Soils range in nature from geological material with only a slight amount of change to soils that have been completely altered, and everything in between. One of the most important distinctions between soil and geological material is the formation of the soil profile. When well developed, the soil profile consists of horizontal layers termed 'horizons', which are physically, chemically, and biologically distinct. The highest amount of change from the original geological material is usually at the soil surface, and changes from that original condition diminish with depth.
There are several possible soil horizons, and their presence, absence, and nature is the main way in which we characterize soils. The primary horizons are termed 'master' horizons, and there are secondary or 'minor' horizons as well. Only the master horizons will be described here, and readers are encouraged to consult a soils text for more detailed descriptions of soil horizonation.
In some cases, there are soil 'O' horizons formed from organic litter derived from plants and animal remains on the surface of the soil. The degree to which the organic matter has decomposed determines the designation. Typically an Oi horizon is composed primarily of nonhumified and mostly recognizable organic material. For instance, in a Douglas-fir forest of the US Pacific Northwest, a layer of Douglas-fir needles that are largely intact would be a good example of an Oi horizon. An Oe horizon has undergone a moderate amount of decomposition, and some of the needles would be recognizable but the rest of the material indistinct of origin. A soil Oa horizon would be nearly completely transformed and it would not be clear what the origin of the organic matter was.
Mineral soil horizons contain relatively low amounts of organic carbon, less than 12-18% organic C depending on the soil particle sizes. The soil A horizon is a mineral horizon partially enriched in organic matter that can be translocated from the overlying O horizon or from below-ground plant biomass (e.g., roots). This organic matter is often referred to as 'humus', or a heterogeneous mixture of complex organic compounds that result from the simultaneous processes of decomposition and synthesis of other organic material in the soil. This organic matter in soils can be classified as either humic or nonhumic substances. Humic substances represent approximately three-fourths of the organic matter in soils and are characterized as complex aromatic structures with high molecular weights and a general resistance to microbial decomposition. Conversely, the remaining one-fourth of organic matter in soils is nonhumic substances. Nonhumic substances are recognizable biochemical compounds with lower molecular weights and a higher susceptibility to microbial degradation. Ultimately, however, the fate of soil humus depends on many factors in addition to the composition of the organic matter itself. The pool of soil humus at any one time represents the balance of multiple competing processes of soil organic matter addition (e.g., exudation, deposition) and loss (e.g., leaching, respiration). The net accumulation or loss of organic matter from soils is dependent upon both the physical and chemical stability of the soil humus. The minerals of soil A horizons often differ a great deal from the original parent materials, and interactions between soil particles (especially clays) and soil humus are an important factor in the stabilization of organic matter and the formation of A horizons.
The soil E horizon is a mineral horizon from which iron and aluminum have been 'eluviated', or lost from the soil horizon. Usually E horizons have a lighter color than other soil horizons due to this loss which occurs as organic acids from overlying horizons solubilize, chelate, and leach out the metals which provide soil with much of its characteristic color. This also leads to a decrease in clay content, and normally soil E horizons have coarser textures than either the A or B horizon.
A soil B horizon is a mineral horizon that has undergone processes that result in changes in the physical or chemical nature of the soil from the original parent material. This can result from an accumulation of minerals that have leached out of the A horizon, or changes within the B horizon itself. There is no single change that identifies all soil horizons.
The soil C horizon is little changed by soil-forming processes, and lacks the properties that define either O, A, E, or B horizons. This designation is largely used for physically weathered rock, but also is used in some cases to describe rock that has weathered chemically in very well-developed soils.
Sometimes an R layer is described which is unweath-ered, consolidated bedrock. Cracks in the R layer can sometimes contain more well-developed soil material. Any or all of these horizons and layers can be present and determine to a great extent the properties of the soils as well as their classification. Transitional horizons that have properties of more than one horizon can also be present. For instance, an AB horizon would have properties of both the A and B horizons. Normally the order from top to bottom is O, A, E, B, C, R (Figure 1), but 'buried' soil horizons can exist where soil is either perturbed by inversion or material is deposited onto an existing soil horizon.
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