Topsoil the surface layer of the soil. In cultivated soils it includes the cultivated layer and an underlying ploughpan, if present; in uncultivated soils the uppermost soil layer or layers darkened by organic matter. In Bangladesh the black Terai soils have a very dark brown to black topsoil, one-third to one metre thick. Noncalcareous brown floodplain soils usually have a dark brown or dark greyish brown topsoil about 30 cm thick and grades into yellowish-brown subsoil. Calcareous brown floodplain soils have neutral to moderately alkaline topsoil. The cultivated topsoil and ploughpan of grey floodplain soils are grey with strong yellow or brown stains along root channels and some cracks. The topsoil becomes near neutral after continuous submergence for about two weeks, but becomes moderately to strongly acid on drying out. The calcareous dark grey floodplain soils also behave in the same manner.
The topsoil of acid basin clay is grey to dark grey, often strongly stained red or brown along root channels and cracks. In this case also the topsoil becomes near neutral after submergence of about two weeks. peats occur regularly in the central portion of the delta and are usually overlain by clayey topsoil. Deep red-brown terrace soils comprise brown loamy topsoil grading into a reddish-brown to yellow-brown friable subsoil. The topsoil of the madhupur tract (shallow red-brown terrace soil) is usually grey and mixed brown. On the other hand, the topsoil and ploughpan of the brown mottled terrace soils are mixed grey and brown with prominent red or reddish-brown mottles. As with many of the other soils, the reaction of topsoils becomes near neutral if submerged for about two weeks. In the case of brown hill soils the properties of the topsoil vary according to the vegetation. Under forests it is dark grey-brown to grey-brown and consists mainly of earthworm casts for 5 to 7 cm at the surface. Under tea it is usually darker and thicker. Under arable crops or grassland it is paler. [Sifatul Quader Chowdhury]