Acidity and Magnetic Susceptibility of Soils in the Central Plain of Thailand

Abstract

Acidity and magnetic susceptibility of acid sulfate soils were investigated in the central plain of Thailand. Soil pH (H₂O) and pH (H₂O₂) of samples collected from surfaces of various landforms were measured in the laboratory. Magnetic susceptibilities were measured at each sampling point. On the basis of the combination of pH (H₂O) and pH (H₂O₂), two classification of acid sulfate soils were made: AASS, actual acid sulfate soils, pH (H₂O) <4.0 and pH (H₂O₂) <3.0; PASS, potential acid sulfate soils, pH (H₂O) >4.0 and pH (H₂O₂) <3.0. AASS is mostly distributed along the paleo-shoreline of the Holocene transgression at 7 - 8 cal kyr BP. Furthermore, the AASS develops on landforms of relatively low altitudes such as back marshes and deltas. Oxidation of pyrite derived from mangrove vegetation at the paleo shoreline and periodic wetting and drying at these landforms involves acidity in soils.