Soil Formation and Soil Horizons

Soil Study Areas



Soil Formation and Soil Horizons

Soils develop as a result of the interactions of climate, living organisms, and landscape position as they influence parent material decomposition over time. These five factors are called the factors of soil formation. Differences in these factors influence the soil-forming process. The four major processes that change parent material into soil and develop soil horizons are additions, losses, translocations, and transformations.

Soil horizons, or layers in the soil, can be seen where roads have been cut through hills, where streams have scoured through valleys, or in other areas where the soil is exposed. Where soil-forming factors are favorable, five or six master horizons may be in a mineral soil profile. These master horizons are identified by the capital letters O, A, E, B, C, and R. The thickness of each horizon varies with location. Under disturbed conditions, such as intensive agriculture, or where erosion is severe, not all master horizons will be present. Young soils, such as bottom land soil, have fewer major horizons. Some master horizons may be transitional horizons, such BA, BC, or A/E. These are considered Master Horizons if both letters are capitalized.

Each master horizon may be subdivided into specific subordinate layers that have a unique identity. These subordinate layers are identified by using lowercase letters as suffixes to the master horizons. Nine of these suffixes are commonly used with master horizons in mineral soil materials and three are used with organic (O) horizons. The letters are a, b, e, g, h, i, p, r, s, t, w, and x. They are defined below. The most common subhorizons in West Virginia are e, g, i, p, t, w, and x.

  • Highly decomposed organic material - This symbol is used with O (Oa) to indicate the most highly decomposed organic materials, which have a fiber content of less than 17 percent (by volume) after rubbin

  • Buried genetic horizon - This symbol is used in mineral soils to indicate identifiable buried horizons with major genetic features that were developed before burial. Genetic horizons may or may not have formed in the overlying material, which may be either like or unlike the assumed parent material of the buried soil. This symbol is not used in organic soils, nor is it used to separate an organic layer from a mineral layer. (Examples: Ab, Eb, and Btb)

  • Organic material of intermediate decomposition - This symbol is used with O (Oe) to indicate organic materials of intermediate decomposition. The fiber content of these materials is 17 to 40 percent (by volume) after rubbing.

  • Strong gleying - This symbol indicates either that iron has been reduced and removed during soil formation or that saturation with stagnant water has preserved it in a reduced state. Most of the affected layers have chroma of 2 or less (gray color) as the predominant color of the horizon, and many have redox concentrations (reddish or brownish spots within the gray). The symbol g is not used for materials of low chroma that have no history of wetness, such as some shales or E horizons. If g is used with B, pedogenic change in addition to gleying is implied. If no other pedogenic change besides gleying has taken place, the horizon is designated Cg. (Examples: Bg, Cg)

  • Illuvial accumulation of organic matter - This symbol is used with B (Bh) to indicate the accumulation of illuvial, amorphous, dispersible complexes of organic matter and sesquioxides (oxides of Fe and Al) if the sesquioxide component is dominated by aluminum but is present only in very small quantities. The symbol h is also used in combination with s as "Bhs" if the amount of the sesquioxide component is significant but the color value and chroma, moist, of the horizon are 3 or less.

  • Slightly decomposed organic material - This symbol is used with O (Oi) to indicate the least decomposed of the organic materials. The fiber content of these materials is 40 percent or more (by volume) after rubbing.

  • Tillage or other disturbance - This symbol indicates a disturbance of the surface layer by mechanical means, pasturing, or similar uses. A disturbed organic horizon is designated Op. A disturbed mineral horizon is designated Ap even though it is clearly a former E, B, or C horizon

  • Weathered or soft bedrock - This symbol is used with C (Cr) to indicate cemented layers (moderately cemented or less cemented). Examples are weathered igneous rock and partly consolidated sandstone, siltstone, or shale. The excavation difficulty is low to high.

  • Illuvial accumulation of sesquioxides and organic matter - This symbol is used with B (Bs) to indicate an accumulation of illuvial, amorphous, dispersible complexes of organic matter and sesquioxides if both the organic-matter and sesquioxide components are significant and if either the color value or chroma, moist, of the horizon is 4 or more. The symbol is also used in combination with h as "Bhs" if both the organic-matter and sesquioxide components are significant and if the color value and chroma, moist, are 3 or less.

  • Accumulation of silicate clay - This symbol indicates an accumulation of silicate clay that either has formed within a horizon and subsequently has been translocated within the horizon or has been moved into the horizon by illuviation (moved down from horizon above), or both. At least some part of the horizon should show evidence of clay accumulation either as coatings on surfaces of peds or in pores, as lamellae, or as bridges between mineral grains.

  • Development of color or structure - This symbol is used with B (Bw) to indicate the development of color or structure, or both, with little or no apparent illuvial accumulation of material. It should not be used to indicate a transitional horizon.

  • Fragipan character - This symbol indicates a genetically developed layer (Bx) that has a combination of firmness and brittleness and commonly a higher bulk density than the adjacent layers. Some part of the layer is physically root-restrictive.