Soil Clasification
Soil Surveys
Soil survey is an inventory of the properties of the soil (such as texture, internal drainage, parent material, depth to groundwater, topography, degree of erosion, stoniness, pH, and salinity) and their spatial distribution over a landscape (often portrayed in a map). Soils are grouped into similar types and their boundaries are delineated on a map. Each soil type has a unique set of physical, chemical and mineralogical characteristics and has similar reactions to use and management. The information assembled in a soil survey can be used to determine potential uses and limitation of soils. As such, soil surveys can be used to plan the development of new lands or to evaluate the conversion of land to new uses. Soil surveys also provide insight into the kind and intensity of land management that will be needed. For example, farmers and ranchers can use it to help determine whether a particular soil type is suited for crops or livestock and what type of soil management might be required.
Soil Survey Components
Typical information in a county soil survey includes:
The Pedon, a Unit of Sampling
Few soil properties can be determined from the surface. To determine the nature of a soil, one must study its horizons, or layers. This study requires pits or some means of extracting samples of material from the surface to the base of the soil. The visible and tactile properties of samples can be studied in the field.
Limitations of Soil Surveys
Soil survey maps are not without limitations. Although the map may say that a discrete area of land contains a certain soil type, it must be understood that the reliability of that information is a function of the map scale. All soil delineations contain small areas of dissimilar soils that are not identified (called inclusions).
Web Soil Survey
The Web Soil Survey (WSS) is operated by the USDA Natural Resources Conservation Service (NRCS) and provides access to the largest natural resource information system in the world. The WSS is a web application that provides customers (producers, agencies, technical service providers, and others) electronic access to relevant soil and related information needed to make wise land use and management decisions. Users can get information on only what they want by highlighting areas on the map and collect information relevant to their land use concerns. NRCS has soil maps and data available online for more than 95 percent of the nation’s counties and anticipates having 100 percent in the near future.
Soil Maps
A soil map shows the boundaries of the mapping units, with each unit identified by codes that vary from state to state (Figure 3.14). These codes may have one, two, or three parts. The first group of digits or letters (in this instance, letters) refers to the soil series.
Land Capability Classification
Soil surveys and maps provide the basis for placing soils into a variety of capability and limitation classes. Land capability classification classifies soils according to their suitability for various uses, such as agriculture, forest, drain fields, landscaping, or others. The soils are grouped according to their limitations for field crops, the risk of damage if they are used for crops, and the way they respond to management.
Capability Classes
The NRCS recognizes eight land capability classes (Table 3.1) based on the degree of specific limitations of the land such as erosion (e), excess wetness (w), problems in the rooting zone (s), and climatic limitations (c). Capability classes are numbered by Roman numerals I (1), II (2), III (3), IV (4), V (5), VI (6), VII (7), and VIII (8) are used to represent both irrigated and non-irrigated land capability classes. Class I soils have the fewest limitations and Class VIII soils are so limited as to be totally unsuitable for agriculture.
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