The authors used soil columns constructed from a Maine Spodosol and Illinois Alfisol to investigate the retention of SO4(2-) added as Na2SO4 or H2SO4. Both organic and inorganic S pools were examined to determine how retention of added SO4(2-) was influenced by both mineralization/immobilization and adsorption/desorption processes. Forty columns were leached weekly for a year with simulated throughfall solutions containing base cations (Ca(2+), Mg(2+), K(+), Na(+), NH4(+), NO3(-), Cl(-), and either 80, 280, or 1080 microeq SO4(2-)/L for varying periods. At the conclusion of the experiment acid and control columns were destructively sampled by depth increments to examine organic (C-bonded S and ester sulfate) and extractable SO4(2-) concentrations, as well as soil pH. For all Spodosol columns, SO4(2-) adsorption by the B horizon was the dominant process of SO4(2-) retention; no changes in organic S pools were observed. Soils in acid columns retained greater SO4(2-) than salt columns, most likely due to pH dependent adsorption. However, all Spodosols retained large amounts of SO4(2-). In the Alfisol SO4(2-) retention was lower than in the Spodosols (<25%) due to a limited SO4(2-) adsorption capacity; mineralization of C- bonded S resulted in S inputs nearly equaling outputs. Although organic S was the dominant S pool in both soils, there was little mineralization overall, and inorganic adsorption appeared to be the primary process of SO4(2-) retention. (Copyright (c) 1991 by Williams & Wilkins.)