Effects of Flooding on pH of Rice‐Producing, Acid Sulfate Soils in Indonesia

Abstract

Abstract Suitability of acid sulfate soils for lowland rice cultivation in coastal swamps in Kalimantan, Indonesia, is limited by low pH and associated high levels of dissolved Al and Fe. In the dry season, oxidation of pyrite lowers the pH wherever the groundwater table drops below the depths 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 parameters were monitored for 2 yr at four representative sites. Dominant redox processes varied with physiography and drainage of the soils. In acidified surface horizons of potential acid sulfate soils of coastal backswamps, pH rose quickly to 5 to 6.5 after flooding. In inland backswamps, 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 with pH values ranging between 3 and 4. Low concentrations of Fe(III) oxides and SO 4 relative to the soil acidity appear to be a major cause of the static pH following soil reduction after flooding. Horizons with an accumulation of Fe(III) oxides and Jarosite, which is characteristic of acid sulfate soils of monsoonal and dry regions, were not observed in Kalimantan. The perhumid climate, in combination with a highly organic parent material, probably prevents sufficient oxidation for the accumulation of large amounts of Fe(III) oxides and jarosite and permits the leaching of iron as Fe 2+ .