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This article in SSSAJ

  1. Vol. 57 No. 3, p. 691-696
    Received: June 26, 1991

    * Corresponding author(s):
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Calcium-Induced Sulfate Adsorption by Soils

  1. N. S. Bolan,
  2. J. K. Syers  and
  3. M. E. Sumner
  1. Fertilizer and Lime Research Centre, Dep. of Soil Science, Massey Univ., Palmerston North, New Zealand
    Dep. of Agricultural and Environmental Science, The University, Newcastle upon Tyne, NE1 7RU, England
    Dep. of Agronomy, Univ. of Georgia, Athens, GA 30602



Gypsum (CaSO4·2H2O) is used in agriculture both as a source of calcium (Ca2+) and sulfate (SO2−4) and as an amendment to improve soil structure. We examined the effect of Ca2+ on the adsorption of SO2−4 in variable-charge soils. Sulfate adsorption measurements from batch and column experiments showed that SO2−4 adsorption increased with increasing adsorption of Ca2+. The increase in SO2−4 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 SO2−4. At low levels of solution Ca2+ (<0.003 mol L−1), most of the increase in SO2−4 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 SO2−4 adsorption. The remaining increase in SO2−4 adsorption is attributed to the coadsorption of Ca2+ and SO2−4 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 SO2−4 adsorption in these soils.

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

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