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Solubility Criteria for the Existence of Calcium and Aluminum Phosphates in Soils¹

ABSTRACT

The solubilities of calcium phosphates may be represented on a single solubility diagram in which functions of the chemical potentials of Ca(OH)₂ and Ca(H₂PO₄)₂ are used as co-ordinates. In the presence of solid phase CaCO₃, the chemical potential of lime (the value of pH - pCa) depends upon the partial pressure of CO₂ in the atmosphere. Solubility determinations showed that neither dicalcium phosphate nor hydroxyapatite was present in the soil or clay systems studied.

A similar solubility diagram may be constructed for the aluminum phosphates using functions of the chemical potentials of Al(OH)₃ and Al(H₂PO₄)₃ as co-ordinates. In the absence of reliable solubility data, the existence of the aluminum phosphates in the soils studied could not be established.

¹ Contribution from the Department of Agronomy, Cornell University, Ithaca, N. Y. The present address of the senior author is Chemistry Division, Science Service, Department of Agriculture, Ottawa, Canada.