Acid Basin Clay  grey to dark grey heavy clays, which are strongly to extremely acid. The topsoil is grey to dark grey, often stained deep red or brown along root channels and cracks. It becomes hard and cracked when dry. There are peaty layers and silty substrata in some places. The topsoil becomes nearly neutral after submergence of about two weeks, but is extremely acid on drying out. The subsoil is strongly to extremely acid.

They are Dystric and Eutric Fluvisols in the FAO-UNESCO classification. They have a strong prismatic and blocky structure in soils, which dry out seasonally. There are about 348,994 ha of Acid Basin Clays found in Bangladesh. These soils occur mainly in the haor basin and more clearly in the Eastern Surma-Kusiyara floodplain, the lower Atrai Basin, barind tract, madhupur tract and Brahmaputra and Ganges floodplains. [Md Khurshid Alam]