Causes of changes in pH in acidic mineral soils

Seasonal variation in soil pH and long-term decreases in pH have been observed for the soils of the south-western slopes of New South Wales. We have determined and measured some processes which are likely to contribute to pH changes within these acidic, mineral soils. The studies were undertaken on closed systems in the absence of plants. Changes in soil pH, in the absence of plants, were controlled by the relative rates of alkali-producing (acid-consuming) reactions and of acid-producing (alkali-consuming) reactions. The alkali-producing reactions studied were ammonification, reduction of Mn-oxides, oxidation of organic anions and SO42− adsorption. The acid-producing reactions were nitrification, oxidation of Mn2+ and oxidation of organic S. Measurements of changes in the concentration of the chemical species involved in these reactions were converted to the corresponding changes in μmol H+ or OH− g−1 soil. The sum of the H+ budget for each soil was transformed to change in pH by using the soilʹs titration curve. With this procedure 96% of the variance of the observed changes in soil pH was explained. On the basis of our results we propose that changes in temperature and water potential determine changes in microbiological activity which in turn determine changes in the H+ budget and its physicochemical consequences.