Acid Sulphate Soils

• Definition

• Further description

• General environment

• Global extent and location

• Land use

• Main production constraints

• Summary

Definition

Acid sulphate soils have a sulfuric horizon or sulphuric material or both at less than 125 cm under the soil surface. In the World Reference Base (WRB) soil classification system these soils are considered in the Soil Reference Group of the Fluvisols as Thionic Fluvisols.

Further description

Acid sulphate soils develop by the drainage of coastal marshes and swamps that contain pyrite which is then oxidised to jarosite resulting in the production of sulphuric acid.  In addition the acidity dissolves aluminium which is toxic to plant growth.

General environment

Acid sulphate soils occur in the tropics, in low-lying coastal land formerly occupied by mangrove swamps.

Global extent and location

The area of potential and actual acid sulphate soils is estimated at approximately 12 million hectares, mainly in coastal tropical lowlands.  They occur in the coastal lowlands of Southeast Asia (Indonesia, Vietnam, Thailand), West Africa (Senegal, the Gambia, Guinea Bissau, Sierra Leone, Liberia) and along the north-eastern coast of South America (Venezuela, the Guyanas).

Land use

Acid sulphate soils are normally left under natural vegetation or used for mangrove forestry.  With good water management they can be used for oil palm and rice production.

Main production constraints

On acid sulphate soils strong acidity and toxic levels of aluminium as well as the low availability of phosphorus are also hampering plant growth.  Additionally the soils are often affected by high salinity and bear the risk of strong acidification during droughts or after rapid drainage when the pyrite may be oxidised to sulphuric acid.  On the other hand, they may be permanently flooded, or large seasonal fluctuations in ground water levels may occur.

Summary

Acid sulphate soils are very poor for crop growth.  The presence of sulphuric soil material or a sulfuric horizon strongly limits the potential for agricultural development.  Agricultural use of these soils has to be based on cautious water management in order to prevent oxidisation processes of pyrite.