EFFECTS OF FLOODING ON PH OF RICE-PRODUCING, ACID SULFATE SOILS IN INDONESIA

Suitability of acid sulfate soils for lowland rice cultivation in coas tal swamps in Kalimantan, Indonesia, is limited by low pH and associat ed high levels of dissolved Al and Fe. In the dry season, oxidation of pyrite lowers the pH wherever the groundwater table drops below the d epths at which the soil contains pyrite. Reduction and leaching in the wet season deacidifies the soil at some sites but not at others. This study was conducted to examine the factors that determine the rise of pH after flooding. Seasonal dynamics of soil physical and chemical pa rameters were monitored for 2 yr at four representative sites. Dominan t redox processes varied with physiography and drainage of the soils. In acidified surface horizons of potential acid sulfate soils of coast al backswamps, pH rose quickly to 5 to 6.5 after flooding. In inland b ackswamps, pH rose much more slowly to 4 to 5.5. Acid sulfate soils in artificially drained backswamps retained their low dry-season pH of 3 .5 to 4 in the wet season. On levees and ridges, the pH rose to 5.5 in the peaty topsoil only, while the mineral soil remained very acidic w ith pH values ranging between 3 and 4. Low concentrations of Fe(III) o xides and SO4 relative to the soil acidity appear to be a major cause of the static pH following soil reduction after flooding. Horizons wit h an accumulation of Fe(III) oxides and jarosite, which is characteris tic of acid sulfate soils of monsoonal and dry regions, were not obser ved in Kalimantan. The perhumid climate, in combination with a highly organic parent material, probably prevents sufficient oxidation for th e accumulation of large amounts of Fe(III) oxides and jarosite and per mits the leaching of iron as Fe2+.