The dissolution of rock phosphate is a primary concern in its direct application as a fertilizer. By examination of the solubility product of hydroxyapatite [Ca10(PO4)6(OH)2], it can be shown that total P = K‘ (Ca)-5/3(OH)1/3. To test this relationship on rock phosphates, we performed a dissolution study in N NH4 citrate (pH 7.0), in which the Ca ion activity (Ca2+) in solution was varied. The results show a linear relationship between P and Ca, but the relationship was different for different rock phosphates. A preliminary extraction in N NH4 citrate (pH 8.1) was used in an attempt to remove oxides and carbonate coatings, and the study was repeated. The results show an increased uniformity in the relationship of P to Ca for the different rock phosphates. The (Ca2+) and pH were measured on several acidic, aluminous soils, and the results show that Ca activity in solution varies independently of pH. This indicates that the (Ca2+) of the soil plays a role independent of pH for these soils and needs to be considered in evaluating rock phosphate solubility. A curvilinear relationship was shown between surface area and solubility in N NH4, citrate (pH 7) of the −150 + 270 mesh fraction. The exception to this was the Tennessee rock phosphate, found to contain significant quartz, which would increase the surface area measured without increasing P released.
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Wilson et al. (1984) studied this question.