Laboratory column-leaching experiments were carried out by using pyritic sediments of Kojima lake (Okavama, Japan) to evaluate the influence of soil microorganisms (SMO), length of oxidation, rainfall, and groundwater on the formation and development of acid sulphate soils. The soil contained 6.5 g kg−1 pyrite, 43 g kg−1 organic carbon, and had pH value of 8.0 (sticky). Two weeks after incubation, the pH of the soils with SMO (set I: T1-T6) turned towards acidic range and the acute drop of the pH (3–4) were detected during 32-weeks, while it turned towards more alkaline range (pH > 8.5) with time in the soil columns without SMO (set II: T7-T10). At about 200 per cent water contents of the soils, the pH of the soils were detected less than 6 from the 16-weeks after incubation with SMO. While at the same level of water contents during 32nd week, the pH of the soils were about I unit lower than those of the 16th week. But in set II (absence of SMO), the pH of the soils were detected less than 6 after 32-weeks of incubation at about 150 per cent of water. Whereas at these (about 150%) water contents, the pH of the soils dropped to about 3 in all the treatments with SMO. The formation of acid sulphate soils at more than 150 per cent water contents (40±2% air v.) was mainly controlled by SMO but less than 150% water content and/or beyond the pH range of 3.5 to 5.8, it depends mainly on chemical oxidation and its length. Rainfall and groundwater enhanced the development of acidity in the surface soils through leaching losses of Ca2+ and Mg2+, and striking increment of Fe3+ and Al3+ by exchange reactions.
Acid sulphate soils, formation, development
