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9.5: Soil Classification

  • Page ID
    14474
  • Soils in the United States are classified according to the USDA Soil Taxonomy. Soil Taxonomy includes the system of soil classification published by the Soil Survey staff of the U.S. Department of Agriculture’s Natural Resource Conservation Service (NRCS), formerly the Soil Conservation Service (SCS). This system provides a comprehensive natural classification of soils based upon measurable and observable soil morphological properties. The current Soil Taxonomy is the result of many years of observations on many different soils. Probably the greatest utility of the taxonomy is for soil management and land use planning. The nomenclature used is designed to fit into any modern language. Terms were coined mainly from Greek and Latin roots, and are used as mnemonic devices for remembering the names. Many soil properties can be described by the use of the formative elements. This system of classification is hierarchical with 12 Orders subdivided into 47 Suborders, 225 Great Groups, 970 Subgroups, more than 5,000 Families and more than 15,000 Series.

    The Orders (the highest level of the classification) are distinguished on the basis of properties associated with soil-forming processes on the grand scale. One soil Order is based on the absence of any distinctive characteristics. The soils in these other 11 Orders have some diagnostic features known as diagnostic surface horizons and diagnostic subsurface horizons.

    The diagnostic surface horizons are called epipedons, from the Greek words epi (over), and pedon (soil). The epipedon includes the upper part of the soil darkened by organic matter, the upper eluvial horizons, or both. Six epipedons are recognized, but only two (Mollic and Ochric) are common near Laramie, WY.

    Mollic A horizon—thick ( >7 inches thick ), dark (Munsell value darker than 3, moist chroma) surface horizon with a high percent ( > 50 %) basic cation saturation, high in calcium, good structure, and not both hard and massive when dry. The horizon contains at least 1 % organic matter.

    Ochric

    A horizon too light, too low in organic matter, or too thin to be mollic; or both hard and massive when dry.

    The diagnostic subsurface horizons are called endopedons and characterize different soils in the system. Several subsurface horizons are described in the U. S. Soil Taxonomy. Two of these (Argillic and Cambic) are common in the Snowy Range Mountains area. Not all soils have a diagnostic subsurface horizon.

    Argillic

    This is a Bt horizon of illuviation (accumulation) of clay. If the soil is not eroded, this horizon is in the position of the B horizon and it has more clay than the overlying A or E horizon. The Argillic horizon is generally equivalent to the Bt horizon. Field clues for identifying argillic horizons include the following factors:

    Higher in clay than the horizon above and generally higher than horizon below; horizon has soil structure, often being blocky or prismatic, clay films visible to the naked eye or with hand lens. Clay films are smooth, shiny clay coatings on the surface of the soil peds or aggregates.

    Cambic

    This is an altered or changed horizon in the position of a B horizon (an A horizon above and a C horizon below). This horizon has been changed by internal physical movement or by chemical reactions to such an extent it no longer retains the original nature of the rocks or sediments in the C horizon. Some chemical changes have occurred, but some easily weathered minerals are usually still present.

    Usually, some leaching has occurred in the Cambic horizon and color changes have resulted, but no evidence exists of any clay movement or illuviation (i.e. clay films). The cambic horizon is generally equivalent to a Bw horizon.

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