A soil horizon is a specific layer in the land area that is parallel to the soil surface and possesses physical characteristics which differ from the layers above and beneath. Horizon formation (horizonation) is a function of a range of geological, chemical, and biological processes and occurs over a long period of time. Soils vary in the degree to which horizons are expressed. Relatively new deposits of soil parent material, such as alluvium, sand dunes, or volcanic ash, may have no horizon formation, or only the distinct layers of deposition. As age increases, horizons generally are more easily observed. The exception occurs in some older soils, with few horizons expressed in deeply weathered soils, such as the oxisols in tropical areas with high annual precipitation.
The term 'horizon' describes each of the distinctive layers that occur in a soil. Each soil type has at least one, usually three or four horizons and these are described by soil scientists when seeking to classify soils (Soil-Net). Horizons are defined in most cases by obvious physical features, colour and texture being chief among them. These may be described both in absolute terms (particle size distribution for texture, for instance) and in terms relative to the surrounding material, ie, ‘coarser’ or ‘sandier’ than the horizons above and below.
The term 'horizon' describes each of the distinctive layers that occur in a soil. Each soil type has at least one, usually three or four horizons and these are described by soil scientists when seeking to classify soils (Soil-Net). Horizons are defined in most cases by obvious physical features, colour and texture being chief among them. These may be described both in absolute terms (particle size distribution for texture, for instance) and in terms relative to the surrounding material, ie, ‘coarser’ or ‘sandier’ than the horizons above and below.
The following horizons are listed by their position from top to bottom within the soil profile. Not all of these layers are present in every location – for instance, P horizons only form in areas which have been waterlogged for long periods of time. Soils with a history of human interference, for instance through major earthworks or regular deep ploughing, may lack distinct horizons almost completely. When examining soils in the field, attention must be paid to the local geomorphology and the historical uses to which the land has been put in order to ensure that the appropriate names are applied to the observed horizons.(The one horizon not listed is O horizon which is grass and animal/plant life.) Soil has three main horizons (A, B, and C), which will be explained below along with other layers.
Soil classification:
- O) Organic matter: Litter layer of plant residues in relatively undecomposed form.
- A) Surface soil: Layer of mineral soil with most organic matter accumulation and soil life. This layer eluviates (is depleted of) iron, clay, aluminum, organic compounds, and other soluble constituents. When eluviation is pronounced, a lighter colored "E" subsurface soil horizon is apparent at the base of the "A" horizon. A-horizons may also be the result of a combination of soil bioturbation and surface processes that winnow fine particles from biologically mounded topsoil. In this case, the A-horizon is regarded as a "biomantle".
- B) Subsoil: This layer accumulates iron, clay, aluminum and organic compounds, a process referred to as illuviation.
- C) parent rock: Layer of large unbroken rocks. This layer may accumulate the more soluble compounds .
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