A light-colored Andosol previously applied with limed sewage sludge was used as a test soil to evaluate acid-buffering mechanisms. Columns filled with the soil were continuously eluted with either 1) constant (pH 3) or 2) stepwise increase (pH 3→2.5→2) of H_2SO_4 strength, and changing levels of pH and chemical elements were monitored. The acid load of the first 72-76 cmol(+) kg^<-1> did not alter the pH (7-8) of eluted water while alkalinity, and Ca and Mg concentrations gradually decreased. Increase in the strength of the loaded acid resulted in an increase in the levels of these constituents without changing the pH values of the eluted water, indicating that the contribution of carbonate dissolution and cation exchange reactions to the acid buffering capacity of the soil. This buffering region was followed by a second one of pH 4.1-4.2,which was characterized by the disappearance or decreased levels of alkalinity, Ca and Mg, and simultaneous increase in the Al level in eluted water. The third buffering region around pH 3.2 was observed only in the 2nd series of the experiment where the Fe level was increased. The ratio of Mg/Ca and Si level in eluted water and input-S/ouput-S ratio became higher with the progressive load of acid, suggesting the destruction of clay minerals. Spiked elution of Zn and Cd was found at around 4 cmol(+) kg^<-1> of the acid load. These metals in addition to Mn appeared again in the 2nd and 3rd buffering regions.
