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This article in SSSAJ

  1. Vol. 50 No. 3, p. 623-627
    Received: Aug 12, 1985

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Effects of Ionic Strength, Calcium, and Citrate on Orthophosphate Solubility in an Acidic, Montmorillonitic Soil1

  1. Samuel J. Traina,
  2. Garrison Sposito,
  3. Dean Hesterberg and
  4. U. Kafkafi2



A recent study by the authors of the effects of pH and organic acids on residual orthophosphate solubility in an acidic, montmorillonitic soil (Typic Chromoxererts) was extended to include variations of ionic strength (NaCl background) and Ca2+ concentration at fixed ionic strength. The objective of the experiments was to elucidate further the mechanism of orthophosphate solubility in a montmorillonitic soil containing exchangeable Al(III) and added citric acid. At ionic strengths between 20 and 100 mol m−3 (NaCl) and Ca2+ concentrations between 1 and 5 mol m−3 at 20 mol m−3 ionic strength (NaCl background), additions of citric acid at low concentrations (<0.1 mol m−3) reduced orthophosphate solubility. The solubility then gradually returned to its value in the absence of added organic acid as the citric acid concentration was increased to 0.6 mol m−3. The only significant effect of increasing the ionic strength or the Ca2+ concentration was to decrease orthophosphate solubility at any level of citric acid addition. Orthophosphate-citric acid interactions in the soil were influenced only quantitatively, probably through the displacement of exchangeable Al(III) by Na+ and/or Ca2+, by increasing the concentration of these two metal cations in the soil solution.

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