Decay of Phosphate Fertilizer Reaction Products in an Acid Soil

Complex iron and aluminum phosphates that may be formed by the reaction of concentrated acidic phosphate solutions with soil minerals were incubated in a moist acid soil for 10 months. Potassium ferric phosphate, H 8 KFe 3 (PO 4 ) 6 ·6H 2 O, and calcium ferric phosphate, H 4 CaFe 2 (PO 4 ) 4 ·5H 2 O, dissolved incongruently with release of part of their phosphate to the soil and formation of strengitic residues. Potassium taranakite, H 6 K 3 Al 5 (PO 4 ) 8 ·18H 2 O, was more stable; it persisted throughout the incubation period and released smaller amounts of phosphate. Amorphous iron and aluminum phosphates also persisted throughout the incubation period, and their rates of dissolution appeared to be controlled by the rate of diffusion of their phosphate into the surrounding soil.