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Calcium-Induced Sulfate Adsorption by Soils

Fertilizer and Lime Research Centre, Dep. of Soil Science, Massey Univ., Palmerston North, New Zealand

J. K. Syers^*

Dep. of Agricultural and Environmental Science, The University, Newcastle upon Tyne, NE1 7RU, England

M. E. Sumner

Dep. of Agronomy, Univ. of Georgia, Athens, GA 30602

*Corresponding author.

ABSTRACT

Gypsum (CaSO₄·2H₂O) is used in agriculture both as a source of calcium (Ca²⁺) and sulfate (SO^2–₄) and as an amendment to improve soil structure. We examined the effect of Ca²⁺ on the adsorption of SO^2–₄ in variable-charge soils. Sulfate adsorption measurements from batch and column experiments showed that SO^2–₄ adsorption increased with increasing adsorption of Ca²⁺. The increase in SO^2–₄ adsorption per unit increase in Ca²⁺ adsorption was 12 times more in soils containing Fe and Al hydrous oxides as the major variable-charge component than in soils dominated 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 SO^2–₄. At low levels of solution Ca²⁺ (<0.003 mol L^–1), most of the increase in SO^2–₄ adsorption (85–98%) due to Ca²⁺ adsorption could be attributed to the increase in positive charge. At higher Ca²⁺ concentration (0.003–0.015 mol L^–1), the increase in positive charge accounted for up to only 75% of the increase in SO^2–₄ adsorption. The remaining increase in SO^2–₄ adsorption is attributed to the coadsorption of Ca²⁺ and SO^2–₄ as a CaSO⁰₄ ion pair. In soils with organic matter as the major variable-charge component, Ca²⁺ is complexed by organic ligands. Calcium complex formation through electrostatic attraction does not create positive sites and this may be the reason for the absence of Ca²⁺-induced SO^2–₄ adsorption in these soils.

Contribution from the Dep. of Agricultural and Environmental Science, Univ. of Newcastle upon Tyne.

Received for publication June 26, 1991.

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