Metastable triple-point solution (MTPS), the solution saturated with respect to Ca(H2PO4)2·H2O and CaHPO4·2H2O, was reacted with a series of soil samples in order to simulate the initial reaction zone that occurs adjacent to a dissolving granule of superphosphate in soil.
As this acidic solution (initially pH 1.48) contacted successive soil increments, appreciable quantities of Al, Fe, Mn, K, and Ca were dissolved or exchanged from the soil. The solution formed during this series of reactions gradually became supersaturated with respect to certain Ca and Fe-Al phosphate compounds. As the pH rose above 2.3, more rapid precipitation of phosphates occurred and was accompanied by the simultaneous removal of Ca, Al, Fe, and K from solution. The crystalline compound, CaHPO4·2H2O, CaHPO4, and H6K3Al5(PO4)8·18H2O, were identified petrographically and by X-ray as precipitates that later formed in solutions filtered during the reaction sequence. The results are compared to those of a previous study in which soil was reacted with TPS (the stable equilibrium solution saturated with respect to Ca(H2PO4)2·H2O and CaHPO4).