Soil Survey Manual - Chapter Six (Part 1 of 4)

Interpretations

Table of Contents

Page 1
Approaches to Generalizing Relative Soil Behavior
Interpretive Systematics
    Management Groups
    National Specific-use Placements
    Local Relative Placements

Page 2
Interpretive Soil Properties
    Assignment Guidelines
    Setting
    Field Water Characterization
    Particle Size Distribution
    Fabric-Related Analysis
    Engineering Classification
    Chemical Analysis
    Physical Features or Processes
    Erosion
    Corrosivity

Page 3
Interpretative Applications
    National Inventory Groupings
    Land-Use Planning
    Farmland
    Rangeland
    Forest land
    Windbreaks
    Recreation
    Wildlife Habitat
    Construction Materials
    Building Sites
    Waste Disposal
    Water Management

Page 4
Areal Application of Interpretations
    Map Units
    Areal Extension of Interpretations
    Areal Generalization
Illustrative Map Units

Approaches to Generalizing Relative Soil Behavior

This chapter explains the concepts and principles used in the interpretation of data to evaluate or predict suitabilities, limitations, or potentials of soils for a variety of uses (see appendices for examples). Interpretation is a process that continues as long as a soil survey is in use.

Soil survey information answers a wide range of soil-related questions. There is also a great range in complexity as the soils information is sometimes used alone and sometimes as one layer of information in integrated systems that also consider other natural resources, demographics, climate, and ecological and environmental factors in decision making. Soil survey data make up a growing number of geographic information systems and models that deal with regional planning, erosion prediction, crop yields, and even modeling of global change.

Historically, soil survey interpretations have been concerned primarily with soil interpretative predictions for the public that are specific to a land use. This contrasts with genetic or taxonomic evaluation of soils by scientists. The level of data collection needed to execute the current interpretations program of the National Cooperative Soil Survey is in relevant parts of the National Soils Handbook (Soil Survey Staff).

Generally, preparation of interpretations involves the following steps: (1) assembling information about the soils and the landscapes in which they occur, (2) modeling other necessary soil characteristics from the soil data, (3) deriving inferences, rules, and guides for predicting soil behavior under specific land uses, and (4) integrating these predictions into generalizations for the map unit.

Soil interpretations provide numerical and descriptive information pertaining to a wide range of soil interpretative predictions. This information can be expressed in classes and units of measure of other disciplines. For example, presentation of particle size data includes both the soil separates of sand, silt, and clay, and the USDA Texture or UNIFIED classes. Generally, evaluations are made for specified uses. Soil properties that limit the land use or establish the severity of the limitation are usually indicated. Relative suitability of the soil and characteristics that determine the suitability may be given. In addition, soil interpretations may provide displays of soil interpretative evaluations for different uses on an areal basis at scales that pertain to a specific application.

Alternative management decisions can be derived from soil behavior information. For a particular land use, this requires information on soil response to management alternatives, identification of the kinds of management needed, and information about the benefit-to-cost relationship for the management selected.

A number of considerations should be kept in mind in the use of soil survey interpretations:

• An interpretation, such as suitability for septic tank filter fields, has a specific purpose and rarely is adaptable without modification to other purposes.
• Application of interpretations for a specific area of land has an inherent limitation related to the variability in the composition of delineations within a map unit. The limitation is related to how soil surveys are made and to the size of the area of interest relative to that of map unit delineations. These concerns are particularly significant for areas of land for which large capital expenditures are contemplated, as for example, house sites. These areas are usually small relative to the size of map unit delineations and may fall within a dissimilar inclusion of soils that have interpretations completely different from those of the major components of the unit. These concerns are increased for multitaxa units.
• The inherent variability of soils in nature defines the restraints in soil interpretations and the precision of soil behavior predictions for specific areas. Interpretations based on soil surveys are rarely suitable for such onsite evaluations as home sites 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, thereby, they 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 in terms of limitations imposed by one or a few soil properties. The limitations posed by a particular soil property must be considered along with those of other soil properties to determine the property that poses the most serious limitation. Shrink-swell, for example, may be the only limiting soil property for building houses with basements on some soils. Other soils, however, which have high shrink-swell, may also have bedrock at shallow depths. The shallow depth to bedrock may represent a greater limitation than does shrink-swell. Relatedly, some soils have low shrink-swell which is favorable to house sites, but they have limitations because of wetness, flooding, slope, or some other reason.
• Certain considerations that determine the economic value of land are not a part of soil interpretations but are an integral part of developing soil potentials for a given land use. For example, the location of an area of land in relation to roads, markets, and other services is considered by local groups when developing soil potential ratings based on costs to maintain the soil resource versus benefit derived.
• Some interpretations are more sensitive to changes in technology and land uses than others. Crop yields have generally increased over time and new practices may reduce limitations for nonagricultural uses. An example of the latter is the change to reinforced concrete slab-on-ground house construction, which has markedly reduced the limitation of shrink-swell for small building construction. Additionally, new uses of land will require new prediction models for soil interpretations.
• Finally, interpretations based on properties of the soil in place are only applicable if characteristics of the area of land are similar to what they were when the soil mapping was accomplished. Physical movement, compaction or bulking of soil material, or changes in the patterns of water states by irrigation, drainage, or alteration of runoff by construction may require that new interpretations be made.

Interpretative Systematics

Interpretations involve predictions about soil behavior or soil attributes. Interpretations are commonly made separately for all components in the map unit name. A summary rating for the whole unit may also be given.

The generalizations are based largely on a known or obtainable set of soil properties that are maintained or predicted for each kind of soil. These known or obtainable sets of properties or characteristics of soils are used to predict other attributes of soil, such as shrink-swell potential or potential for frost heave. In addition, documented experience with soils having certain sets of properties are used to generalize or predict. These generalizations are commonly formalized in interpretative criteria tables for computer-generated ratings.

The interpretative criteria may range from a narrow set of inferences for specific uses or applications (for example, limitation of the soil for trench-type sanitary landfill) to a highly integrative set of inferences about complex practices that are based on a large number of considerations, only some of which are interpretative soil properties (such as the Land Capability Classification System). The criteria may be based on knowledge of how soils perform under different uses or on research inferences.

Highly integrative generalizations are made for what are called management groups. Groupings of soils may be made for the purposes of various national inventories. These groupings may be highly integrative (as for example, prime farmland) or be based on a few, quite specific criteria (such as highly erodible lands). Because such interpretative groups as prime farmland, highly erodible land, and hydric soils are referenced in legislation, their care has become important in national environmental objectives.

The soil properties selected and the criteria employed for making the interpretative generalizations are applicable to a very wide range of soils on a national basis. The system is not sensitive to locally important differences among soils with the same interpretative placement. For local decisions, relative rankings within the same interpretative placement may be extremely important because frequently the question is how to make the best decision within a locale. Interpretative criteria may have to be adjusted to reflect regional or local peculiarities. Soil potentials attempt to rationalize between the constraints of a national interpretative system and the advantages of making decisions locally on a relative basis.

Management Groups

Management groups identify soils that require similar kinds of practices to achieve acceptable performance for a soil use. In practice, management groups are limited to uses that involve the growth of plants; in theory, management groups could pertain to nonagricultural uses. All of the soils in a management group are not expected to have identical management needs; although, the needs defined for each management group must apply to all of the included soils. The broader the groups, generally the less specific are the descriptions of the management needs. The number of classes for a management group depends on the range of soil properties, the intensity of use, the purpose of the grouping, the audience for which it is intended, and the amount of pertinent information available. The number of classes must balance between the need for homogeneity within a class and the complexity that results from increasing the number of classes. The advantages of management groups can be destroyed by making the classes either so broad that the 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 is the land capability classification system for farming. Other management groups are the woodland suitability groups and range sites. Recently, management groups have been defined for purposes of a national soil inventory. Prime farmland, for example, is a kind of management group. More recent groups include the Fertility Classification System (Sanchez et al., 1982) and methods to group soils based on productivity indices and resilience of soils to certain uses.

National Specific-Use Placements

This section considers nationally developed evaluations for closely defined soil uses. Most of the interpretations for narrowly defined objectives can be stated as either limitations or suitabilities. Some users may prefer an expression that employs both approaches, such as stating the suitability and also listing the limiting properties according to severity or difficulty to overcome.

Historically, limitations have been the favored form for prediction about the interpretations of a soil for a particular use. Actually, the expression of interpretations may take any form that suits the needs of the user. Some users prefer a positive statement with a listing of limiting properties. The septic tank absorption system suitability rating is an example of a prediction based on limitations.

Interpretations that involve the soil as a source of something or as a material (for example, top soil) have been framed in terms of relative suitabilities rather than limitations. In addition to limitations and suitabilities, interpretative generalizations employ the probability of occurrence (such as source of sand and gravel) and a listing of the restrictive interpretative soil properties without a class placement (such as aspects of water management).

Limitation ratings.—Soils may be rated according to limitations for soil uses. Limitation ratings are usually based on hazards, risks, or obstructions presented by properties or characteristics of undisturbed soil. Limitation ratings use terms of severity such as slight, moderate, or severe.

Slight. Presents, at most, minor problems for the specified use. The soil gives 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.

Moderate. Does not require exceptional risk or cost for the specified use, but the soil does have certain undesirable properties or features. Some modification of the soil itself, special designs, or maintenance are required for satisfactory performance over an acceptable period of time. The needed measures usually increase the cost of establishing or maintaining the use, but the added cost is generally not prohibitive.

Severe. Requires unacceptable risk to use the soil if not appreciably modified. 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 severe. 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.

Some soils have such extreme limitations that they should be avoided for certain uses unless no reasonable alternatives are available. Such soils have one or more features that are so unfavorable for the use that the limitation is extremely difficult and expensive to overcome. For example, shallow bedrock or inundation for a long duration are extreme limitations for onsite sewage disposal and for underground utilities. The rating of very severe is sometimes used for such extreme cases.

Suitability ratings.—Soils may be rated according to the degree of suitability for specific uses. Suitability ratings are based on the characteristics of the soils that influence the ease of using or adapting a soil for a specific use. A three class suitability system is commonly used.

Good. Includes soils that have properties favorable for the specified use. Satisfactory performance and low maintenance cost can be expected.

Fair. Includes soils that have one or more properties that make the soil less suitable than those rated good.

Poor. Includes soils that have one or more properties that are unfavorable for the specified use. Overcoming the unfavorable properties requires special design, extra maintenance or cost, or field alteration.

A fourth class, unsuited, is sometimes used for soil or soil material that is unacceptable for the specific use unless extreme measures are employed to alter the undesirable characteristics.

Suitability ratings may also be supplemented with the restrictive features that affect the performance of a soil for a specific use. These restrictive features may be a list of soil properties that are important for a specific use and may be listed with each class for which they apply. An example is, fair—watertable at depths of 25 to 50 cm, 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.

Other generalizations.—Evaluation of limitation or suitability is not feasible for some uses. For sand and gravel as construction material, a kind of soil may be rated only to show the probability of finding the material in suitable quantity. For some uses, the restrictive features may be given without a rating; for example, the features affecting use of the soil for irrigation or drainage of cropland. Commonly, the location is fixed for such soil uses and alternative sites are less of a consideration. The question then becomes what are the problems rather than whether to use the soil for the desired purpose. Merely noting features can be helpful to users, especially if important interactions are recorded.

Local Relative Placements

If interpretations are locally made, it becomes feasible 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 soil potential has been used to describe locally controlled numerical ratings that give the relative ranking of soils for a given use. This is in contrast to the national specific-use interpretative system which emphasizes criteria that apply nationwide and thus are more general than rankings based on local data that include costs to overcome limitations and costs to maintain a system.

The process of determining a soil potential requires an evaluation of the capacity of the soil to produce a crop or support a given structure or activity at a cost expressed in economic, social, and environmental units. Determination of potentials usually cannot be accomplished by soil scientists working alone. In particular, identification of corrective measures requires other disciplines.

Soil potentials for a soil survey area are of greatest value in 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 the plans, or to find another area that has soils with higher potential. The best soils in the specific tract for the particular use may be among those with low potential in the soil survey area overall, although this fact has no bearing on the relative evaluation for the specific tract.

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 properties of the soil. 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, then these soils may be decidedly limiting for foundations. The process of determining soil potentials, which involves the interaction of knowledgeable local people, makes it possible to use more sophisticated criteria than is feasible for the national specific-use program.

Soil potentials are presented either as a set of qualitative, relative classes or in a numerical scale. The first step is to identify for a particular use the soil properties of significance. These properties may be the same as the basic soil interpretative attributes but are not limited to these properties. Critical values for each property are defined and may include properties that are not limiting. For example, the occurrence of free water below 60 cm may not interfere with the production of soybeans; but the critical depth may be 120 cm for the production of alfalfa. Thus, soil potentials are crop and property specific.

The second step in rating soil potential is to identify the corrective measures. Alternatives that appear to be applicable for each soil use should be listed. The most common sources of information are examples where the practices have been successfully used. Soil research and field trials may identify new practices. A single practice may not be fully effective unless it is used in combination with other practices. This may require broadening the definition of a single practice to include interrelated practices. Alternative practices can commonly be substituted. For example, dwellings can be built without basements on integral slab foundations and avoid the necessity of reinforced basement walls.

The third step in rating soil potential is to determine the cost or difficulty of overcoming soil limitations. Relative rather than absolute costs of corrective measurements are generally desirable. If the cost of overcoming the limitation is judged to be prohibitive, the soil is rated to have low potential for that use. Information on cost provides a guide to landowners and local planners.

The fourth step is to identify the limitation that would exist after the corrective measures have been installed. Certain practices are fully successful in overcoming limitations on some soils. Performance is as good as that of soils that do not have the limitations or even better. For other soils and uses, however, no corrective measures are available at an acceptable cost. For many situations, practices may substantially lessen the effects of soil properties, but problems still remain.

To express soil potentials numerically, values are assigned to those soil properties and site conditions that influence performance. Some approaches assign penalty points to limiting soil properties or site conditions, and others assign points based on favorable properties or conditions.

An illustrative formula for the Soil Potential Index (SPI) is:

SPI = P - CM - CL

P is the standard of performance or yield as locally defined. An index value of 100 for P is commonly used, but it may be any value; for example, the potential yield in kilograms per hectare may be employed. CM is an index of cost of the needed corrective measures. Finally, CL is an index of the extent to which the feasible corrective measures are not fully successful. It reflects costs for annual or periodic maintenance, for inconvenience or aggravation, or for substandard yield.

Alternatively, qualitative ratings of soil potential may be employed. An example of a three class set follows:

High potential. The soil meets or exceeds the requirements stipulated in one or more of the following statements:

1. The soil has few limitations, or practices for overcoming the limitations are available at a reasonable cost.
2. Crop production is profitable and at least average for the area.
3. For a particular use, performance of the soil is satisfactory or as good as, or better than, local standards.
4. Environmental quality, both on and off the site, is maintained at a level that is better than the average for the area.
5. After corrective measures have been installed, any continuing soil limitations do not appreciably reduce production, performance, or environmental values.

Medium potential. The soil has a combination of properties intermediate between those qualifying for high potential and those qualifying for low potential. Production is somewhat below local standards, cost of corrective measures is high, or continuing limitations after measures have been installed detract from environmental quality or economic return.

Low potential. The soil has a combination of properties, including one or more of the following situations:

1. Serious soil or site limitations exist and measures for overcoming them are not available or are considered locally to be too expensive.
2. Crop production is substantially below the local average and is economically marginal or submarginal.
3. Performance of the soil is below local standards or below the normal expectations of the land user, even under the best available management.
4. Environmental quality, both on and off the site, is considerably degraded by the use.
5. Serious soil limitations continue to affect the use even after corrective measures have been installed.

A five-class system may be needed. Very high potential distinguishes soils that have few or no limitations affecting the land use. Only standard practices, systems, or designs are needed on these soils; no special practices involving additional cost are needed. Very low potential applies to soils so severely limited that they cannot be made to even marginally perform for the use.

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