Nature of the Reactions of Monocalcium Phosphate Monohydrate in Soils: IV. Repeated Reactions With Metastable Triple‐Point Solution

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Abstract
A series of soil samples was reacted repeatedly, first with solution saturated with respect to Ca(H2PO4)2 · H2O and CaHPO4 · 2H2O (MTPS), and later with water, in an attempt to simulate the changing chemical environment of soil surrounding a superphosphate granule.The reaction of MTPS with soil was accompanied by increase in pH and precipitation of Fe, Al, and Ca phosphates from solution. Soil repeatedly contacted by MTPS gradually became more acidic and showed continued dissolution of Fe and Al. Subsequent additions of water to the soil residues remaining after reaction with MTPS increased the pH and caused further precipitation of phosphate from solution. Many filtrates obtained during these reactions yielded precipitates upon standing. The following crystalline compounds were identified from these precipitates: CaHPO4 · 2H2O, H6K3Al5(PO4)8 · 18H2O, H8K(Al,Fe)3(PO4)6 · 6H2O, and CaHPO4. The indications are that these compounds may form as initial phosphate reaction products of superphosphate fertilizers in soils. Tentative conclusions are presented regarding the nature of the reactions of Ca(H2PO4)2 · H2O in soils.

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