Soil Taxonomy and Classification

A soil taxonomist distinguishes soils partly on the basis of the kind of diagnostic horizon(s) present in each soil. The current soil taxonomy (classification) was adopted in 1965; a simplified account of this classification system follows below (see US Soil Survey Report 1972, 1975).

Order. This is the most general category. All soils fit into one of ten orders. Suborder. Suborders within a soil order are differentiated largely on the basis of soil properties and horizons resulting from differences in soil moisture and soil temperature. Forty-seven suborders are presently recognized. Great group. Soil great groups are subdivisions of suborders. The 185 great groups found in the US, and 225 worldwide, have been established largely on the basis of differentiating soil horizons and soil features. The soil horizons include those that have accumulated clay, iron, and/or humus, and those that have pans (hardened or cemented soil layers) that interfere with water movement or root penetration.

Subgroup. Each soil great group is divided into three kinds of subgroups: one representing the central (typic) segment of the soil group; a second that has properties that tend toward other orders, suborders, or other great groups (intergrade group); and a third that has properties that prevent its classification as typic or intergrade. About 970 subgroups are known in the United States. Family. Subgroups contain soil families, which are distinguished primarily on the basis of soil properties important to the growth of plants or the behavior of soils when used for engineering purposes. These soil properties used include texture, mineral reactions (pH), soil temperature, precipitation pattern of the area, permeability, horizon thickness, structure, and consistency. About 4,500 families have been identified in the United States. Series. Each family contains several (similar) soil series. The 10,500 or more soil series in the United States have narrower ranges of characteristics than a soil family. The name of the soil series has no pedogenic (i.e., related to soil formation) significance; instead, it represents a prominent geographic name of a river, town, or area near where the series was first recognized. Soil series are differentiated on the basis of observable and mappable soil characteristics, such as color, texture, structure, consistency, thickness, reactions (pH), and the number and arrangement of horizons in the soil pedon as well as their chemical and mineralization properties. Terms describing surface soil texture, percentage slope, stoniness, saltiness, erosion, and other conditions are called phases. Mapping units are created by adding phase names to series names. All mapping units are polypedons. Prior to 1971, soil type was a mapping unit that was used to denote a subdivision of a series indicating the series name and surface texture. Soil type is no longer official nomenclature; it has been replaced by series phase.

The prime land means the best land. The definition of prime land will change depending on the use of the land, and full agreement as to exactly how "prime" should be defined is unlikely, even for a specific land use. For farmland use, it is proposed that prime land should meet all the following requirements: adequate natural rainfall or adequate and good-quality irrigation water for intended use; mean annual temperature >32°F (0°C) and mean summer temperature >46°F (8°C); lack of excessive moisture - flooding should not occur more often than once every two years; water table should be below the rooting zone; soil should not be excessively acidic, alkaline, or saline; soil permeability should be at least 0.38" h-1 (1.0 cm h-1) in the upper 20" (51 cm); the amount of gravel, cobbles, or stones should not be excessive enough to seriously interfere with power machinery; any restricting layer in the soil should be deep enough to permit adequate moisture storage and unhampered root growth, and; the soil should not be excessively erodible.

The objectives of soil surveys and taxonomy are to facilitate growth on soils that have never been grown on before, and to learn enough about certain soils to predict how they would respond when irrigated with a specific quantity of irrigation water of known quality. This objective also emphasizes the inclusion of a rational means of transferring technology from one soil to another, interpretations that allow the prediction of land use for every soil mapped, and that the survey should serve as a scientific database. Soil surveying has developed into a specialized subject. A survey report contains information on not only the characteristics of the soil and its profile, but also the existing and potential uses of the land, the yields obtained by the farmer or by experimental stations under different systems of management, erosion and drainage conditions, and the potential for reclamation or its suitability for irrigation, where these are necessary. Soil maps and survey reports form the basis for planning the utilization of the land, and they have also been found useful in road and building projects.

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