The Origins of Soil
Soil Profile
Master Horizons
The action and interaction of soil-forming processes as influenced by parent material, climate, topography, soil biota, and time give rise to distinct soil horizons. The arrangement of soil horizons in a soil is known as a soil profile. Soil horizons are defined by features that reflect soil-forming processes. A horizon is a layer of soil approximately parallel to the land surface which can be differentiated from adjacent layers, or horizons, by identifiable physical, chemical, and biological characteristics. These layers are assigned distinctive alphabetic symbols as a form of shorthand for their characteristics. The six soil horizons are labeled with a letter denotation and are O, A, E, B, C, and R (Figure 2.9). Each of these soil horizon layers are unique.
Soil Horizon O
The uppermost layer, the O horizon, consists primarily of organic material. Forested areas usually have a distinct O horizon. However, in some settings such as a grassland or cultivated field, there may be no O horizon present. Factors such as erosion or constant tillage contribute to the lack of organic matter. The entire soil profile in an organic soil such as peat soil may constitute the O horizon. Two main scenarios result in the formation of an O horizon: saturated, anaerobic conditions (wetlands) or high production of leaf litter in forested areas.
Soil Horizon A
The organically enriched A horizon at the soil surface is sometimes referred to as topsoil. In cultivated soils, the majority of plant roots can be found in the topsoil as that is the zone in which the cultivator can most readily enhance the supply of nutrients, air, and water by mixing in organic and inorganic amendments, loosening the structure, and applying irrigation. In cultivated soils productivity is often correlated with the thickness of the topsoil layer.
Soil Horizon E
The E horizon appears lighter in color than an associated A horizon (above) or B horizon (below). Clay, iron, and/or aluminum are removed from the E horizon via leaching, which causes its light color compared to the adjacent horizons. Leaching is the loss of nutrients from the root zone due to the movement of water through the soil profile. The loss of the above materials is known as eluviation, which entails that these substances and dark minerals have been stripped from the soil particles.
Soil Horizon B
Soil horizon B is the subsoil layer. The B horizon is typically a mineral subsurface horizon and is a zone of accumulation, called illuviation. B horizon usually develops below the A or E horizon, but sometimes E may be absent. In highly eroded soils, the B horizon may be exposed to the surface. Commonly B horizon exists in the middle part of the soil profile. Accumulation of silicate clays, iron and aluminum oxides, and sometimes humus takes place in humid climate.
Soil Horizon C
Below the A and B horizons is the C horizon, a zone of little or no humus accumulation or soil structure development. It is the parent material that has not yet been altered by pedogenic processes. The C horizon often is composed of unconsolidated parent material from which the A and B horizons have formed.
Soil Horizon R
Under the C horizon comes the R horizon, or bedrock. The rocks typically found in this layer include limestone, quartzite, sandstone, basalt, and granite. Depending on the geographic location, environmental conditions, and landscape position, bedrock may be found in excess of 100 feet deep or merely inches or centimeters from the soil surface. In areas where the bedrock is exposed at the surface of the ground, these materials will weather. The materials that weather off of the bedrock become the soil horizon C.
Vertical Subdivisions
Horizons or layer designated by a single combination of letter symbols can be subdivided using Arabic numerals, which follow all the letters. For example, within a C, successive layers could be C1, C2, C3, etc.; or if the lower part is gleyed and the upper part is not, the designations could be C1-C2-Cg1-Cg2 or C-Cg1-Cg2-R. These conventions apply whatever the purpose of subdivision.
Transitional Horizons
In some soil profiles, a horizon may occur in between two master horizons and having characteristics of both the overlying and underlying horizons. Such horizons are transitional horizons and are designated by capital letters of the two master horizons in between those it occurs. Usually, characteristics of one horizon dominate over the other. The first letter indicates the characteristics of the dominating horizon and the second by the subordinate horizon characteristics. Common examples are AB, AE, EB, BE, and BC horizons. The AB horizon, for instance, contains characteristics of both A and B horizons, and it is more like A than B.
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