can be stated as either limitations or suitabilities. 

Some users may prefer interpretive expressions that use both approaches, 

such as a statement of the suitability and also a listing of limiting 

properties according to severity or difficulty to overcome.

Computer-generated interpretations are commonly made separately 

for each component in a map unit for any size of area. An aggregated 

summary rating for each map unit may also be given. Current technologies 

permit users to map interpretive output for the most limiting component, 

least limiting component, dominant condition, weighted average, or a 

specific limiting soil property. Current geographic information systems 

(GIS) also permit interpretive results to be displayed over broad 

geographic areas and in a variety of ways, including thematic maps, 

charts, and standard tables.

Map Units and Soil Interpretations

This section discusses the relationships between the terminology and 

conventions employed to define and describe map units (see chapter 4) 

and soil interpretations. The components of map units are the entities 

for which interpretations are provided. The application of interpretive 

information to areas of land is through map unit descriptions and 
Soil Survey Manual 4

depends on an understanding of the map unit concept as it applies to 

interpretations.

Consociations, Associations, and Complexes

For map units that are consociations, the interpretations generally 

pertain to a single, named soil and are applicable throughout the delineation, 

although minor components may be rated if the associated data is deemed 

reliable. For associations and complexes, the interpretations may be 

given for each named component as well as the unnamed components or 

may be given for the map unit as a whole, depending on the objective. In 

the description of the map unit, information is commonly provided about 

the geographic occurrence on the landscape of the named components. 

From this information, interpretations for each of the named components 

of the map unit may be applied to the portion of the landscape on which 

it occurs. However, such an application requires information beyond 

what the soil map alone can provide. The location of each soil within 

the map unit delineation is needed. The map unit description provides 

information on the location and extent of each named component of the 

map unit.

Map units differ in specificity of the named soils and therefore 

in the broadness of the ranges for various interpretive soil properties. 

Phases of soil components that are based on series are more specific soil 

concepts than are phases of soil components that are based on a higher 

categorical level, such as a great group, e.g., Haplaquods. Consequently, 

the interpretive information for a phase of a soil component based on 

soil series has narrower ranges than one based on a higher taxonomic 

category.

Similar Soils

Similar soils differ so little from the named soil in the map unit that 

there are no important differences in interpretations. These soils are not 

named components in the map unit. Recognition is limited to a brief 

description of the feature or features by which the soil in question differs 

from the soils in the map unit name. For example: “In places, the upper 

part of the material is silty clay. In a few areas, the underlying material 

contains a few lime concentrations.”

Dissimilar Soils

Map units are permitted to have certain proportions of included soils 

that differ sufficiently from the named soil to affect major interpretations. 
448 Chapter 

These soils are referred to as dissimilar soils (see chapter 4). Typically, 

the dissimilarities are such that the soils behave differently. Dissimilar 

soils are named in the map unit description if they are part of the name 

of another map unit in the soil survey area. Otherwise, the dissimilar 

soil is briefly described in a generic fashion, for example, “medium 

textured soil with bedrock at a depth of less than 50 cm.” Location 

of the dissimilar soils relative to landscape position may be given. 

Inferences as to the influence of the dissimilar soils on behavior of the 

map unit may be obtained from their interpretive properties and their 

location on the landscape. The map unit descriptions may state how the 

dissimilar soils affect soil behavior. Tabular soil properties and related 

interpretations do not include properties and interpretations of dissimilar 

soils. Yield estimates are, in principle, influenced by the occurrence of 

dissimilar soils if based on field-scale measurement. However, if yields 

were significantly affected, the dissimilar soil would likely be a named 

component of the map unit.

For consociations, the interpretations pertain to a single, named soil 

and soils similar to the named soil. Thus, they have a higher possibility 

of being applicable throughout the delineation than map units named for 

more than one taxon. For associations and complexes, the possibility of 

different kinds of interpretations is higher than for consociations, unless 

the soils are similar. The interpretations may need to be presented on a 

probability or possibility basis. Where the soils are related to specific 

landforms or parts of landforms, interpretations can be related to soils 

and landforms.

Aggregation

In the context of the modern soil survey database, very few map 

units are composed entirely of one component; some minor components 

almost always occur and are interpreted. This presents a challenge for 

displaying interpretive output in a geographic information system, since 

only one value can be tied to a polygon. Some method of aggregating 

the data across components is needed. Depending on the context 

of the interpretation and what makes sense to display, one of several 

methods can be used on either the rating classes or the membership 

values. Historically, for example, the rating class (e.g., slight, moderate, 

or severe) of the dominant component (component having the highest 

component percentage) was displayed in either green, yellow, or red for 

the map unit delineation. For multi-taxa map units, this may represent 

as little as 40 percent of the map unit area. In the case of multi-taxa map 

units, a dominant condition aggregation can be used to describe more of 
Soil Survey Manual 4

the map unit. In this method, the rating class associated with the highest 

sum of the component percentages is displayed. In some cases, it makes 

sense to display either the least limiting or most limiting condition for a 

map unit. It is also possible to reclassify the membership values to make 

more classes for mapping to represent a gradation of the moderately 

limited class. If a large proportion of the area of the map unit will be 

used in the context of the land use, such as in agricultural applications 

like productivity indices, a weighted average of the membership values 

by component percentage may be most appropriate. (For additional 

information, see appendix 4, table A-4.) 

Interpretive Soil Properties

Soil survey interpretations are provided for specific soil uses. 

Interpretations for each soil use are based on a set of interpretive soil 

properties. These properties include site generalities (e.g., slope gradient), 

measurements on individual horizons (e.g., particle-size distribution), 

and temporal repetitive characteristics that pertain to the soil as a whole 

(e.g., depth to free water). 

Abbreviated descriptions for many commonly used interpretive 

soil properties used in the NCSS are explained below. For logical 

presentation, they are grouped into categories: site, component, and 

horizon data; physical features or processes; erosion; and corrosivity. 

Formal classes have been assigned to several interpretive soil properties. 

These classes generally are not given unless they are used in field 

morphological descriptions. All classes are described in the National 

Soil Survey Handbook (USDA-NRCS). Local conditions may dictate 

other interpretive soil properties or a greater emphasis on a subdivision 

of some of the interpretive properties here listed. 

Site Data

Climate

Mean annual air temperature.—The mean air temperature for the 

calendar year. 

Frost-free period.—The average length of the longest time period per 

calendar year that is free of killing frost. 

Mean annual precipitation.—The mean annual moisture received per 

calendar year, including rainfall and solid forms of water. 
450 Chapter 

Landscape

Slope.—The range in slope gradient, in percent.

Slope aspect.—The direction in which the slope faces, in degrees.

Slope shape.—Whether the land surface is convex, concave, or linear 

in the up-down or across planes.

Elevation.—The height above sea level. 

Geomorphic component.—The part of the landform the soil occupies 

(e.g., interfluve, head slope, nose slope, side slope).

Hillslope position.—The position the soil occupies on the landscape 

(e.g., summit, shoulder, backslope, footslope, toeslope).

Component Data

Field Water Characterization

Available water capacity (AWC).—The volume of water that a soil 

layer retains between the tensions of 10 kPa (sandy soils) or 33 kPa and 1500 

kPa. The water is considered to be available to most common agronomic 

plants. The amount of water is reduced depending on the volume of rock 

fragments and the osmotic effects of high salt concentration. Volumes 

are expressed both as a volume fraction and as a thickness of water. The 

standard of reference is the water retention difference (under 4C in Soil 

Survey Staff, 2014a). Reductions are made in water retention difference 

for incomplete root ramification that is associated with certain taxonomic 

horizons and diagnostic and/or restrictive features (such as fragipans) 

and for chemical properties that are indicative of root restriction (such 

as high content of salts, low levels of available calcium, or high levels 

of extractable aluminum). The amount of available water to the expected 

maximum depth of root penetration (commonly either 1 or 1.5 m) or to 

a physical or chemical root limitation, whichever is shallower, has been 

formulated into a set of classes for root-zone available water storage. For 

the class sets, the depth of rooting that is assumed and the class limits that 

are stipulated differ among the taxonomic moisture regimes.

Hydrologic soil groups (HSG).—Interpretive classes that have 

similar runoff potentials under conditions of maximum yearly wetness. 

It is assumed that the ground surface is bare and that ice does not impede 

infiltration and transmission of water downward. In some cases, HSG is 

used as a soil property.

Flooding.—Inundation by flowing water. The frequency and duration 

of flooding are placed in classes.

Ponding.—Inundation by stagnant water. The duration and month(s) 

of the year that ponding occurs are recorded.
Soil Survey Manual 4

Moisture status.—The thickness of the zone with a particular water 

state, the kind of water state, and the months of year that the water state 

is present within the soil. Three general water state classes are used in 

the soil survey database—dry, moist, and wet. Chapter 3 presents more 

refined classes. In the soil survey database, the wet class is wet-satiated 

and the moist class includes wet-nonsatiated. Both wet-satiated and 

wet-nonsatiated are subclasses of wet in chapter 3. There is also a set of 

classes (see chapter 3) for the occurrence of internal free water. These 

classes include depth to, kind, and months of the year that a zone of free 

water is present within the soil. Free water is defined as satiated through 

saturation.

Horizon Data

Particle Size and Fragments > 2 mm

USDA texture classes and modifiers.—Texture is the relative 

proportion, by weight, of sand-, silt-, and clay-sized particles (texture 

classes). The texture classes are modified by adjectival classes based on 

proportion, size, and shape of rock fragments and by the proportion of 

organic matter, if the content is high. 

Particle-size separates (based on < 2 mm fraction).—The particlesize separates recorded in the soil survey database are percent total sand 

(2.0–0.05 mm), very coarse sand (2.0–1.0 mm), coarse sand (1.0–0.5 

mm), medium sand (0.5–0.25 mm), fine sand (0.25–0.10 mm), very fine 

sand (0.10–0.05 mm), total silt (0.05–0.002 mm), coarse silt (0.05–0.02 

mm), fine silt (0.02–0.002 mm), total clay (< 0.002 mm), and carbonate 

clay. Percentages are expressed as a weight percent and are based on 

the < 2 mm fraction. For soils that disperse with difficulty, the total clay 

percentage is commonly evaluated based on the ratio of 1500 kPa water 

retention to clay.

Soil fragments > 250 mm (based on whole soil).—This quantity is 

expressed as a weight percent of the horizon occupied by fragments up 

to an unspecified upper limit (size of rock fragments does not exceed 

the size of the pedon). Fragments include pieces of bedrock, bedrocklike material, durinodes, concretions, nodules, and woody materials 

(organic soils). Fragments larger than 250 mm are not included in the 

determination of Unified or AASHTO class placements, but they may 

significantly influence suitability for certain soil uses.

Soil fragments 75–250 mm (based on whole soil).—This quantity 

is expressed as a weight percent of the horizon occupied by fragments 

75–250 mm in size. Fragments include pieces of bedrock, bedrock-like 

material, durinodes, concretions, nodules, and woody materials (organic 
452 Chapter 

soils). The upper fragment size limit cannot exceed the size of the pedon. 

Fragments greater than 75 mm do not affect the Unified and AASHTO 

class placements, but they may have a large influence on suitability for 

certain uses.

Soil fragments > 2 mm (based on whole soil).—This quantity is 

expressed as a volume percent (whole soil base) of the horizon occupied 

by the > 2 mm fragments. Associated data include the kind, size, shape, 

roundness, and hardness of the fragments. Fragments include pieces of 

bedrock, bedrock-like material, durinodes, concretions, nodules, and 

woody materials (organic soils).

Percent passing sieve numbers 4, 10, 40, and 200 (based on < 75 mm 

fraction).—The weight percentage of material passing each sieve. Sieve 

openings are 4.8 mm (no. 4), 2.0 mm (no. 10), 0.43 mm (no. 40), and 

0.075 mm (no. 200) in diameter. Quantities are expressed as a percentage 

of the < 75 mm material. Material passing the number 4 and 10 sieves 

may be estimated in the field (see chapter 3) or measured in the office 

or laboratory. Material passing the number 40 and 200 sieves may be 

measured directly in the laboratory. Percent passing sieves also may be 

estimated from USDA particle-size and rock fragment measurements 

made in the field or laboratory.

Soil Fabric-Related Analyses

Moist bulk density.—The oven-dry weight in megagrams divided by 

the volume of soil in cubic meters at or near field capacity, exclusive of 

the weight and volume of fragments > 2 mm. 

Linear extensibility percent (LEP).—The linear reversible volume 

difference of a natural clod between field capacity and oven dryness, 

inclusive of rock fragments. The volume change is expressed as a 

percent change for the whole soil. Actual LEP (shrink-swell), in 

contrast, is dependent on the minimum water content that occurs under 

field conditions. Organic soils typically do not have reversible volume 

changes when oven dried. Shrink-swell classes are defined based on 

LEP.

Water retention (10, 33, and 1500 kPa).—The water content that 

is retained at 10, 33, and 1500 kPa tension, expressed as a percentage 

of the oven-dry soil weight inclusive of rock fragments (whole soil). 

Measurements are conducted in the laboratory on clods (for 10 and 33 

kPa tension) and sieved samples (for 1500 kPa tension). Pedotransfer 

functions are also used to estimate the water content at 10, 33, and 1500 

kPa tensions.

Available water capacity.—This is defined in the section “Field Water 

Characterization” above as the volume of water that should be available 
Soil Survey Manual 4

to plants if the soil, inclusive of rock fragments, were at field capacity. 

Field capacity is the volume of water that remains in the soil 2 or 3 days 

after being wetted and after free drainage becomes negligible. Contents of 

water are expressed both as a volume fraction and as a thickness of water. 

Available water is estimated as the amount of water held between 10 or 

33 kPa and 1500 kPa tension. Reductions in water retention difference 

should be made for root-restricting layers that are associated with certain 

taxonomic horizons and features (such as fragipans) and for chemical 

properties that are indicative of root restriction (such as low levels of 

available calcium and high levels of extractable aluminum). Adjustments 

may also be made for the osmotic effect of high salt concentrations, if 

present.

Saturated hydraulic conductivity (Ksat).—The amount of water that 

would move downward through a unit area of saturated in-place soil in 

unit time under unit hydraulic gradient. It is used to convey the rate of 

water movement downward through the soil under saturated conditions 

(and unit hydraulic gradient). Saturated hydraulic conductivity classes 

are defined in chapter 3.

Engineering Classification

Liquid limit (LL) .—The water content at the change between liquid 

and plastic states. It is measured on thoroughly puddled soil material that 

has passed a number 40 sieve (0.43 mm) and is expressed on a dry weight 

basis. Values are typically placed in interpretive classes.

Plasticity index (PI).—The range in water content over which soil 

material is plastic. The value is the difference between the liquid limit 

and plastic limit of thoroughly puddled soil material that has passed a 

number 40 sieve (0.43 mm). The plastic limit is the water content at the 

boundary between the plastic and semisolid states. Values are typically 

placed in interpretive classes.

Unified classification.—An interpretive classification system of soil 

material designed for general construction purposes. It is dependent 

on particle-size distribution of the < 75 mm, liquid limit, and plasticity 

index and on whether the soil material has a high content of organic 

matter. There are three major divisions: mineral soil material having less 

than 50 percent particle size < 0.074 mm (passing 200 mesh), mineral 

soil material having 50 percent or more particle size < 0.074 mm, and 

certain highly organic soil materials. The major divisions are subdivided 

into groups based on liquid limit, plasticity index, and coarseness of the 

material more than 0.074 mm in diameter (retained on 200 mesh).

AASHTO classification.—An interpretive classification system of 

soil material for highway and airfield construction (Procedure M 145-
454 Chapter 

91; AASHTO, 1997). It is based on particle-size distribution of the 

< 75 mm fraction and on the liquid limit and plasticity index. The system 

separates soil materials having 35 percent or less particles passing the 

no. 200 sieve (< 0.074 mm in diameter) from those soil materials having 

more than 35 percent. Each of these two divisions is subdivided into 

classification groups based on guidelines that employ particle size, 

liquid limit, and volume change. A group index may be computed based 

on the liquid limit and plasticity index in addition to percent of particles 

< 0.074 mm. The group index is a numerical quantity based on a set of 

formulas.

Chemical Analysis

Calcium carbonate equivalent.—The quantity of carbonate in the 

soil expressed as CaCO3

 and as a weight percentage of the < 2 mm 

fraction. The available water capacity and availability of plant nutrients 

are influenced by the amount of carbonates, which affect soil pH.

Cation-exchange capacity (CEC).—The amount of exchangeable 

cations that a soil can adsorb at pH 7.0. Effective CEC (ECEC) is reported 

in soils where the pH in 1:1 water is 5.5 or less.

Gypsum.—The gypsum content pertains to amount in the < 20 mm 

fraction. The methods of reference are under 6F (Soil Survey Staff, 

2014a).

Organic matter.—Measured organic carbon is multiplied by the Van 

Bemmelen factor of 1.72 to obtain organic matter content.

Reaction (pH).—The standard method for pH is the 1:1 water 

extraction. For organic soil materials, the pH in 0.01M CaCl2

 is used. 

Typical agronomic classes are in discussed chapter 3.

Salinity.—A set of classes is used to indicate the concentration of 

dissolved salts in a water extract. Classes are expressed as electrical 

conductivity (EC). The measurement of reference is made on water 

extracted from a saturated paste. Units are decisiemens per meter 

(dS/m).

Sodium adsorption ratio (SAR).—SAR is evaluated for the water 

extracted from a saturated soil paste. The numerator is the concentration 

of water-soluble sodium, and the denominator is the square root of 

half of the sum of the concentrations of water-soluble calcium and 

magnesium. 

Sulfidic materials.—Upon exposure to air, soil materials that contain 

significant amounts of reduced monosulfides develop very low pH. The 

requirements are defined in the latest edition of the Keys to Soil Taxonomy 

(Soil Survey Staff, 2014b). Direct measurement of the pH after exposure 

to air is also used.
Soil Survey Manual 4

Physical Features or Processes

Depth to Restrictive Horizons or Layers

Depth to bedrock.—The depth to unweathered, continuous bedrock. 

The bedrock is commonly indurated but may also be strongly cemented, 

and excavation difficulty is very high or higher (see chapter 3). 

Depth to cemented pan.—The depth to a pedogenic zone that is 

weakly cemented to indurated (see chapter 3). Thin and thick classes are 

distinguished. The thin class indicates a pan that is less than 8 cm thick 

if continuous and less than 45 cm thick if discontinuous or fractured. 

Otherwise, the thick class applies.

Depth to permafrost.—The critical depth is determined by the active 

layer (the top layer that thaws in summer and freezes again in fall). 

Utilities, fencing, footings, etc. are placed below the active layer. The 

minimum depth is affected by depth of annual freezing. Permafrost depth 

may be strongly influenced by soil cover. 

Process Features

Total subsidence.—The potential decrease in surface elevation 

resulting from the drainage of wet soils having organic layers or 

semifluid mineral layers. Subsidence may result from loss of water and 

resultant consolidation, mechanical compaction, wind erosion, burning, 

or oxidation (of particular importance for organic soils). 

Potential frost action.—The likelihood of upward or lateral movement 

of soil caused by the formation of ice lenses and the subsequent loss 

of soil strength upon thawing. Large-scale collapse that forms pits is 

excluded and considered mass movement. Predictions are based on soil 

temperature, particle size, and pattern of water states.

Erosion

Factors and Groupings Related to Water or Wind Erosion

The K factor.—A relative index of susceptibility of bare, cultivated 

soil to particle detachment and transport by rainfall. This interpretive 

factor is used in the Revised Universal Soil Loss Equation (Renard et al., 

1997). Measurements are made on plots of standard dimensions. Erosion 

is adjusted to a standard of 9 percent slope. K factors are currently 

measured by applying simulated rainfall on freshly tilled plots. Earlier 

measurements integrated the erosion for the year for cultivated plots under 

natural rainfall. The K factor may be computed from the composition of 

the soil, saturated hydraulic conductivity, and soil structure.
456 Chapter 

The T factor.—The maximum rate of annual soil erosion that will 

permit crop productivity to be sustained economically and indefinitely 

(the soil loss tolerance). It can be used in the Revised Universal Soil 

Loss Equation (Renard et al., 1997). T factors are integer values from 1 

through 5 indicating tons per acre per year. The factor of 1 ton per acre 

per year is used for shallow or otherwise fragile soils, and that of 5 tons 

per acre per year is used for deep soils that are least subject to damage 

by erosion.

Wind erodibility groups.—A set of classes, using integer designations 

from 1 through 8, based on compositional properties of the surface 

horizon that affect susceptibility to wind erosion. Texture, presence of 

carbonates, content of iron oxides, materials with andic soil properties, 

and the degree of decomposition of organic soils are the major interpretive 

criteria. Each wind erodibility group is associated with a wind erodibility 

index, expressed in tons per acre per year. The wind erodibility index

is the theoretical, long-term amount of soil lost per year through wind 

erosion. It assumes a soil that is bare, lacks a surface crust, occurs in an 

unsheltered position, and is subject to the weather at Garden City, Kansas 

(Woodruff and Siddoway, 1965). Tillage frequency and practices are not 

specified. 

Corrosivity

Corrosivity Ratings for Steel or Concrete Structures in 

Contact with the Soil

Uncoated steel.—This rating depends on soil texture, drainage class, 

extractable acidity, and either resistivity of a saturated soil paste or 

electrical conductivity of the saturation.

Concrete.—This rating depends on soil texture, occurrence of organic 

horizons, pH, and amounts of magnesium and sodium sulfate or sodium 

chloride in the saturated soil paste.

Dynamic Soil Properties

The previous section dealt almost entirely with soil properties that 

do not typically change dramatically with use and management. Some 

soil properties are sensitive to use and management and may change 

temporally and spatially. These properties are termed dynamic soil 

properties (DSP) and discussed thoroughly in chapter 9. DSPs are valid 

and useful as variables in soil interpretations, especially if the outcomes 

of various management options are being predicted.
Soil Survey Manual 4

Interpretive Applications

In this section, kinds of soil interpretations or groupings of soils 

are presented. Soil interpretations may be developed at many levels of 

generalization or abstraction. Commonly, standard interpretations have 

been developed for wide use and application. Because many soil survey 

professionals use these interpretive criteria, interpretive results can 

be consistently produced from place to place. These standard criteria, 

however, may be too general for applications at some local or regional 

levels. If appropriate, the standard criteria may provide an effective 

template from which to adjust interpretive limits or add further criteria to 

better address local conditions.

Local Relative Placements

The soil properties and model criteria used in making interpretive 

generalizations are applicable to a very wide range of soils on a regional 

or national basis. For local decisions, relative rankings within the same 

interpretive placement may be extremely important. The interpretive 

model may have to be adjusted to reflect regional or local requirements, 

legislation, or land use codes. If interpretations are made locally, it is 

possible to rank soils on a strictly relative basis and to introduce local 

knowledge about soil behavior that may have been excluded from more 

general national ratings. The term “local interpretations” is used to 

describe locally controlled numerical ratings that give relative ranking 

of soils for a given use. In contrast, the national specific-use interpretive 

system emphasizes criteria that apply nationwide and thus provides more 

general rankings. 

Local soil interpretations are of greatest value in implementing 

ordinances for the local planning of specific tracts of land. If comparative 

ratings of every soil in a specific tract for a particular use are available, 

then a rational decision can be made whether to proceed, to change plans, 

or to find another area that has soils with higher potential. In some cases, 

the best soils in the specific tract for the particular use may be among 

those with low potential in the soil survey area overall.

The extent to which a given property is limiting and, in many 

cases, the practices that can be used to overcome the limitation are 

influenced by other soil properties. An example is the low strength of 

some soils in coarse-silty families. Such soils may not be limiting for 

dwelling foundations if the shallowest depth of free water exceeds 2 m. 

If, however, the shallowest depth of free water is within 25 to 50 cm 

of the base of the foundation, these soils may be decidedly limiting for 
458 Chapter 

foundations. Because the process of determining soil potentials involves 

input from knowledgeable local people, local interpretations can use 

more sophisticated criteria.

Steps for Developing Local Interpretations

Local soil interpretations are presented either as a set of qualitative 

classes, as a numerical index, or as both. The first step is to define the 

local interpretive product and the information that will be provided to the 

user. For example, a local sanitary district may request soil interpretations 

that are based on their sanitary codes. Is the information to be provided as 

discrete classes or as membership values? Are the coded criteria such that 

the first requirement can be met or are changes needed? What is the exact 

intent of each requirement contained in the local code? One requirement 

may be “depth to water table.” What is the local code’s definition of 

water table? What is meant by depth? What months, if any, can the water 

table be present? Is a layer of near saturation considered a water table for 

the specified use?

The second step is to identify soil properties that significantly impact 

or effect the particular use or management of the soil. Critical values 

for each property are defined locally and are generally based on local 

code, laws, or administrative regulations, for example, “depth to water 

table will not be less than 16 inches.” Is water table depth of 17 inches 

significant? Working with the local interpretation sponsor, these and 

other questions need to be addressed.

The third step is to develop the interpretive model. In this step, the 

effect of each criterion on the overall rating is described along with the 

interpretive output. A criterion can be weighted or given precedence 

over another criterion, or criterion interaction can be described. Once 

the model is created, extensive testing and a complete technical review 

are needed before the interpretive products are delivered to the sponsor. 

Management Groups

Management groups identify soils that require similar kinds of 

practices to achieve acceptable performance for an identified use. 

Historically in the U.S., management groups were limited to uses that 

involve the growth of plants. Management groups, however, can pertain 

to both agricultural and nonagricultural uses. The major advantage of 

management groups is that a user only needs to understand the concepts 

embodied in a relatively few groups of soils to make management 

decisions rather than understand and evaluate specific details of all the 

individual soils in the area. Not all soils in a management group are 
Soil Survey Manual 4

expected to have identical characteristics or management needs; however, 

the requirements of each management group must apply to all included 

soils. Generally, the broader the groups the less specific the descriptions 

of management needs. The number of classes for a management group 

depends on the range of soil properties, intensity of use and scale, purpose 

of the grouping, intended users, and availability of pertinent information. 

The number of classes must balance the need for homogeneity within a 

class against the complexity that results from increasing the number. The 

advantages of management groups are diminished if the classes are so 

broad that soils within a group differ greatly or so narrow that the number 

of classes is large and the differences among classes too small.

The most generally applied soil management group in the U.S. is 

the land capability classification system, which is widely used in the 

development of conservation plans for farming. Other management 

groups common in the U.S. are woodland suitability groups, pasture 

and hayland groups, and ecological sites. Recently, management groups 

have been defined for purposes of a national soil inventory. Prime 

farmland, for example, is a kind of management group. Highly integrated 

generalizations are made for so-called management groups. Groupings 

of soils may be made for various national land management programs 

and inventories. These groupings may be highly integrated (such as 

prime farmland) or be based on a few, quite specific criteria (such as 

highly erodible lands). Because such interpretive groups are frequently 

referenced in legislation, their applicability and maintenance have 

become important in achieving national environmental objectives in the 

United States. As a result, the official NCSS soil survey database has 

been designated as the only source of these and other data. 

Current U.S. Inventory Groupings

Technical soil groupings have been developed as criteria for 

application of national legislation concerned with the environment and 

with agricultural commodity production. Groupings may pertain to 

agricultural productivity and diversity, erosion potential, quality of surface 

and ground waters, maintenance of wetlands, or other national needs. 

Four national groupings are described below: prime farmland, unique 

farmland, hydric soils, and highly erodible land. Refer to the National 

Soil Survey Handbook to see how various map unit criteria, coupled with 

interpretive soil properties, have been employed to construct definitions 

for national inventory purposes.

Prime farmland.—Land that has the best combination of physical 

and chemical characteristics for producing food, feed, forage, fiber, and 

oilseed crops. It must also be available for these uses. It has the soil 
460 Chapter 

quality, growing season, and moisture supply needed to economically 

produce sustained high yields of crops when treated and managed 

according to acceptable farming methods, including water management. 

In general, prime farmland has an adequate and dependable water supply 

from precipitation or irrigation, favorable temperatures and growing 

season, acceptable acidity or alkalinity, acceptable salt and sodium 

content, and few or no rocks. It is permeable to water and air. Prime 

farmland is not excessively erodible or saturated with water for a long 

period of time, and it either does not flood frequently or is protected 

from flooding. 

Unique farmland.—Land other than prime farmland that is used 

for the production of specific high-value food and fiber crops. It has 

the special combination of soil quality, location, growing season, and 

moisture supply needed to economically produce sustained high quality 

and/or high yields of a specific crop when treated and managed according 

to acceptable farming methods. Examples of crops are tree nuts, olives, 

cranberries, citruses and other fruits, and vegetables. 

Hydric soils.—Soils that formed under conditions of saturation, 

flooding, or ponding long enough during the growing season to develop 

anaerobic conditions in the upper part. They make up one of three criteria 

needed for qualification as wetlands. 

Highly erodible land.—This land has been defined in order to identify 

the areas on which erosion-control efforts should be concentrated. The 

definition is based on erosion indexes derived from certain variables of 

the Revised Universal Soil Loss Equation (Renard et al., 1997) and the 

Wind Erosion Equation (Woodruff and Siddoway, 1965). The indexes 

are the quotient of tons of soil loss by erosion predicted for bare ground 

divided by the sustainable soil loss (T factor).

Land Use Planning

Land use planning is the formulation of policies and programs for 

guiding public and private land use in areas of any size where different 

uses compete for land. The word “land” in this context implies attributes 

of place and other factors besides soil. Planners must consider place, size 

of area, relation to markets, social and economic development, skill of the 

land users, and other factors. Soil surveys can help in land use planning 

by identifying soil resources in the area and providing information for 

the evaluation of environmental and economic effects of proposed land 

uses. They can be interpreted for land use planning through groupings or 

ratings of soils according to their limitations, suitabilities, and potentials 

for specified uses.
Soil Survey Manual 4

Local Planning

Local government units, such as those of cities, towns, and counties, 

do local planning. The planning applies to complexes of farms and 

ranches, to housing developments, to shopping centers, to industrial 

parks, and to entire communities or political units.

Local planners use soil interpretations and other information to 

develop recommendations on alternatives for land use, patterns of 

services, and public facilities. Planners may need interpretive maps at 

different scales, depending on their objective. Interpretations of small 

areas for local planning can rate limitations, identify management or 

treatment needs, and predict performance and potential of individual kinds 

of soils identified on detailed soil survey maps. Interpretations of areas 

that include entire governmental units evaluate soils for all competing 

uses within the planning area. These maps are smaller in scale, and the 

map units are associations of soil series or of higher taxa. Local planners 

commonly need ratings of the whole association for alternative uses. 

Special maps showing the location of areas having similar potentials 

or limitations for certain uses may be helpful for planners. Information 

about amounts and patterns of soils having different potentials within 

each association can be given in tables or in the text of a soil survey 

report.

Regional Planning

Geographically extensive soil-limiting factors may pertain to areas 

that cover several political units. For these situations, regional planning 

is appropriate. Principal functions of regional planning are collection, 

analysis, and dissemination of planning and engineering information, 

preparation of long-range plans, and coordination among the agencies 

involved.

Most soil maps for regional planning are medium-scale maps 

generalized from detailed soil survey maps. Soil interpretations show 

differences between map units in terms of suitabilities and limitations 

for the principal competing uses. The distribution of map units having 

similar behavior for a given use is commonly shown on special maps. An 

accompanying text describes the units, explains the basis for the ratings, 

and may also describe effects of the pattern of associated soils on the use 

of specific parcels. Regional planners commonly need information about 

the suitability of small parcels that is more specific than that provided 

by generalized soil maps. For example, they may locate an area that 

is generally good for recreation but also need to know that a potential 

site for a reservoir has soils suitable for storing water before they can 

complete the regional plan.
462 Chapter 

Farmland

Soil surveys in agricultural areas identify soil characteristics that 

determine suitability and potential of soils for farming. Interpretations 

for farming involve placement of soils into management groups (such 

as the land capability classification system) and identification of 

important soil properties that pertain to crop production, application of 

conservation practices, and other aspects of agriculture. Other aspects 

of agriculture include yield potential, susceptibility to erosion, depth to 

layers that restrict roots, available water capacity, saturated hydraulic 

conductivity, annual pattern of soil water states (including soil drainage 

class, inundation, and free water occurrence), qualities that describe tilth, 

limitations to use of equipment (including slope gradient and complexity, 

rock fragments, outcrops of bedrock, and stickiness), salinity and sodium 

adsorption ratio, presence of toxic substances, deficiency of plant 

nutrients, capacity to retain and release plant nutrients, capacity to retain 

soluble substances that may cause pollution of ground water, capacity to 

absorb or deactivate pesticides, and pH as related to plant growth and the 

need for liming.

The fate of added nutrients and pesticides, as related to farm 

management and cropping systems, is an important consideration in 

nonpoint water pollution. Identification of critical soil properties as 

related to resource management systems is crucial in the wise use of land. 

The land capability classification system indicates suitability of soils 

for agricultural uses (Klingebiel and Montgomery, 1961). The system 

classifies soils for mechanized production of more commonly cultivated 

field crops—corn, small grains, cotton, hay, potatoes, and field-grown 

vegetables. It does not apply directly to farming systems that produce 

crops, such as some fruits and nuts, that require little cultivation or to 

crops that are flooded, such as rice and cranberries. It also cannot be used 

for farming systems that depend on primitive implements and extensive 

hand labor.

Soil productivity.—Soil productivity is the output of a specified 

plant or group of plants under a defined set of management practices. 

It is the single most important evaluation for farming. In general, if 

irrigation is an optional practice, yields are given for both irrigated and 

non-irrigated conditions. Productivity can be expressed in quantity of a 

product per unit land area, such as kilograms or metric tons per hectare. 

For pasture, productivity can be expressed as the carrying capacity of 

standard animal units per unit area per season or year, or as live-weight 

gain. Productivity may be expressed as a rating or index related to either 

optimum or minimum yields, or it may be indexed to a set of soil qualities 

(properties) that relate to potential productivity. Productivity indices 
Soil Survey Manual 4

have the advantage of being less vulnerable to changes in technology 

than expressions of productivity based on yields.

Productivity ratings express predicted yields of specified crops 

under defined management as percentages of standard yields. They are 

calculated as follows:

Productivity rating = predicted yield per unit area x 100 standard yield per unit area

Such a rating provides a scale for comparing productivity of different 

kinds of soils over large areas. Ratings lend themselves to numerical 

treatment. Productivity ratings permit comparison of the productivity 

of crops having yields that differ markedly in numerical values. For 

example, a certain soil has a yield of 60,000 kg/ha for silage corn and 

of 9,000 kg/ha for grain corn. Because these quantities represent similar 

levels of production, the productivity ratings are similar. Selection of the 

standard yield of a crop depends on the purpose of the rating. For national 

comparison, standard yields should be for a high level of management on 

the best soils of the region for the crop. For potential production, yields 

under the best combination of practices are used.

Productivity ratings for individual crops can be combined to obtain 

a general rating for soil over its area of occurrence. Individual ratings are 

weighted by the fraction of the area occupied by each crop, and a weighted 

average is calculated that characterizes the general productivity of the soil. 

Productivity indices tied to soil properties are used as a relative 

ranking of soils. Typically, soil properties important to favorable rooting 

depth and available water capacity are chosen. Some productivity models 

rely on a few critical soil properties, such as pH and bulk density, to rate 

soils (Kiniry et al., 1983). The National Commodity Crop Productivity 

Index (Dobos et al., 2012) uses soil, site, and climatic information to 

provide an array of the soils of the United States on the basis of their 

inherent ability to foster crop growth.

Resiliency.—Resiliency of soils is an interpretation that relates 

to the ability of a soil to rebound from depletion of plant nutrients or 

organic matter or to rebound from degradation of physical or chemical 

soil properties (Seybold et al., 1999). Resiliency ratings are based on 

estimates of the natural fertility of the soil, soil carbon content, available 

water capacity, favorable rooting depth, particle-size distribution, and 

distribution of salts in the profile, if present. Resiliency ratings are 

important in evaluating alternative management systems that are based 

on lower chemical and energy inputs. Traditional practices that use high 

inputs of chemical fertilizers and pesticides commonly offset deficiencies 

in some soil properties that are important to crop production. Resiliency 
464 Chapter 

of soils is also important in evaluating long-term effects of management 

systems on soils.

Rangeland

Rangeland is land on which the historic climax vegetation was 

predominantly grasses, grass-like plants, forbs, or shrubs as the consequence of a dry climate. It includes land revegetated naturally or 

artificially to provide a plant cover that is managed like native vegetation 

(introduced forage species are also managed as rangeland). The vegetation 

is suitable for grazing and browsing by animals. Rangeland includes 

natural grasslands, savannahs, many wetlands and deserts, tundra, and 

certain shrub and forb communities.

Soil-ecological site correlation within a soil survey gives the suitability 

of the soil to produce various kinds, proportions, and amounts of plants. 

This knowledge is important in developing management alternatives 

needed to maintain site productivity. Rangeland interpretations in the 

U.S. are normally produced as ecological site descriptions.

Ecological site descriptions (ESD).—An ESD commonly contains 

the following information:

1. Physiographic features that describe the position of the site on 

the landscape and whether the site generates or receives water 

runoff. 

2. Climate factors that typify the site, as well as characterize the 

dynamics of the site, including storm intensity, frequency of 

catastrophic storm events, and drought cycles. 

3. Influencing water features where the site is associated with 

wetlands or streams. 

4. Representative soil features that significantly affect plant, soil, 

and water relationships and site hydrology, such as major soil 

families, geologic formation, soil surface features, surface 

horizon and texture, soil depth, thickness and available water 

capacity of major root zone, kind and amount of accumulations, 

rock fragments in the profile, reaction, salinity, sodicity, soil 

water states, water table, and flooding.

5. Plant communities of the site, including a description of the 

vegetation dynamics, the common vegetative states of the site, 

and the transitions between states. Thresholds are identified 

as boundaries of the vegetative states. Other plant community 

information includes a state-and-transition diagram, plant 

community composition, ground cover and structure, annual 

production, growth curves, and photos of each vegetative state 

(see appendix 4).
Soil Survey Manual 4

6. Site interpretations for the animal community (livestock and 

wildlife), hydrologic functions, recreational uses, wood products, 

and other potential uses.

Forestland

Forestland is land dominated by native or introduced trees with an 

understory that commonly consists of many kinds of woody plants, forbs, 

grasses, mosses, and lichens. Some forest communities produce enough 

understory vegetation to provide forage. 

Soil-ecological site correlation within a soil survey gives the 

suitability of the soil to produce wood products or other ecosystem 

services. If forestland is part of a soil survey, estimated productivity of the 

common trees is given for each individual soil. The understory vegetation 

is described at the expected canopy density most representative of the 

site. Determination of the soil’s productivity requires close collaboration 

between foresters and soil scientists.

Wood production or yield is commonly expressed as the site index

or as some other measure of the volume of wood produced annually. Site 

index is the average height of dominant and codominant trees of a given 

species at a designated age. Measurements of site index are typically 

extended to several similar soils for which data are unavailable. The site 

index is correlated to each soil and may be further interpreted in terms of 

cubic meters per hectare.

Soils may be grouped using the woodland ordination system. 

This system uses symbols to indicate productivity potential and major 

limitations for the use and management of individual soils or groups 

of soils. The first part of the ordination symbol, a number, is the class 

designator. It denotes potential productivity in terms of the nearest whole 

cubic meter of wood growth per hectare per year for the soil, based 

on the site index of an indicator tree species. For several species, data 

are available for converting site index to average annual wood growth. 

The second part of the ordination symbol (the subclass) indicates soil 

or physiographic characteristics that limit management—stoniness or 

rockiness, wetness, or restricted rooting depth. The ordination symbol 

may also have a third part to distinguish groups of soils that respond 

similarly to management. Soils with the same group symbol have about 

the same potential productivity, are capable of producing similar kinds 

of trees and understory vegetation, and have similar management needs.

Soils may be rated for such factors as susceptibility to mechanical 

compaction or displacement during forestry operations, limitations due 

to burning, hazards from soil-borne pests and diseases, and limitations 

due to specific soil properties such as wetness. In the management of 
466 Chapter 

trees, one must first understand the soil on which the trees grow or are to 

be grown. Soil surveys include information that can be used effectively 

in the management of forestland. This information includes:

Erosion hazard.—The possibility that erosion damage may occur 

as a result of site preparation and the aftermath of cutting 

operations, fires, and overgrazing. 

Equipment limitations.—Limits on the use of equipment either 

seasonally or year-round due to soil characteristics such 

as slope, surface rock fragments, wetness, and surface soil 

texture. 

Seedling mortality.—A rating that considers soil properties that 

contribute to the mortality of naturally occurring or planted 

tree seedlings, such as droughtiness, drainage class, and 

slope aspect. It does not consider plant competition. 

Windthrow hazard.—A determination based on soil properties 

that affect the likelihood of trees being uprooted by wind 

as a result of insufficient depth of the soil for adequate root 

anchorage. A fragipan, bedrock, gravel, or high water table 

may affect soil rooting depth. Differences in root systems 

related to tree species are not considered. The rating is 

typically independent of the probability of high winds unless 

the soil is typically in landscape positions susceptible to high 

winds. 

Plant competition.—The likelihood of invasion or growth of 

undesirable plants in openings within the tree canopy. Depth 

to the seasonal water table and available water capacity are 

the soil properties having the greatest effects on natural 

regeneration or suppression of the more desirable plant 

species. 

Windbreaks

Windbreaks are made up of one or more rows of trees or shrubs. 

Well placed windbreaks of suitable species protect soil resources, control 

snow deposition, conserve moisture and energy, beautify an area, provide 

wildlife habitat, and protect homes, crops, and livestock. The plant species 

used in windbreaks are not necessarily indigenous to the area. Because 

each tree or shrub species has certain climatic and physiographic limits, 

a particular species may be well suited or poorly suited based on soil 

characteristics. Therefore, correlation of soil properties and adaptable 

windbreak species is essential.

A listing of adaptable species is given for each kind of soil, or 

grouping of soils by ecological site or suitability group, where windbreaks 
Soil Survey Manual 4

can serve a useful purpose—such as open field-planting, interplanting in 

existing woodland, and environmental modifications like wind or water 

barriers and development of wildlife habitat. The plant species identified 

for these purposes are grouped by height classes at 20 years of age.

Recreation

Interpretations in urban and suburban areas are made for golf 

fairways, picnic sites, playgrounds, paths, trails, and campsites. 

Interpretations for ski slopes, snowmobile trails, and off-road vehicles 

are made in some places. Ratings are typically based on restrictive soil 

interpretive properties, such as slope, occurrence of internal free water, 

texture of surface horizons, and soil resiliency.

Interpretations for recreation must be applied cautiously. Many 

recreational areas in the U.S. that are on large tracts of publically 

owned lands have only order 3 or higher soil surveys. Map units for 

such soil surveys are commonly associations or complexes of soils that 

may differ markedly in their limitations and suitabilities. Furthermore, 

general suitability of the map unit must take into consideration not 

only the qualities of the individual kinds of soil but also the soil pattern 

and potential interactions. Suitability may depend on a combination of 

several kinds of soil in a pattern appropriate to the intended use. Finally, 

factors other than soils are important in recreational planning. Aesthetic 

considerations, accessibility, land values, access to water and public 

sewer lines, presence of potential impoundment sites, and location 

relative to existing facilities may be important even though none of these 

factors is evaluated for map units.

Wildlife Habitat

Soils influence wildlife primarily through control over vegetation 

diversity. Descriptions of the soil as wildlife habitat have two parts. In one 

part, suitability class for different vegetation groups is recorded. These 

vegetation groups are called habitat elements. Each habitat element is 

a potential component of the environment of wildlife. Hardwood trees 

and shallow water areas are examples of habitat elements. In the other 

part of the description, soils are rated separately for several kinds of 

wildlife, including animals adapted to openland, woodland, wetland, and 

rangeland. Current land use and existing vegetation are not considered 

because these factors are subject to change and cannot be determined 

from a soil map. Wildlife population is also not considered because of 

the mobility of wildlife and the possibility of changes in population 

during the year. The ratings show where management for wildlife can be 

applied most effectively and which practices are appropriate. The ratings 
468 Chapter 

may also show why certain objectives (e.g., the production of pheasants) 

may not be feasible. Some soil surveys include explicit management 

recommendations. 

Construction Materials

Soil survey interpretations estimate suitability of the soil as 

construction material and show where to locate material that can be 

mined. Material that compacts readily and has high strength and a 

low shrink-swell potential is preferred as base material for roads and 

foundations. Material for fill has to be evaluated for the potential for 

acid-sulfate formation, which can corrode steel and concrete and form 

unfavorable pH conditions for lawns and landscaping. Gravel and sand 

are used for concrete, road surfacing, and filters in drainage fields. Organic 

soil material is used widely as horticultural mulch, potting soil, and soil 

conditioner. Mineral soil is generally rich in organic matter and is applied 

to lawns, gardens, and roadbanks. Soils can be rated as probable sources 

of these materials. The quality of a particular site, however, typically 

cannot be specified.

Building Sites

Interpretations are made for construction of small buildings; for 

installation of roads, streets, and utilities; and for establishment of lawns, 

landscaping, and stormwater management. Such soil uses involve high 

capital expenditures in relatively small areas. Onsite evaluation typically 

is necessary.

Soil survey interpretations are useful for comparing alternative sites, 

in planning onsite investigations and testing, and in land use planning. 

Soil maps can assist in selecting building sites that are near areas suitable 

for utilities, parks, and other needs.

The preparation of building sites may alter soil properties markedly. 

As a result, some interpretive soil properties for the undisturbed sites 

must be applied cautiously. Upper horizons may have been removed and 

locally translocated, and the depth to horizons important to soil behavior 

may have been increased or decreased. The pattern of soil water states 

may have changed. Areas may have been drained and, therefore, are 

not as wet as indicated in the survey. Irrigation may have been used to 

establish and maintain vegetation and resulted in a more moist soil and 

deep movement of water. Pavements, roofs, and certain other aspects 

of construction increase runoff and may cause inundation at lower 

elevations where such hazards are not indicated in the survey.

Building construction.—Construction and maintenance of buildings belongs primarily to the fields of architecture and engineering. 
Soil Survey Manual 4

Additionally, large multistory structures are generally supported by 

footings placed below the depth of soil survey examination. Therefore, 

soil survey interpretations are not a definitive source of information for 

building construction. Important interpretive soil properties for small 

buildings and accessory installations, such as roads and utilities, include 

slope, inundation, mass movement, potential frost action, depth to 

bedrock and cemented pans, shrink-swell potential, rock fragments > 75 

mm, erodibility, subsidence, and soil strength.

Roads, streets, and utilities.—Performance of local roads and streets, 

parking lots, and similar structures is directly related to performance of 

the underlying soil in many cases. Pipelines and conduits commonly 

are buried in soil at shallow depth. Soil properties may affect cost of 

installation and rate of corrosion. Soil material is used directly as topsoil, 

roadfill, and aggregate for concrete. Soil interpretations can predict 

some suitabilities and limitations of different kinds of soil for these uses, 

although they cannot predict performance of highways, major streets, 

and similar structures. For these structures, onsite testing is necessary. 

Use of soil survey information, however, may reduce the number of 

borings and engineering tests needed. 

Soil information in conjunction with engineering testing can identify 

soils that can be stabilized in place for a road base and establish where 

gravel or crushed stone will be needed. Soil surveys can be helpful 

in deciding methods of stabilizing cuts and fills. Soil properties may 

affect the cost of installation and length of service of buried pipelines 

and conduits. For example, shallow bedrock greatly increases the 

cost of installation. Rate of corrosion is related to wetness, electrical 

conductivity, acidity, and aeration. Differences in properties between 

adjacent horizons, including aeration, increase corrosion in some soils. 

Soil properties affect the cathodic protection provided by sacrificial 

metal buried with pipes. Rock fragments can break protective coatings 

on pipes. Shrinking and swelling of some soils may preclude the use of 

certain kinds of utility pipe. 

Soil survey interpretations may be particularly useful in the prediction 

of potential problems along proposed routes. Hydrologic information and 

other data combined with interpretive soil properties, such as hydrologic 

groups, can be helpful in estimating potential runoff for designs of 

culverts and bridges. The probability of bedrock and unstable soils that 

require removal or special treatment can be determined from soil surveys. 

Lawns and landscaping.—Soil survey interpretations give general 

information about sources of fill and about planning, planting, and 

maintaining grounds, parks, and similar areas. Particularly important 

are the suitability of the soil for turf, ornamental trees, and shrubs; the 
470 Chapter 

ability to withstand trampling and traffic; the suitability for driveways 

and other surfaced areas; and the ability to resist erosion. A number of 

soil chemical properties may be critical, especially for new plantings. 

Interpretations for particular plants and the treatments for a specific site 

require input from other disciplines. 

Many lawn and ornamental plantings are made in leveled areas 

on an exposed subsoil or substratum or on excavated material that 

has been spread over the ground. Interpretations can be made for the 

suitability of such soil materials for lawns and other plantings, the 

amount of topsoil that is necessary, and other treatments required for 

satisfactory establishment of vegetation. Highway departments use soil 

interpretations when establishing and maintaining plantings on subsoil 

material in rights-of-way.

Stormwater management.—Building of infrastructure (such as 

roads, sidewalks, and rooftops) creates impervious surfaces, which greatly 

increase runoff and can contribute to flooding. Soils can be interpreted 

for various practices for stormwater retention and infiltration that can 

reduce the threat of flooding and the pollution of surface waters. The 

ability of the soil to transmit water and retain harmful materials while not 

contributing to landscape instability are important site considerations.

Waste Disposal

Waste disposal practices either place the waste in a relatively small 

area of soil or distribute the waste at low rates over larger areas. 

Localized placement.—In this context, waste includes a wide range 

of material, including household effluent, solid waste, and industrial 

wastes of various kinds. Effluent from septic tanks is distributed in filter 

fields. Liquid wastes are stored and treated in lagoons constructed in soil 

material. Solid wastes are deposited in sanitary landfills and covered 

with soil material.

Extremes in saturated hydraulic conductivity and free water at 

a shallow depth limit the use of soil for septic tank absorption fields. 

(Table 8-1 shows the criteria for septic tank absorption fields.) Sewage 

lagoons require a minimum saturated hydraulic conductivity to prevent 

rapid seepage of water, a slope within certain limits, and a slight or 

no possibility of inundation or the occurrence of free water at shallow 

depths. 

Soils are used to dispose of solid wastes in landfills, either in trenches 

or in successive layers on the ground surface. For trench disposal, 

properties that relate to the feasibility of digging the trench (i.e., depth 

to bedrock and slope) and factors that pertain to the likelihood of 
Soil Survey Manual 4

pollution of ground water (i.e., shallow zone of free water, inundation 

occurrence, and moderate and high saturated hydraulic conductivity) 

have particular importance. For disposal on the soil surface, saturated 

hydraulic conductivity, slope, and inundation occurrence are important. 

Low-intensity distribution.—Soil is used to render safe either solid 

or liquid waste that is spread on the ground surface or injected into the 

soil. This waste includes manures, sewage sludge, and various solids and 

wastewaters (particularly from factories that process farm products). In 

general, the physical process of distribution is limited by steep slopes, 

rock fragments > 75 mm, rock outcrops, and wetness. The rate at which 

wastes can be applied without contaminating ground water or surface 

water is called the “loading capacity.” Low infiltration values limit the rate 

at which liquid wastes can be absorbed by soil. Similarly, low saturated 

hydraulic conductivity through most of the upper meter limits the rate 

at which liquid wastes can be injected. Shallow depth of a hardpan or 

bedrock or coarse particle size reduces the amount of liquid waste that a 

soil can absorb in a given period. The time that wastes can be applied is 

reduced by frozen soil or occurrence of free water at shallow depths. Low 

soil temperatures reduce the rate at which the soil can microbiologically 

degrade the material. 

Soils differ in their capacity to retain pollutants until they are 

deactivated or used by plants. Highly pervious soils may permit 

movement of nitrates to ground water. Similarly, saturated or frozen soils 

allow runoff to carry phosphates absorbed on soil particles or in waste 

deposited on soil directly to streams without entering the soil. Soils that 

combine a limited capacity to retain water above slowly permeable layers 

and a seasonal water excess may allow water that is carrying pollutants to 

move laterally at shallow depths. Such water may enter streams directly. 

Large quantities of waste may change the soil. Heavy loading with 

liquid waste may reduce the oxygen supply so that yields of certain crops 

decrease. Conversely, heavy loadings can provide beneficial irrigation 

and fertilization for other kinds of soil and crop combinations. Animal 

wastes improve most soils, but effects differ according to the kind of soil. 

Typically, the first step in making soil interpretations for disposal 

of wastes is to determine how disposal systems for each kind of waste 

have performed on specific kinds of soil in the area. Data may come 

from practical operations or from research. Which properties are critical 

and how to appraise the effects of the properties need to be determined. 

Limiting values of critical properties can be determined through 

experience and may be used in making interpretations where data on soil 

performance are scarce or lacking.
472 Chapter 

Water Management

Water management, as discussed here, relates to construction of 

relatively small- or medium-sized impoundments, control of waterways 

of moderate size, installation of drainage and irrigation systems, and 

control of surface runoff to minimize erosion. These activities may 

require large capital expenditures. In most cases, onsite evaluation should 

be conducted, particularly for soil properties at depth. Order 2 or order 

3 soil surveys can be helpful in evaluation of alternative sites, but onsite 

investigations are required to design engineered projects.

Ponds and reservoirs.—Soil information is used in predicting soil 

suitability for ponds and reservoir areas. Impoundments contained by 

earthen dikes and fed by surface water have somewhat different soil 

requirements than those that are excavated and fed by ground water. 

Separate interpretations are commonly made. 

Soil seepage potential, as determined by the minimum saturated 

hydraulic conductivity and the depth to pervious soil material, is an 

important factor for design of ponds and reservoirs. Slope is also 

important because it affects the capacity of the reservoir. The soil’s 

hydrologic group (see chapter 3) pertains to the prediction of runoff into 

a pond or reservoir. 

Embankments, dikes, and levees.—These are raised structures 

made of disturbed soil material constructed to impound water or to 

protect land from inundation. Soils are evaluated as sources of material 

for construction. Particle-size distribution and placement in the Unified 

system are important considerations. Interpretations do not consider 

whether the soil in place can support the structure. Performance and 

safety may require onsite investigation to depths greater than are typically 

considered in a soil survey. 

Irrigation.—Important considerations for the design of irrigation 

systems are feasible water application rates, ease of land leveling and 

the resultant effect on the soils, possibility of erosion by irrigation 

water, physical obstructions to use of equipment, and susceptibility 

to flooding. An order 1 soil survey may be needed for observations 

and measurements of infiltration rates at depths greater than typically 

surveyed. The interpretations may be based on various soil properties, 

including saturated hydraulic conductivity, available water capacity, 

erodibility, slope, stoniness, effective rooting depth, salinity, sodium 

adsorption ratio (SAR), gypsum content, and other properties that may 

affect the level of crop response. 

Interpretations for irrigation in arid and semiarid regions may be 

more complex than in humid regions, because irrigation changes the 
Soil Survey Manual 4

soil water regime more in arid and semiarid areas. Salinity and SAR of 

soils can be particularly significant, as can the quality of irrigation water. 

In arid and semiarid areas, small differences in slope and elevation can 

lead to an accumulation of salt-laden drainage water in low places or 

to development of a high water table if a proper drainage system is not 

provided. 

Drainage.—Drainage refers to the removal of excess water from 

soils for reclamation or alteration. Engineers establish the criteria 

for drainage construction. The criteria include spacing and depth of 

subsurface drains, depth and width of open ditches and their side slopes, 

and allowable gradient. Soil properties important to drainage include 

water transmission, soil depth, soil chemistry, potential frost action, 

slope, and presence of rock fragments > 75 mm.

Public Health and Safety

Soil and site properties can profoundly influence the distribution 

of pathogenic organisms, the risk of mass movement and earthquakeinduced hazards, and disease vectors related to mosquito habitat. The 

suitability of soils as habitat for soil-borne fungi and bacteria that affect 

human or animal health may be determined with increased resolution of 

maps showing various hazards and propensities of soils at soil survey 

scales. 

Soil-Borne Diseases

Valley Fever is an example of a soil-borne disease. It is caused by 

the fungi Coccidioides immitis and Coccidioides posadasii. Because 

these fungi have very specific soil and climate requirements, areas that 

are suitable as habitat for these organisms can be predicted. Therefore, 

areas of likely habitat can be avoided or measures can be taken to prevent 

creating dust during times that the fungi are releasing spores. 

Mass Movement

The likelihood of soil slippage using shear strength and shear stress 

concepts can be inferred from the slope, land surface shape, and soil depth 

to planes of weakness. The propensity of some soils to liquefy during 

earthquake events is influenced by the age and wetness of the landscape. 

These attributes and their relationships can be modeled using soil survey 

data. Care is needed in evaluating the relevance of the predictions if the 

depth of inference for the soils data is not deep enough to characterize 

the affected soil material. 
474 Chapter 

Geophysical Tools and Site Suitability

Ground-penetrating radar (GPR) and other geophysical tools 

(discussed in chapter 6) are widely used for locating underground 

infrastructure, soil features, and burial sites and other applications in 

which large areas must be investigated without disturbing the soil. Soil 

properties such as electrical conductivity, clay content, and mineralogy 

influence the attenuation and penetration of electromagnetic energy. 

Where and how well GPR will work can be predicted using soil properties. 

The U.S. has developed a series of interpretive maps illustrating soil 

suitability for GPS use throughout the country (USDA-NRCS, 2009).

Subaqueous Soils

Subaqueous soils form in water-deposited material and can be 

mapped, characterized, and interpreted like terrestrial soils. These 

deposits undergo pedogenic processes (Demas and Rabenhorst, 2001). 

They occur in predictable patterns and have predictable soil properties 

that are useful for interpretation. This section discusses some soil 

interpretations that have been developed for subaqueous soils in the 

United States. Chapter 10 provides more information on the nature and 

properties of subaqueous soils. Because land use does not end at the 

water’s edge, interpretations have been developed for the subaqueous 

environment. Mapping and characterizing subaqueous soils helps ensure 

the wise use of the near offshore soil resource. Below are a few examples 

of interpretations for subaqueous soils.

Moorings 

A stable place to tie up watercraft is essential during a storm. The 

type of mooring that can be used for securing watercraft depends on 

the nature of the subaqueous soil (Surabian, 2007). In areas where the 

bottom is fluid (soft bottom), a mushroom anchor will suffice to hold the 

vessel in place. In areas where the bottom is composed primarily of sand 

and gravel (hard bottom), a deadweight anchor is needed.

Eelgrass Restoration

Eelgrass is an important species in the subaqueous environment 

because it supplies food and cover for desirable fish and shellfish. It 

requires a sandy soil matrix free of reduced monosulfides. The water 

column must be shallow enough to allow light penetration but deep 

enough to avoid freezing.
Soil Survey Manual 4

Land Disposal of Dredged Material

Sediment is removed from navigation channels to facilitate the 

movement of vessels. If this material is placed on land, it will oxidize 

in the subaerial environment. If reduced monosulfides are present in the 

dredged material, these compounds will oxidize and form sulfuric acid, 

which can have severe environmental effects.

Hard Clam Substrate

Aquaculture is an important agricultural sector in coastal areas. Hard 

clams require a sandy substrate since fine soil particles can clog their 

filtering apparatus. 

Areal Application of Interpretations

The objective of soil surveys is to provide interpretations for areas 

delineated on soil maps. This section discusses the relationship of 

interpretations to map unit terminology and conventions (described in 

detail in chapter 4), the interpretive basis of map unit design, and the 

uncertainty of interpretive predictions for specific areas within the map 

unit. 

Polygon-Based Soil Interpretations

Polygon-based interpretations are applied uniformly to an entire map 

unit delineation. The top image in figure 8-4 is a soil map that shows the 

delineations of map units and depicts other features on the landscape. 

The bottom image in figure 8-4 shows the same area as the top image 

and illustrates the ratings of the soil map units for local roads and streets. 

Green indicates not limited, yellow somewhat limited, and red very 

limited. Gray areas are not rated because there were not enough data to 

derive a rating (in this case they are water bodies). Table 8-3 indicates 

which soil properties (in the column “Rating reasons”) are limiting for 

local roads and streets for the Albrights map unit (AbB). The numeric 

values give an estimate of the degree of limitation posed by each reason. 

Note that even though one component is given in the map unit name (i.e., 

Albrights), it is understood that more than one component exists in the 

map unit. In this case, the included Brinkerton soil is estimated to make 

up about 5 percent of the map unit (listed in the column “Component 

name”). 
476 Chapter 

Figure 8-4

Soil map (top) showing the distribution of mapping units on the landscape and 

interpretive map (bottom) showing limitations for local roads and streets.
Soil Survey Manual 4

Table 8-3

Limitation Ratings for Local Roads and Streets for the 

Albrights Map Unit (AbB)

Local Roads and Streets—Summary by Map Unit—Bedford County, Pennsylvania (PA009)

Map unit 

symbol

Map unit 

name Rating

Component 

name 

(percent)

Rating reasons 

(numeric values)

Acres 

in AOI

Percent 

of AOI

AbB Albrights silt 

loam, 3 to 

8 percent 

slopes

Very limited Albrights 

(90%)

Depth to thick 

cemented pan 

(1.00)

43.6 3.1%

Depth to thin 

cemented pan 

(1.00)

Frost action 

(0.50)

Depth to saturated 

zone (0.48)

Brinkerton 

(5%)

Depth to thick 

cemented pan 

(1.00)

Depth to saturated 

zone (1.00)

Depth to thin 

cemented pan 

(1.00)

Frost action 

(1.00)

Low strength 

(1.00)

Raster-Based Soil Interpretations

The processes used in digital soil mapping (see chapter 5) present 

intriguing possibilities for the future development and display of 

spatially explicit soil interpretations. The current U.S. interpretation 

system has two primary shortcomings that limit the precision and 

accuracy of the derived predictions. First, the system is constrained to 

use only data from within the database. While this is reasonable for 

the soil attribute data, ideally climatic and geomorphic data would 

be obtained from more authoritative sources. Second, the interpretive 

output can only be displayed as aggregated values for the polygons 

of the original mapping. Any fine detail of the landscape cannot be 

represented. Digital soil mapping (DSM) offers the opportunity to 

overcome both of these limitations by allowing the use of authoritative 

data layers and displaying results at the resolution of the digital soil map. 

The interpretive models themselves are generally scale-independent, 

and higher resolution input data would allow greater confidence in the 

spatial location of the results.
478 Chapter 

The advantages of raster-based interpretations relate to the scale 

of the land use. Land uses such as farming, ranching, and forestry are 

relatively extensive operations (10 to 1,000 hectares) with a relatively 

low investment per hectare (although some farming systems are 

more intense than others). For these uses, a scale of 1:20,000 may 

be adequate. Other land uses, such as homesites and animal waste 

facilities, are on a more intensive scale (0.1 to 1 hectare) and have a 

higher monetary investment per hectare. They occupy a discrete portion 

of the landscape, which may fall into an area that is not accounted 

for on an aggregated 1:20,000 soil map. A linear land use, such as a 

pipeline or road, may involve a long, narrow segment of the landscape 

that encompasses several kilometers of length and traverses portions 

of many map units. Accounting for the inherent homogeneity of the 

landscape for these types of land use could allow routing the right-ofway to avoid obstacles and sensitive areas that might not be displayed 

on a soil map.

In a raster environment, continuous soil data would allow depiction 

of interpretive results limited only by the pixel size of the DSM. 

Environmental covariates, such as climate and topographic data, as 

well as the soil attribute data would be processed by the interpretation 

modeling system for each pixel (fig. 8-5). These data would already be 

available from the DSM process.

Spatially explicit raster-based interpretations would be subject to 

the same issues of data quality and confidence as the DSM from which 

it was derived. The confidence level would be indicated in the DSM, 

and the interpretive results would also have a reportable confidence 

interval. The processing workload would be much larger than what is 

currently needed and would vary depending on the resolution of the 

DSM.

References

American Association of State Highway and Transportation Officials 

(AASHTO). 1997. The classification of soils and soil-aggregate 

mixtures for highway construction purposes. AASHTO designation 

M145–91. In Standard specifications for transportation materials and 

methods of sampling and testing, Part 1: Specifications (18th ed.). 

Cox, E., and M. O’Hagan. 1998. The fuzzy systems handbook: A 

practitioner’s guide to building, using, and maintaining fuzzy 

systems, 2nd edition. Morgan Kaufmann Publishers.
Soil Survey Manual 4

Figure 8-5

Raster-Based Soil Interpretation

Land Cover Spatially Explicit Soil 

Interpretation Engine

Interpretation Output 

Raster

Raster Soil Data Digital Soil 

Mapping Technique

Wetness Index, Restriction Depths, 

Rule Sets ...

Horizon Data from Suitable Source

Sand, Silt, Clay, Unified, AASHTO, 

AWC, Ksat, Bulk Density, LEP, CEC, 

pH, EC, SAR ...

Continuous Landscape Data

Digital Elevation Model, Slope, Surface 

Shape, Aspect, Elevation ...

High-Resolution Climate Data

Precipitation, Growing Degree Days, 

PAR, Day Length, Evapotranspiration, 

Wind ...

Conceptual framework of raster-based soil interpretation.

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formation: A system of quantitative pedology for submersed environments. Geoderma 102:189-204.

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National Commodity Crop Productivity Index (NCCPI), version 2.0. 

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fundamental component of soil quality. Soil Science 164:224-234.

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R. Burt and Soil Survey Staff (eds.). USDA Natural Resources 

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Surabian, D.A. 2007. Moorings: An interpretation from the coastal zone 

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U.S. Department of Agriculture, Natural Resources Conservation 

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U.S. Department of Agriculture, Natural Resources Conservation 

Service. 2009. Ground-penetrating radar soil suitability maps. 

Available at http://www.nrcs.usda.gov/wps/portal/nrcs/detail/soils/

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Woodruff, N.P., and F.H. Siddoway. 1965. A wind erosion equation. Soil 

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