Sulfate Retention by an Acid BE Horizon of an Ultisol

Abstract Many apparently conflicting reports on sulfate (SO 2‐ 4 ) retention by soils suggest that a single, inclusive mechanism may not be adequate in explaining SO 2‐ 4 ‐soil reactions and that additional experimentation is needed for different soils. An acid Benndale (Typic Paleudults) BE horizon, predominantly kaolinite and gibbsite in the clay fraction, was initially treated with either five lime rates or five gypsum rates. After 14‐d of incubation at field capacity, previously limed soils were amended with gypsum, and previously gypsum‐treated soils were limed. All samples were again incubated moist for 14 d, after which soil solutions were displaced and their chemical compositions determined. The sequence of Ca(OH) 2 and CaSO 4 additions did not affect SO 2‐ 4 retention. Furthermore, SO 2‐ 4 retention isotherms indicated that multiple mechanisms were involved because at low pH (<5.1) an adsorption‐precipitation type mechanism dominated, while at high pH (>5.6) electrolyte adsorption was dominant. This was further supported by the decrease in the ratio of OH ‐ gain to SO 2‐ 4 retention as soil pH increased. Sulfate retention was also accompanied by an increase in total exchangeable cations. Cation increases, however, varied with pH and SO 2‐ 4 rates. The combined increases in OH ‐ and exchangeable cations, particularly at low pH and high SO 2‐ 4 rates, are inexplicable with current SO 2‐ 4 ‐soil reaction models.