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This article in SSSAJ

  1. Vol. 24 No. 3, p. 177-182
    Received: Sept 28, 1959
    Accepted: Dec 28, 1959

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Phosphate Phase Equilibria in Soils1

  1. W. L. Lindsay and
  2. E. C. Moreno2



The activity isotherms for AlPO4 · 2H2O (variscite), FePO4 · 2H2O (strengite), Ca10(PO4)6F2 (fluorapatite), Ca10(PO4)6(OH)2 (hydroxyapatite), Ca4H(PO4)3 · 3H2O (octocalcium phosphate), and CaHPO4 · 2H2O (dicalcium phosphate dihydrate) were represented on a single solubility diagram in which a function of the phosphate activity in solution was plotted against pH. This plot is useful for assessing the relative stabilities of these phosphate compounds and for predicting their transformations in soils upon the application of fertilizer or lime.

For this development, the assumptions were made that Al3+ activity in soils was limited by the solubility of gibbsite, the Fe3+ activity by the solubility of goethite, F- activity by that of fluorite, and the Ca++ activity was arbitrarily selected as that of a 0.005M solution of a monovalent salt. Equations and graphs are included for modifying the basic diagram so as to correspond to any Al3+, Fe3+, F-, or Ca++ activity that may be found in soils or that may be established during a particular soil extraction.

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