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

ABSTRACT

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.

¹ Contribution from the Dep. of Soil and Environmental Sciences, Univ. of California, Riverside, CA 92521, and A.R.O. Inst. of Soils and Water, Volcani Center, Bet-Dagan 50250, Israel.

² Assistant Research Soil Chemist, Professor of Soil Science, and Graduate Fellow (Riverside), and Research Scientist (Bet-Dagan), respectively. The senior author is currently Assistant Professor of Soil Physical Chemistry, Dep. of Agronomy, Ohio State Univ., Columbus, OH 43210.

Received for publication August 12, 1985.