Cultivated acid sulfate soils have limited microbial activities and populations related to nitrogen transformations, particularly to N₂ fixation. Nitrogen fixation activity by blue green algae was much weaker than in most of the other soils in Thailand.
The addition of phosphorus increased rice yield by about 1 ton/ha without nitrogen fertilizer. Field acetylene reduction assays in a long-term fertility plot in Thailand demonstrated the enhancement of N₂ fixation by the application of phosphate.
The technique of growing Azolla in acid sulfate soil needs to be improved. Blue green algae grow epiphytically on the aquatic parts of deepwater or floating rice, which is widely grown in flooded acid sulfate soils. The nitrogen fixation by those epiphytic blue green algae and its transfer to aerial parts were demonstrated by ¹⁵N technique.
Roles of phosphate to stimulate nitrogen fixation are discussed.In Southeast and East Asia, 60% of about 5 million ha of actual and potential acid sulfate soils are distributed along the sea coast of Indonesia, Thailand and Vietnam. West Africa may have 6.6 million ha of actual and potential acid sulfate soils (Van Breemen and Pons, 1978).
There is adequate information available about genesis, classification, chemistry, toxic properties, and reclamation method of acid sulfate soils, but research on the microbiological aspects, excluding sulfur metabolism, is limited.
Extensive surveys on microbial properties and their activities in acid sulfate soil were reported in Thailand (Araragi and Tongcham, 1974; 1979; Matsuguchi et al., 1970; 1975) and Senegal
(Garcia et al, 1973)
Discussion here focuses on rice yield in acid sulfate soil areas. Potential acid sulfate soils, which will undoubtedly become noxious after reclamation as rice fields, are not discussed.