Gypsum (CaSO4.2H2O) is used in agriculture both as a source of calcium
(Ca2+) and sulfate (SO42-) and as an amendment to improve soil struct
ure. We examined the effect of Ca2+ on the adsorption of SO42- in vari
able-charge soils. Sulfate adsorption measurements from batch and colu
mn experiments showed that SO42- adsorption increased with increasing
adsorption of Ca2+. The increase in SO42- adsorption per unit increase
in Ca2+ adsorption was 12 times more in soils containing Fe and Al hy
drous oxides as the major variable-charge component than in soils domi
nated by organic matter. In soils containing Fe and Al hydrous oxides,
specific adsorption of Ca2+ increased the positive charge and thereby
induced further adsorption of SO42-. At low levels of solution Ca2+ (
< 0.003 mol L-1), most of the increase in SO42- adsorption (85-98%) du
e to Ca2+ adsorption could be attributed to the increase in positive c
harge. At higher Call concentration (0.00-30.015 mol L-1), the increas
e in positive charge accounted for up to only 75% of the increase in S
O42- adsorption. The remaining increase in SO42- adsorption is attribu
ted to the coadsorption of Ca2+ and SO42- as a CaSO40 ion pair. In soi
ls with organic matter as the major variable-charge component, Ca2+ is
complexed by organic ligands. Calcium complex formation through elect
rostatic attraction does not create positive sites and this may be the
reason for the absence of Ca2+-induced SO42- adsorption in these soil
s.