A comparison of potential, active and post-active acid sulfate soils in Thailand

Acid sulfate soils in Thailand have been modified by drainage, liming, irrigated agriculture and paddy rice growing over a long period of time generating potential (PASS), active (AASS) and post-active (PAASS) acid sulfate soils which have different soil properties and extents of soil development. This research compares Thai PASS, AASS and PAASS using field observations, mineralogy and chemical properties. Eighteen acid sulfate soil profiles, representing PASS, AASS and PAASS located in the Lower Central Plain and the Southeast Coast regions, were investigated. All three types of acid sulfate soils (ASS) contain a reduced layer below the water table. The soils are dominated by quartz, feldspars, kaolinite and illite with the Southeast Coast ASS being more sandy than the Lower Central Plain ASS. The labile minerals pyrite, jarosite, goethite, hematite, gypsum and halite are present in variously colored redox concentrations and have resulted from variations in drainage and oxidation status induced by land management including addition of lime. Mineralogical and geochemical properties of PASS, AASS and PAASS mostly reflect the nature of parent materials, but the ASS can be discriminated by the presence of labile minerals and by the labile chemical properties pH, sulfur content, organic carbon, electrical conductivity, actual acidity and total acidity. This research has clearly identified the labile properties that reflect the transformation from potential through active and into post-active acid sulfate soils. In addition, the transformation from PASS to fully oxidized PAASS is not a simple one direction oxidation process in ASS which have experienced cyclical freshwater-reflooding for rice cultivation. (c) 2016 Elsevier B.V. All rights reserved.