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Soil Profile

The soil profile is one of the most important concepts in soil science. It is a key to understanding the processes that have taken in soil development and is the means of determining the types of soil that occur and is the basis for their classification. The soil profile is defined as a vertical section of the soil from the ground surface downwards to where the soil meets the underlying rock. The soil profile can be as little as 10 cm thick in immature soils and as deep as several metres in tropical areas where the climate is conducive to rapid alteration of the underlying rock to form soil. In temperate areas, the soil profile is often around a metre deep and in arid areas somewhat shallower than this.

Virtually all soil profiles are composed of a number of distinctive layers, termed horizons, interpretation of which is the key to understanding how the soil has formed. Most soils will have three or more horizons. Soils that have not been cultivated will normally have L, F and H layers at the surface. These layers largely represent different degrees of decomposition of organic matter, the L layer representing the litter layer formed of recognisable plant and soil animal remains, the F layer below, the fermentation layer, usually consisting of a mixture of organic matter in different stages of decomposition, and the H layer, the humose layer, consisting largely of humified material with little or no plant structure visible. Below these, and in cultivated soils occupying the surface layer, is the A horizon composed of a more or less intimate mixture of mineral and organic matter. The A horizon is often referred to as the ‘ploughed layer’ in cultivated soils. It is an important part of the soil because it is a source of plant nutrients and contains the majority of plant roots. The A horizon may lie directly on the B horizon or, in well developed soils, there may be an intermediate leached horizon, termed E or A2, depending on the nomenclature system used. The E/A2 horizon is usually paler in colour than the horizons above and below because it is a horizon that has been subject to leaching and loss of components compared to the A and B horizons.

The B horizon is the horizon most widely used to identify soil types. Its morphology is important in supporting the classification of soils. In some soils the B horizon results purely from the weathering of the underlying rock whereas in other soils this weathering is supplemented by the translocation of materials from overlying horizons. Thus the B horizon needs to be inspected carefully in order to understand the genesis of the soils. B horizons may have a number of different subscripts indicative of the nature of the materials that have moved into the horizon, e.g. Bh indicates the translocation of humus into the horizon, Bs, the translocation of sesquioxides. These subscripts will vary according to the nature of the soil component that has accumulated but also with the system of nomenclature of soil types (See Section on Soil Types).

Below the B horizon is the C horizon. This latter horizon is often consistent with the parent material and may have been little altered from the material in which the soil originally formed.

Most soils have A, B and C horizons. Some, generally weakly developed, soils may have A horizons lying directly on C horizons. When next you see a profile down through the soil, perhaps in an excavated pit or in a roadside cutting, take time to look at the profile and see if you can identify some of the different soil layers that make up the profile. (For some examples of soil profiles and soil types, see section of Soil Types).