Soil Genesis and Classification Volume 143 (Buol/Soil Genesis and Classification) || Interpretations of Soil Surveys and Technical Soil Classification

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    Soil Genesis and Classification, Sixth Edition. S. W. Buol, R. J. Southard, R. C. Graham and P. A. McDaniel. 2011 John Wiley & Sons, Inc. Published 2011 by John Wiley & Sons, Inc.

    Interpretations of Soil Surveys and Technical Soil Classification

    Interpretations of soil surveys are the proof of the pudding as the theories, hypotheses, and knowledge expressed in taxonomic systems are put to the test through applied, practical uses. The best test of our principles and technologies of soil genesis, classification, and surveys is how well they can be applied in the generalization of useful, easily understood descriptions of soil qualities and behavior important for various uses. Technical soil classifications are designed for a specific use and purpose, using criteria and characteristics appropriate for that purpose. These criteria and characteristics are generally a restricted subset of the properties used in the natural classification system.

    Soil properties can be interpreted in the context of soil quality. Soil quality is a basic, intrinsic characteristic of a soil that is a reflection of several soil properties and cannot be directly characterized in one measurement, but ordinarily is estimated from a number of different measurements and/or observations. The concept of soil quality continues to evolve, but soil properties and environmental factors that are often included in an assessment of soil quality include soil fertility, potential productivity, resource sustainability, environmental quality, contaminant levels and their effects on use, the environmental cost of agricultural production, and the potential for reclamation of degraded soil (Kellogg 1961; Singer and Ewing 2000).

    Soil Survey InterpretationsSoil survey interpretations help predict potentials, limitations, problems, and management needs for soils (Kellogg 1966; Soil Survey Division Staff 1993). Aandahl (1958) defined soil survey interpretation as the organization and presentation of knowledge about characteristics, qualities and behavior of soils as they are classified and outlined on maps. Soil survey interpretations are prepared to help land users, planners, policy makers and analysts, legislative officials, engineers, and other scientists to transfer technology about the use and management of soilsboth agricultural and nonfarmmore accurately. These interpretations are made to accompany or be used with detailed soil surveys and larger-scale maps prepared by the generalization process previously described (Chapter 20), or they are made to be


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  • 426 Soil Genesis and Classification

    used with the schematic or technical single-purpose maps (Soil Survey Staff 1978, 1983a; USDA-NRCS 2009).

    Interpretations of detailed soil surveys (those that have not been generalized) have a number of uses (USDA-NRCS 2009): indicating soil potentials for various land uses, pointing out limitations and potential problems with soil qualities (especially for conservation and environmental protection), predicting potential production of soils under various management systems, indicating types of management needed for different soils, and recasting the technical soil information into forms and terminology useful in other scientific disciplines and to land users. The soil information used for these detailed interpretations is contained in published county soil surveys, in digital soil surveys at the NRCS Web Soil Survey site (, or in the USDA-NRCS Soil Survey Geographic (SSURGO) Data Base ( For statistical evaluation of crop response on adjacent mapping units, experimental designs that consider the soil patterns within fields are required (Nelson and Buol 1990).

    Interpretations of generalized large-scale maps are usually designed for uses such as regional planning, setting priorities for action programs, depicting regional problems in conservation and environmental protection, estimating productivity on a regional basis, preparing and implementing regional development programs, determining feasibility and potential benefits of proposed irrigation and watershed programs, and assessing regional environmental protection needs and concerns. The soil information used for these regional interpretations is contained in soil maps generalized from county soil survey maps or in the USDA-NRCS State Soil Geographic (STATSGO) Data Base (

    Interpretations are prepared for taxonomic soil components of soil map units or of the soil map unit as a whole. Most commonly they are prepared for phases of taxonomic units (such as slope and eroded phases) if used with a detailed survey or with maps generalized according to the graphic generalization strategy. If used with taxonomically generalized, schematic, or technical maps, interpretations are generally prepared for mapping units, but may also be prepared from the properties of dominant taxonomic components identified in the map unit. As indicated in Chapter 20, both the preparers and the users of soil interpretations need to be aware of, and cautioned about, the differences between taxonomic units and soil mapping units. This is especially true if taxa with different interpretation requirements are included in a soil map unit. This is particularly important when the interest focuses on small sites such as house lots smaller than 1 ha.

    Interpretive Uses of Taxonomic Information and Soil MapsSome interpretations are for specific soil materials present in soil horizons, but most soil uses involve an area of land. For these uses, the nature and composition of map units, as discussed in Chapter 20, are the basis of soil interpretations. Soil map and

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    taxonomic information is put to a number of agricultural and nonagricultural interpretive uses (Bartelli et al. 1966; Simonson 1974). For agricultural uses these include recommended conservation practices; identification of prime farm land; predictions of erosion losses under various cropping systems; development of productivity ratings of soils; soil potentials and crop yield; land evaluations and tax assessments; soil and farm management recommendations, including those on application of fertilizer; programs for protection of environmental quality from damage by sedimentation and agricultural chemicals; and improvements in agricultural energy conservation. Nonagricultural interpretations of soils are many, and include forest, range, and wildlife management; mined land reclamation; local, state, and regional planning, zoning, and other land use concerns; identification of hydric soils, suitability for locating septic tank filter fields and for municipal sewage effluent and sludge disposal; environmental quality protection with respect to off-farm sediment, dust, and pollution control; local road and highway route location, location of pipelines and power lines; building and real estate development site location; suitability as sources of building materials; valuation of land for tax assessment purposes; and planning the location and layout of outdoor recreation facilities, including parks, campgrounds, paths, and trails.

    For these purposes it is more desirable and economical to prepare comprehensive soil maps as a base from which the interpretations can readily be made. If a single-purpose technical classification were to be used as the basis for soil mapping, it is highly probable that such a classification would not be suitable for other types of planning and site evaluation, such as the kinds we have described. Then the mapping and classification would need to be redone at considerable unnecessary expense to satisfy each new purpose. A well-prepared soil map, based on a sound general classification system that has quantitatively defined taxa, is useful over a long period of time as a base for a number of interpretation activities for technical and specific objectives, including those not yet foreseen. Such an approach allows (even forces) consideration and description of the dynamic aspects of each soil and the relating of the soils to various uses.

    Technical Soil Maps and Classifications. We distinguish between technical soil maps, classifications, and surveys on the one hand and comprehensive standard soil classification, maps, and surveys on the other. The technical maps and classifications, prepared either by interpretation from the standard maps and classifications or spe-cially and separately prepared, are designed for a specific use and purpose, using criteria and characteristics appropriate for that purpose. The comprehensive standard maps and classifications are general and multipurpose in nature with criteria and char-acteristics selected according to scientific and taxonomic principles. They are capable of being interpreted for any number of specific, single-purpose objectives and uses.

    It is important to understand that there are an almost infinite number of technical systemsone for each particular use of soil. Anyone can make a technical map and classificationeither by interpreting existing standard comprehensive scientific

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    surveys and classifications or by a separate soil survey with accompanying technical classification. In order to be useful and sound, such technical systems must, however, use a quantitative nomenclature with class limits based on measurable properties of the soils and/or of the environmental factors that influence soil behavior. The discussion that follows relates to preparation of technical systems interpreted from the comprehensive scientific classification and accompanying standard soil mapsthe process that we recommend over special purpose technical soil surveys.

    Development of Technical Soil Interpretations. The preparation of interpretations by soil scientists of the National Cooperative Soil Survey ordinarily involves the following four steps (Soil Survey Division Staff 1993): (1) assembling information about the soils and landscapes, (2) modeling other necessary soil characteristics from existing soil data, (3) deriving inferences, rules, and guides for predicting behavior under specific land uses, and (4) integrating these predictions into generalizations for the map unit.

    The essential ingredients in the development of soil interpretations are a clear definition of the specific soil use and quantitative expression of the influence of each applicable soil property on the evaluation or interpretation of a soil, or mapping unit, for that specific use. Each technical soil use (of which there is an infinite number) can and should have its own technical classification in order to facilitate communication between soil scientists and the specialists in the particular subject matter area of the soil use. The soil data used to develop the interpretations are contained in the National Soil Information System (NASIS) as described in the National Soil Survey Handbook (USDA-NRCS 2009). The NASIS database is managed by the USDA-NRCS and is the repository of the attributes of the taxonomic components of soil map units and the geo-referenced site information and pedon data. The database is populated with actual soil property data from pedon descriptions, laboratory characterization, and map unit characteristics, and with inferred soil properties derived from the actual data. Most of the NASIS data are available via the Soil Data Mart ( Laboratory data and pedon descriptions of sampled and characterized pedons are from a separate database maintained by the National Soil Survey Center in Lincoln, Nebraska ( or

    Because it is not possible for anyone to anticipate all of the possible uses of soils information, the following example of a soil interpretation based on criteria outlined in the National Soil Survey Handbook (USDA-NRCS 2009) and on data contained in NASIS, is presented to illustrate the procedure for developing a soil interpretation.

    Consider that you are confronted with the question: What makes a good picnic area? In order to address the question clearly, a number of broad criteria must be established first. For example, will only hikers use the picnic area or will vehicles bedriven to the area? Are garbage facilities to be provided for on the site, or will the garbage be exported? Will extensive revegetation of the site be required following construction activities? Thus, providing a clear definition of the use must be addressed first. In this example, we will use the following definition of picnic areas: picnic areas

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    are natural or landscaped tracts used primarily for preparing meals and eating out-of-doors. These areas are subject to heavy foot traffic, and vehicular traffic is confined to access roads and parking lots. The soils are rated on the basis of soil properties that influence development costs of shaping sites or building access roads and parking areas, trafficability, and growth of vegetation after development.

    The soil properties of concern would appear to be those providing good trafficability: the surface soil in the picnic area should absorb rainfall readily, remain firm to heavy foot traffic, and not be dusty when dry. The second step is to place quantitative limits on the soil properties that should be considered as restrictive. Table 21.1 illustrates one guide to this soil use. The severity of the limitations is given, as well as a common or general expression of the problem. Perhaps most important, however, are the quantitative critical limits that will document the basis for the interpretation should the soil scientist or user be called upon to defend his or her decisions. In this example, the three categories of limitationsslight, moderate, or severeare used. The implication is that the relative cost of developing a picnic area would probably be least for locations with slight limitations and greatest for locations

    Table 21.1. Evaluation of sites for plenic areas

    Limits Restrictive

    FeatureProperty Slight Moderate Severe

    Slope (%) 08 815 > 15 SlopeFlooding None to

    occassionalFrequent Flood hazard

    Depth to high water table (cm)

    > 75 3075 < 30 Wetness

    Fraction 275 mm (i.e., wt % of surface layer)

    < 25 2550 > 50 Small stones

    USDA texturea,c (surface layer)

    SC, SIC, C Too clayey

    USDA texture (surface layer)


    COS, S, FS Too sandy

    USDA texture (surface layer)

    Muck, peat Excess humus

    USDA textureb (surface layer)

    SIL, SI, VFSL, L


    Soil reaction (pH) (surface layer)

    < 3.5 Too acid

    Sodium adsorption ratio > 13 Excess sodiumPermeabilityc

    (cm/hr)(01 m)> 1.5 0.151.5 < 0.15 Slow percolation

    a Texture abbreviations: LCOSloamy coarse sand, COScoarse sand, Ssand, FSfine sand, LFSloamy fine sand, VFSvery fine sand, SCsandy clay, SICsilty clay, Cclay, SILsilt, Lloam, VFSLvery fine sandy loam, LSloamy sand.

    b Disregard unless soil is in Torr, Arid, or Xer suborder, great group, or subgroup.c Soils in Ust, Tor, Arid, or Xer suborders, great groups, or sub groups rate one class better.

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  • 430 Soil Genesis and Classification

    with severe limitations. Potential suitability for a specified use generally includes categories such as high, medium, low; or good, fair, and poor.

    Alternative representations of the limitations or suitability include a verbal rating or a numerical score. The verbal rating no limitations means that the soil has features very suitable for the specified use. A rating of limitations means that the soil has some features that are favorable for the specified use and some that are unfavorable. The interpretation identifies the most significant limitations for any given soil. The limitations can be overcome or minimized by special planning, design, or installation. Fair to poor performance and moderate to high maintenance costs can be expected, depending on the number of limitations and the severity of each limitation. When a numerical score is provided, the rating is provided as a decimal fraction between 0 and 1.00. A score of 0 means that the soil feature is not a limitation (not limited) for the specified use; a score of 1.0 means that the soil feature has the greatest negative impact (very limited) on the specified use.

    Interpretations of Soil Material in Soil Horizons. The following soil properties, qualities, and behaviors are included in most soil survey interpretation tables (USDA-NRCS 2009):

    Soil Material Properties: textural class, stone and gravel percent, soil reaction (pH), salinity, high-water table levels, cemented pans, bedrock depths.

    Qualities and behaviors: Unified Soil Classification or American Association of State Highway and Transportation Officials (AASHTO) classification of liquid and plastic limits, permeability, available water, shrinkswell, corrosivity, erodibility, subsidence, hydrologic soil group, and potential frost action.

    Although soil characteristics important for civil engineering are included in the above list, on-site investigations are necessary on small tracts for which planned use is intense such as construction of heavy buildings, main highways, and large waste disposal sites. This is not only because of the risk of soil spatial variability but also because of the need for licensed engineers to make the interpretations and recommendations for these types of construction. And, as pointed out by Lindsay et al. (1973), soil survey information does not eliminate the need for deep borings supervised by engineers and other specialized tests as part of on-site investigations.

    Interpretations of Soil Map Units. In preparing interpretations and giving advice on their use, keep in mind the minimum decision area concept (Doucette 1983). Aminimum decision area is the smallest feasible size and shape for which a particular management system or land-use alteration can be applied. For example, it could be that the smallest possible area on which a septic tank system could be installed is about 500 m2 (0.12 acre), whereas the smallest area for which it is feasible to design a subsurface soil drainage system for agricultural purposes is approximately 12,150 m2

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    (3 acres). Interpretations of soil map units consider not only the properties of the named soils in that map unit but also the properties of included soils, the slope of the land, and permanent management technologies such as engineered drainage systems. Interpretations related to soil map unit delineations include crop-forest-range production under specified levels of management, flood frequency, and suitability for recreational facilities and wildlife management.

    Examples of Technical Soil Classification Systems. In addition to the soil interpretations described above, each of which can be considered a technical classifi-cation system, a number of more comprehensive technical soil classification systems have been developed that focus on specific land uses. We briefly describe three of them here: the USDA Land Capability Classification system, the Storie Index (for agriculture and timber), and the Fertility Capability Classification system.

    The Land Capability Classification (LCC) system groups arable lands based on potential and limitations for sustained production of commonly cultivated crops that do not require special site conditioning or treatment (Klingebiel and Montgomery 1966). Nonarable lands are grouped based on their ability to support permanent vegetation and on the risk of soil damage if mismanaged. The classification system incorporates both climate and soils information and rates lands for irrigated and nonirrigated management systems. At the most general level, landscape (map) units are grouped into one of eight classes (I is highest; VIII is lowest). Lands in classes Ithrough IV are arable; lands in classes V through VIII are nonarable.

    Class I lands have few limitations for cultivated crop production. Soils are on nearly level parts of the landscape, rainfall or irrigation provides sufficient water, the frost-free season is long enough to allow crops to mature, and salts, flooding, or a high water table do not occur. Class II lands have some limitations, and careful management is required to maintain productivity, but management practices are easy to apply. Class III lands have severe limitations and require special management and crops to maintain productivity. Class IV lands have very severe limitations that generally restrict the range of crops that can be grown and that require intensive management to maintain productivity. Limitations may include, among others, slope, soil thickness, salinity, high water table, and low water-holding capacity. Class V lands have level soils with special management problems (for example, flooding, short growing season, stony and rocky soils, and areas with stagnant surface water that cannot be drained) that limit their use to pasture, range, woodlands, and wildlife habitat. Class VI and VII lands have management problems that restrict use for range, pasture, and woodland. Class VIII lands are nonagricultural lands that are generally used for watershed, recreation, and wildlife.

    The next categorical level is the subclass, which groups classes on the basis of similar kinds of limitations. The subclasses include e where erosion is the main risk; w where wetness caused by impeded drainage or overflow is the main problem; swhere soil limitations, such as shallow soil, salinity, or low water-holding capacity, are the main problems; and c where a climatic limitation is the main problem and when e, w, and s are not appropriate.

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  • 432 Soil Genesis and Classification

    The lowest categorical level is the capability unit. Each state within the United States has its own mechanism for identifying the problems that are most limiting. Generally speaking, the soils in one capability unit are enough alike to be suited to the same crops and pasture plant, to require similar management, and to have similar productivity. Soils in Class I do not have a subclass or capability unit because there are no major limitations. The data elements that drive the LCC ratings are found in NASIS. Details of the LCC criteria are found in Exhibit 622-2 in the National Soil Survey Handbook (USDA-NRCS 2009).

    The Storie Index Rating (Storie 1933) is a quantitative, factorial productivity rating system that was developed for irrigated row crop production in California. This index expresses numerically the relative degree of suitability of a soil for general intensive agricultural uses at the time of the evaluation. The rating is based on soil characteristics and is obtained by evaluating soil surface and subsurface chemical and physical properties, as well as landscape surface features. Not considered in the rating are availability of water for irrigation, local climate, size and accessibility of mapped areas, distance to markets, and other factors that might determine the desirability of growing certain plants in a given location. Therefore, the index cannot be used as theonly indicator of land value. Where the local economic and geographic factors are known to the user, however, the Storie index may provide additional objective information for land tract value comparisons.

    Four general factors are used in determining the index rating: Athe permeability, available water capacity, and depth of the soil; Bthe texture of the surface soil; Cthe dominant slope of the soil body; and Xother conditions more readily subject to management or modification by the land user. These other conditions may include drainage, flooding, salinity, alkalinity, fertility, acidity, erosion, and microrelief. For some soils, more than one of these X conditions is used in determining the rating. Arating of 100% expresses the most favorable condition for general crop production. Lower percentage ratings are assigned for less favorable conditions. Factor ratings, in percentages, are selected from tables prepared from data and observations that relate soil properties to plant growth and crop yields. Certain properties are assigned a range of values to allow for variations in the properties that affect the suitability of the soil for general agricultural purposes.

    The index rating for a soil component of a map unit is obtained by multiplying the percentage rating values (expressed as a decimal fraction) given to its four factors, A,B, C, and X. If more than one condition is recognized for the X factor for a soil, the value for each condition acts as a multiplier. Thus, any of the A, B, C, or X factor conditions may dominate or control the final rating. If a map unit consists primarily of one named soil series (a consociation), the index rating for the named soil component equals the index rating for the map unit. If a map unit consists of more than one named component (a complex or association), ratings are assigned to each named component (soil series or miscellaneous area, such as Rock outcrop), and a weighted map unit index is calculated from the component indexes and the proportion of each of the named components in the map unit. Miscellaneous areas are considered

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    to be unsuited for agriculture, and are assigned a rating of zero. Inclusions of other soils, not named in the map unit name, are ignored in the calculations.

    The final decimal product of the mutliplied factors is converted to a percentage value between 0 and 100. Map units are assigned grades according to their suitability for general intensive agriculture as shown by their Storie index ratings. The six grades and their range in index ratings follow:

    Grade 1: 80 to 100Grade 2: 60 to 80Grade 3: 40 to 60Grade 4: 20 to 40Grade 5: 10 to 20Grade 6: 0 to 10

    Grade 1 soils are well suited to intensively grown irrigated crops that are climatically adapted to the region. Grade 2 soils are good agricultural soils, although they are not as desirable as soils in grade 1 because of a less permeable subsoil, deep cemented layers (such as, duripans), a gravelly or moderately fine textured surface layer, moderate or strong slopes, restricted drainage, low available water capacity, lower soil fertility, or a slight or moderate hazard of flooding. Grade 3 soils are only fairly well suited to agriculture because of moderate soil depth; moderate to steep slopes, restricted permeability in the subsoil; a clayey, sandy, or gravelly surface layer; somewhat restricted drainage; acidity; low fertility; or a hazard of flooding. Grade 4 soils are poorly suited. They are more limited in their agricultural potential than the soils in grade 3 because of restrictions, such as a shallower depth; steeper slopes; poorer drainage; a less permeable subsoil; a gravelly, sandy, or clayey surface layer; channeled or hummocky microrelief; a hazard of flooding; or low fertility; salinity or alkalinity, or acidity. Grade 5 soils are very poorly suited to agriculture and are seldom cultivated. They are more commonly used as pasture, rangeland, or woodland. Grade 6 soils and miscellaneous areas are not suited to agriculture because of very severe limitations. They are better suited to limited use as rangeland, protective wildlife habitat, woodland, or watershed.

    A computerized calculation of the Storie Index has been developed to eliminate some of the subjectivity encountered during manual calculation of the Index (OGeen and Southard 2005). The calculation is driven by data from NASIS, and utilizes discrete and fuzzy logic functions as a means for assigning numerical scores to soil properties (OGeen et al. 2008).

    Storie and Wieslander (1948) developed a Timber Rating using a similar quantitative, factorial approach focused on commercial conifer species of the Sierra Nevada, Cascades, and northern Coast Ranges of California. Five factors are evaluated: Asoil depth and suitable texture, Bsoil permeability, Csoil chemical characteristics (alkalinity, salinity), Dsoil drainage and run off, and Emean annual precipitation. Numerical values are derived from tables based on the relationship

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  • 434 Soil Genesis and Classification

    between tree growth (height-age site index) and site properties as determined from extensive field measurements. The product is expressed as a percentage (up to 120% to accommodate coastal redwoods). Ratings are assigned based on the index rating percentage:

    Redwood: 98 to 120High: 75 to 98Medium: 50 to 75Low: 30 to 50Nontimber: 0 to 30

    Soils rated as Redwood are well suited to intensively grown timber that is climatically adapted to the region. High soils are good timber soils, although they are not as desirable as soils in the Redwood category because of a less permeable subsoil, deep cemented soils layers, a gravelly or moderately fine-textured surface layer, restricted drainage, low available water capacity, or less rainfall. Medium soils are only fairly suited to timber harvesting because of soil depth; restricted permeability in the subsoil; a clayey, sandy, or gravelly surface layer; somewhat restricted drainage; low fertility; flooding hazards; or low rainfall rates. Low soils are very poorly suited for timber production and are more commonly used for pasture, rangeland, or recreational purposes. Nontimber soils and miscellaneous areas are not suited for timber production because of very severe soil or climatic limitations. They are better suited for use as rangeland, wildlife habitat, or watersheds.

    The Fertility Capability Soil Classification (FCC) system is a technical system for grouping soils according to the kinds of problems they present for agronomic management of their chemical and physical properties (Buol et al. 1975; Sanchez etal. 1982). This interpretive system uses readily measurable topsoil characteristics and some subsoil properties important for plant growth. The system is designed to interrelate and tie together the subdisciplines of soil fertility and soil classification by grouping soils according to properties that are related to the response of a soil to fertility management practices. It is best used in conjunction with soil testing practices. The system has three categorical levels: the type (topsoil texture), substrata type (subsoil texture), and 15 modifiers. The modifiers systematically identify soil properties near the soil surface that agronomists use to predict expected responses of individual kinds of soil to fertility related practices. Some examples of modifiers are a: aluminum toxicity (such as, >60% exchangeable aluminum saturation of the effective cation exchange capacity; e: low (ECEC) within 60 cm of soil surface; i: P fixation by iron oxide surfaces; x: P fixation by short-range order (amorphous) aluminum-silicates; g: reducing conditions and potential denitrification. The system uses quantitative ranges of soil properties but accepts modifications to accommodate localized management technologies. It can be used for regional evaluations of fertility requirements when applied to soil survey map units using Soil Taxonomy, FAO world soil maps, or other quantitative systems of soil classification.

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    PerspectiveSoil survey interpretations apply the information and knowledge contained in a soil survey to the evaluation and prediction of the suitabilities, limitations, and potential of soil materials and landscape units for a variety of specific uses. The interpretations are prepared to help land users, public officials, land use planners, scientists, and technologists make better decisions about soil resources. Interpretations are organized in a variety of use-specific technical soil classifications, which rely on a subset of soil properties contained in the natural soil classification system. Soil survey interpretations for the National Cooperative Soil Survey are developed from data contained in the National Soil Information System. This database is populated with actual and derived soil data from pedon descriptions, laboratory characterization, and map unit characteristics. Interpretations are made for a wide variety of soil uses, including agricultural, environmental, and land use-planning purposes. Interpretations are made for both soil materials and areas of land occupied by populations of soils and depicted as map unit delineations on soil maps. The minimum decision area concept is a useful guideline for interpretations of soil map unit delineations. The Land Capability Classification System, the Storie Index Rating, and the Fertility Capability Classification System are examples of technical soil classification systems that use soil and map unit properties to rate soils for agricultural and timber production purposes. Most modern soil survey information is stored in computer databases that are readily accessible online.

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