In acid sulfate soils, the material which causes heavy acidity is sulfate ; mainly pyrite (FeS2), produced from the sediments in the sea that contain a lot of organic matter and sulphuric acid and subjected to heavy reductive conditions. In the process of sulphuric acid production, particularly in the primary acidity stage, chemical acidity and bacterial acidity act together. These bacteria (Thiobacillus ferrooxidans and Thiobacillus thiooxidans) contribute a major role as catalysts. The purpose of this study is to investigate the relationship between the acidification process and the temperature and water content conditions through an incubation experimentation. Results are listed as follows : (i) In 10-50°C temperature conditions, the soil was acidified more strongly at high temperature by chemical oxidation accompanied by high evaporation rate. The acidification was stopped when the quantity of evaporation became lower than the level of soil moisture. (ii) When keeping the water condition with almost no evaporation, the speed of acidification is slow but continuous. At 30°C and at around —31.0 kPa, the acidity is extremely strong. The above results show that the speed of evaporation influences the acidification process and microstructure of the soil matrix. Drying is also an important factor that influences acidity due to bacterial activity.
