Subsoil is the layer of soil under the

topsoil Topsoil is the upper layer of soil. It has the highest concentration of organic matter and microorganisms and is where most of the Earth's biological soil activity occurs. Description Topsoil is composed of mineral particles and organic mat ...

on the surface of the ground. Like topsoil, it is composed of a variable mixture of small particles such as

sand Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is usually defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a textural ...

silt Silt is granular material of a size between sand and clay and composed mostly of broken grains of quartz. Silt may occur as a soil (often mixed with sand or clay) or as sediment mixed in suspension (chemistry), suspension with water. Silt usually ...

and

clay Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolinite, ). Most pure clay minerals are white or light-coloured, but natural clays show a variety of colours from impuriti ...

, but with a much lower percentage of

organic matter Organic matter, organic material or natural organic matter is the large source of carbon-based compounds found within natural and engineered, terrestrial, and aquatic environments. It is matter composed of organic compounds that have come fro ...

and

humus In classical soil science, humus is the dark organic matter in soil that is formed by the decomposition of plant and animal matter. It is a kind of soil organic matter. It is rich in nutrients and retains moisture in the soil. Humus is the Lati ...

. The subsoil is labeled the B Horizon in most soil mapping systems. Because it has less organic matter than

, subsoil soil colour is mainly derived from iron oxides. Iron oxides and clay minerals form due to weathering. Rainfall moves these weathering products downward as solutes and colloids by rainfall. The subsoil is the depth where these weathering products accumulate. The accumulation of clay minerals, iron, aluminum, and organic compounds is called illuviation. Whereas the topsoil tends to be the depth of greatest physical, chemical, and biological activity, the subsoil is the depth of most deposition. Due to physical, chemical, and biological activity, the subsoil generally has a

soil structure In geotechnical engineering, soil structure describes the arrangement of the solid parts of the soil and of the Pore space in soil, pore space located between them. It is determined by how individual soil granules clump, bind together, and Soil a ...

. The presence of structure distinguishes the subsoil from the underlying substratum. Due to human activity, the topsoil and subsoil in many environments have been mixed. Below the subsoil is the soil substratum (or C horizon). Clay-based subsoil has been the primary source of

material A material is a matter, substance or mixture of substances that constitutes an Physical object, object. Materials can be pure or impure, living or non-living matter. Materials can be classified on the basis of their physical property, physical ...

for adobe, cob, rammed earth, wattle and daub, and other earthen construction methods for millennia. Coarse

, the other ingredient in most of these materials, is also found in subsoil. Although not sterile, subsoil is relatively barren in terms of soil organisms compared to humus-rich topsoil.

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* * Types of soil {{Horticulture-stub

See also

Sources