Subsoil the layer below the topsoil which has been changed from its original state (of alluvium or rock) by soil forming processes. In other words subsoil is the layer below plow layer. The B-horizons are sometimes referred to incorrectly as the subsoil. Depending on the depth of A and E horizons the upper part of the B horizon sometimes is included in the plow layer. In that case, B horizon is subsoil. The subsurface B-horizons include layers in which illuviation of materials has taken place from above and even from below. In humid regions, the B-horizons are the layers of maximum accumulation of materials such as iron and aluminium oxides and silicate clays. In arid and semiarid regions, calcium carbonate, calcium sulphate, and other salts may accumulate in the same horizon.
Subsoil is not seen from the surface and is not commonly disturbed by soil tillage but there are a few landmasses that are not influenced by subsoil characteristics. But crop production certainly is affected by root penetration into the subsoil and by the reservoir of moisture and nutrients it represents; likewise, downward movement of water is sometimes impeded by impervious subsoil. The consequent wetness is detrimental to the growth of most crop plants. Areas with impervious subsoil should be avoided in the selection of building sites and the location of roads.
Subsoil of Non-calcareous Brown Floodplain Soils of Bangladesh has yellowish-brown, medium acid to neutral tendency, while the subsoil of Grey Floodplain Soils are usually 15-50 cm thick and are neutral to mildly alkaline, and the subsoil of the Eastern Surma-Kushiyara Floodplain, Chittagong Coastal Plain and some Middle Meghna Floodplain Soils are slightly to moderately acid. Alluvial stratification is usually absent in the subsoil (usually 30-60 cm thick) of Calcareous Dark Grey Floodplain Soils. The subsoil of acid basin clays is strong to extremely acid. Grey Piedmont Soils along the hill ranges of Sylhet have redder subsoil. Deep Red-brown Terrace Soils grade into a reddish-brown to yellow-brown friable subsoil. Shallow Red-brown Terrace Soils have a subsoil 30-60 cm thick of firm clay in redder soils and porous loam in yellow-brown soils. The subsoil of brown hill soils is one-third to one-meter thick and strong brown to yellow-brown in colour, but often quite red in patches. Most of Bangladesh's floodplains soils are coated with grey to mid-grey and dark grey transported material from the soil surface. The effect of these coatings is to increase the mixing of soil material by transferring topsoil material into the subsoil. Such coatings also increase colour contrast in the subsoil. The subsoil of most floodplain soils remains aerated throughout the period of submergence, even in soils that are deeply flooded. Subsoil in uncultivated soils also remains aerated below an anaerobic surface layer, so it is possible that air is also taken down to the subsoil by the roots of plants (including rice) growing on the submerged soils. [Md Didar-Ul-Alam]