Process of Acid Sulfate Soil Formation from the Viewpoint of Moisture Conditions on Coastal Muddy Soil

Abstract

The relation between the process of oxidation of sulfur compounds and the water content (water moisture suction) in coastal muddy soil was examined.
The concentration of sulfuric ions increase as oxidation of sulfur compounds contained in pyrite, etc. occurs, and as a consequence, the soil pH decreases in the coastal muddy soil. Especially, the initial oxidation of sulfur compounds occur at pF values of 4.0-4.4 and the lowest soil pH was observed at pF 2.8 over an elapsed time period.
In spite of the difference in the soil pF values, the soil pH decreased from 7.6 to 3.0-4.0, after the soil was allowed to dry over a two year period.
The soil pH decrease occurred to a much greater extent in soil containing a lower salt concentration.
We considered that the treatment of salt removal enhanced the activity of soil microorganisms helping in bring about sulfur oxidation and the formation of sulfuric ions.
The pH decrease in the soil with autoclaved treatment was only slight at pF values lower than pF 3.0.
Accordingly, we concluded, that the decrease in pH in coastal muddy soil mainly occurred due to soil microorganisms which oxidize sulfur compounds under the condition of higher-water than pF 3.0.