DEVELOPMENT OF ACID SULPHATE SOILS IN NILWALA FLOOD PROTECTION AREA, MATARA, SRI LANKA
Development of acid sulphate soils was observed in some paddy fields of Nilwala flood protection area in Matara district. Field investigations, borehole data and chemical evidence that cause the abandoning of most of the paddy fields showed occurrences of sulphide-bearing sediments at depths ranging from 2 to 5m at Malimboda and Kapuduwa. Soils contain potentially acidic substances such as jarosite, ferric hydroxide, gypsum and aragonite. The pH of these soils is around 4.0. The total potential acidity values of the study area have very much exceeded the standard action criteria levels. The levels of potential and actual acid sulphate soils are also significantly higher than normal values in both study sites. The Eh–pH diagram representing the samples of Malimboda and Kapuduwa have shown that many points in this study fall within the Fe2+ field, indicating possible disequilibrium with respect to Fe-bearing minerals. Preventive measures are suggested to reduce the harmful effects of toxic substances generated in the acid soils. Using proper soil management systems and selection of rice varieties tolerant to this specific environment, the abandoned paddy lands can be re-cultivated to increase the rice production of Matara district.