Phosphate Phase Equilibria in Soils

Abstract

Abstract The activity isotherms for AlPO 4 · 2H 2 O (variscite), FePO 4 · 2H 2 O (strengite), Ca 10 (PO 4 ) 6 F 2 (fluorapatite), Ca 10 (PO 4 ) 6 (OH) 2 (hydroxyapatite), Ca 4 H(PO 4 ) 3 · 3H 2 O (octocalcium phosphate), and CaHPO 4 · 2H 2 O (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 Al 3+ activity in soils was limited by the solubility of gibbsite, the Fe 3+ activity by the solubility of goethite, F ‐ activity by that of fluorite, and the Ca ++ activity was arbitrarily selected as that of a 0.005 M solution of a monovalent salt. Equations and graphs are included for modifying the basic diagram so as to correspond to any Al 3+ , Fe 3+ , F ‐ , or Ca ++ activity that may be found in soils or that may be established during a particular soil extraction.