Extractable sulfate was determined on 74 samples of Japanese forest Andisols from 12 pedons, and sulfate adsorption isotherms were determined on 24 samples from 8 pedons of those soils. The relationship of extractable sulfate content with soil properties and sulfate adsorption capacity was therefore investigated. (1) Sulfate adsorption capacity, which was described by the adsorption isotherm, varied widely among soils and seemed to depend largely on aluminum content in allophane and on the specific surface area of the soils. On most soil, the increase in sulfate adsorption was relatively small at solution sulfate concentrations over 0.02 mmol L^<-1>. (2) Extractable sulfate content varied widely among soils, from 0.8 to 93.0 mmol(-)kg^<-1> and indicated a positive correlation with Al content in allophane. Furthermore, extractable sulfate content was found to be close to the adsorbed amount at 0.1 mmol L^<-1> of solution sulfate and significantly reflected the sulfate adsorption capacity of each soil. (3) The increase in adsorbed sulfate until soil solution sulfate increases by 0.1 mmol L^<-1> was calculated based on extractable sulfate content and adsorption isotherm. The increase did not correspond to the sulfate adsorption capacity of soils, being influenced by already adsorbed sulfate. Acid buffering capacity of forest Andisols due to sulfate adsorption was therefore considered to be affected by accumulated sulfate, and not to correspond to the sulfate adsorption capacity.
