Acid Sulphate Soil  soils of estuaries and deltas of streams in the tropical region subject to the action of tides and usually covered by mangroves. It resembles grey floodplain soils when wet, but becomes toxically acid if allowed to dry out. High quantities of undecomposed organic matter, high proportion of Fe and Al salts in solution and a deficiency of phosphorus and nitrogen are its features. Impeded drainage and undecomposed organic matter may contribute to their formation.

There are about 226,647 ha of Acid Sulphate Soil found in Bangladesh. They occur in the Basin southwest of the Ganges Tidal Floodplain in satkhira, more extensively on Young Tidal Floodplains and in chakaria of cox's bazar on the Chittagong Coastal Plain. They are Thionic Fluvisols. They have either a sulphidic layer between 50 and 125 cm or a sulphuric horizon (pH <3.5) in any part of the profile between the surface and 125 cm. [Md Khurshid Alam]