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(H 2 PO 4 ) 2 are used as co‐ordinates. In the presence of solid phase CaCO 3 , the chemical potential of lime (the value of pH ‐ ½pCa) depends upon the partial pressure of CO 2 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(H 2 PO 4 ) 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.
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Clark et al. (1955) studied this question.