Sulfate transformation under submurged conditions was investigated by the incubation of Submurged soils applied with granular and paste-like high analysis fertilizers. Secondly, soil samples collected from the fixed paddies for the soil fertility survey, both of farmers and experimental stations, were analysed for humus and sulfur. The results obtained were as follows; 1) Paste-like and granular fertilizers were applied in a layer and mixed in the whole layer respectively, followed by the extraction of soluble and available sulfur with appropriate extractants. As a result, both soluble and available sulfur decreased as the Eh of soils decreased, indicating appreciable amounts of sulfate were transformed to sulfide. 2) Five soils of fixed paddies in which the humus contents differed appreciably were selected to check the changes in available sulfur within 15 y. As a result, humus contents showed relative increases within 15 y, however, the content of available sulfur tended to decrease. The ratios of available to total sulfur significantly decreased within 15 y, indicating the influences of soil pH and/or farmers' practices of fertilizer selection upon the ratios. 3) Soil samples, especially of well-drained paddy in an experiment station, decreased the available sulfur within 20 y. It can be concluded that the greater Eh is reduced in the soils, the higher level of sulfide formed, thus the rice plants will become deficient in sulfate due to the formation of unavailable iron sulfides. This will be much enhanced considering the recent trends of lower levels of available sulfur, which coincidentally inhibited the initial growth of rice in early planting cultivation in our district.
