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Soil Survey Manual - Chapter Six (Part 4 of 4)InterpretationsTable of Contents
Areal Application of InterpretationsThe objective of most soil surveys is to provide interpretations for areas delineated on soil maps. This section considers the relationship of interpretations to map unit terminology and conventions, the interpretative basis of map unit design, and the uncertainty of interpretative predictions for specific areas within the map unit. Map UnitsThe purpose of this section is to consider the relationships between the terminology and conventions employed to define and describe map units (ch. 2) and soil interpretations. The components of map units are the entities for which interpretations are provided. The application of interpretative information to areas of land must be through map unit descriptions and 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 pertain to a single, named soil and are applicable throughout the delineation. For associations and complexes the map unit is named for more than one component. For these kinds of map units the interpretations may be given for each named component or may be given for the map unit as a whole, depending on the objective. Information is commonly provided in the description of the map unit about the geographic occurrence of the named components of the map unit on the landscape. 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. Such an application requires, however, additional information beyond what the soil map alone can provide. The illustrative map unit of Bakeoven and Condon soils (app. I) is a complex of two phases of different soil series. The interpretations are applicable to each of the two phases considered separately. To apply these different interpretations separately requires knowledge of the location of each soil within the map unit delineation. The map unit description will provide information as to the location and extent of each named component of the map unit. Map units differ in specificity of the named soils and hence in the broadness of the ranges for various interpretative soil properties. Phases of soil series, for example, are more specific soil concepts than are phases of a higher categorical level. Consequently, in general, the interpretative information for a phase of a soil series has narrower ranges. Similar Soils.—These are soils that 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 named. The following statement from the map unit of Sharpsburg soils (app.I) illustrates the point: —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. These soils are referred to as dissimilar (ch. 2). Usually 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: —medium-textured soil with bedrock at 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 interpretative properties and their location in 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 are significantly affected, the —dissimilar— soil would likely be a named component of the map unit. For map units that are 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 by more than one taxon. For associations and complexes, map units with more than one component, the possibility of different kinds of interpretations are higher than consociations unless the soils are similar. In these units the interpretations may have to be presented on a probability or possibility basis. Where the soils are related to specific landforms or parts of land forms, interpretations can be related to soils and landforms. Areal Extension of InterpretationsThis may be accomplished by interpreting phases of soil series, as has been historically done, or by modifying the interpretative criteria or models to include the probability of occurrence of properties that affect a certain use. Both of the descriptive approaches that follow require the use of geographic information systems and computer technology to perform the necessary calculations and projections of soil properties areally.
Information presented on a probability basis is essential if risk assessment procedures are to be employed in the interpretation of soils for specific land uses. Coupled with a climatic data base, a probability base presents a powerful method from which to predict soil responses and to develop resource management scenarios. Areal GeneralizationThe level of generalization for the application of soil maps and the soil attribute information in soil surveys depends on the scale of the soil map, the taxonomic level used to define the map units, and the combinations of both map scale and taxonomic level. Hole and Campbell (1985) present a detailed discussion of generalizing soil survey information. In the following discussion, these methods of generalization will be discussed. Examples of applications at 3 levels of abstraction are included in the discussion. Map unit information is commonly generalized from the relatively large scale maps in the soil survey reports to smaller scale maps, but phases of soil series are used to name map units. This is done by combining map units according to landscapes or landforms, physiography, use, vegetation, and geology or climate in order to create smaller scale maps. Such smaller scale maps as the general soil maps in published surveys, historically, use associations of soil series to name the map units. Generalization of detailed soil maps can also be accomplished by naming or representing the map units at higher levels in the taxonomic system. Detailed surveys commonly use phases of soil series to name map units. This information, however, can be generalized in successive levels by using families, Great Groups, or Suborders to name the map units. This method is rarely used without an accompanying change in map scale. In addition, generalizations may be made by changing both map scale and level of taxonomic representation. For example, a detailed soil survey (1:24,000 scale) has map units named by phases of soil series. Conversely, a very general map may have small scale such as 1:7,500,000 and map units named at the suborder or order level (highest taxonomic level). Intermediate combinations are possible and must be determined by the purpose for generalizing the information. It may be desirable to have a map scale of 1:7,500,000, but name the map units as associations for phases of soil series. To accomplish this, a method of determining map unit composition from the detailed map must be developed, or the composition is projected from a statistical sampling scheme after Reybold and TeSelle (1989). Interpretive precision is deliberately sacrificed by cartographic or taxonomic generalization. This is done in order to get a summary map that can provide more general information about larger areas. Once cartographic generalization has taken place, the geographic precision has been sacrificed. For example, a 1:63,360 map that shows associations of phases of soil series is generalized from a detailed soil survey. In this case, the range of properties of each component of a map unit is relatively small. Probability statements for limitations, management needs, and performance of each component can be as specific for the 1:63,360 scale map as for the 1:24,000 scale map; although, the map units for the smaller scale map will have more components, thus diminishing geographic precision for the soil interpretations. Conversely, on a 1:63,000 map that shows associations of phases of suborders (generalization of scale and taxonomic level), the range of properties of each constituent is large. Limitations and potentials of each constituent can be predicted only in general terms, and interpretations of their effects on use, management, and performance of the map unit must be correspondingly general. The area of the delineations of interpretative maps should not exceed the area of concern for soil behavior interpretation. Three areas of concern have been given the names operating units, communities, and regions. These terms imply relative size of the delineations for which soil interpretations are needed, not to the area represented by the map as a whole. Operating Units.—These are areas of land that usually are managed as a whole. The most common examples are farms and ranches. The operating units usually range in size from a few hectares to several thousand hectares. In addition to being used by the operators directly, soil maps for such areas are used by farm advisors, credit agencies, planners, and others who are interested in the suitabilities and limitations of soils in individual or contiguous operating units. The map units usually consist of series or associations of soil series. At least two steps are required to interpret the map units. First, the individual kinds of soil are interpreted and rated for a given use. Then the interaction among the soils and the combined effects of all of the soils on the use, management needs, and expected performance of the mapped area are estimated to arrive at a prediction for the map unit overall. Generally, something is known about the local soil pattern from study locations. This information is used in evaluating portions of map unit delineations that are dominated by particular taxa. For soil maps prepared by generalizing Order 1 or Order 2 soil surveys, local associations of soil series can be easily identified and treated as components of map units. Local planners use these maps and interpretations to develop recommendations on alternatives for land use, patterns of services, and public facilities. 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 the amounts and patterns of soils having different potentials within each association can be given in tables or in the text. Communities.—These areas encompass communities, secondary or tertiary watersheds of major local streams, and other large areas. Map delineations may range from as few as 10 to as many as 1,000 square kilometers. The maps are used for regional planning and other purposes that require consideration of areas larger than individual operating units. In developing countries, maps of this kind are used to identify large areas that are suitable for a specific use. The map units are commonly associations of soil families, subgroups, or great groups. The map unit composition is usually quite heterogeneous. Soil properties, consequently, may have a wide range in most delineations. Soil behavior predictions must be general. The basis for the predictions may be intensive studies of relatively small tracts of land that represent extensive map units. Soil behavior can be predicted directly from the taxonomic-based characteristics of named soils. An area might be identified as "Argiustoll-Argiaquoll-Haplustoll association on dissected, undulating loess-mantled plains." For each great group, the characteristics that pertain to soil behavior predictions are recorded and evaluations are made. The use, management, and performance of the map unit as a whole is evaluated based on the proportions and geographic patterns of the great groups. The appraisal for the map unit is necessarily general because much of the local detail is unknown. Regions.—These areas commonly cover continents, large nations, or groups of nations. Soil maps usually have a scale of 1:250,000; although the scale may be as small as 1:1,000,000. The map units are generally associations of soil taxa ranging from the soil series to order levels of taxonomy. These small scale field maps are commonly generalized from soil survey maps at scales of 1:24,000 or larger. The objective of the generalization is to consolidate the soil information of the large areas. For areas that do not have detailed soil surveys, soil maps are made by reconnaissance methods. The information about soils commonly is least abundant and the delineations least precise for Order 5 maps and for schematic maps made without direct field investigations. The units on many maps of regions are associations of suborders, which indicate many soil properties that are important for broad interpretations. The pattern of soil-water states, for example, can be identified or inferred for suborders such as Udults, Ustults, Xerults, and Aquults. The soil temperature can be identified for some suborders, such as Tropepts and Boralfs. This information can be converted to certain broad interpretations—the kinds of plants that would grow well, for instance. If such information as relief, physiography, and parent material is contained in the definition of a map unit, then additional interpretations are feasible. For example, the map unit designation "Tropepts and Udults on maturely dissected basalt plateaus" implies information about soil temperature and the pattern of water states, land surface configuration, extractable acidity at depth, and relative fertility of some of the principal soils. Numerous soil behavior predictions about the map unit can be derived from such information. Behavior prediction at this level must depend heavily on inference. The predictions should be at a level of generalization consistent with the confidence in the original data and in the inferences drawn from them. Soil behavior inferences for map units generalized from more detailed soil maps are more reliable than inferences based on map units formulated without a soil survey. Illustrative Map UnitsThree map units from published soil surveys are illustrated in the appendices: A single soil series (consociation), two soil series as a complex, and a single taxa above the soil series. Tabular information is given for the map units for which the named soils are soil series. Class limits are in either chapter 3 or chapter 6 or are given directly.
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