Phosphorus Chemistry and Mineralogy in Excessively Fertilized Soils: Solubility Equilibria

Eleven samples representing six soil series were selected for this study of excessively fertilized soils. Phosphorus sources included fertilizers, sewage sludges, and manures. The samples had Bray Pl extractable P levels ranging from 29 to 640 mg P/kg and 1:1 soil/water pH values ranging from 5.4 to 7.4 The equivalent of 25 g of dry soil and 50 mL of water were incubated at 298K. Samples were sacrificed for analysis periodically during the 130‐d test period. The activities of most ions, as estimated with GEOCHEM, were relatively constant throughout the course of the experiment. The concentrations of organic and inorganic C for samples that had been amended with sewage sludges or manures varied considerably during the course of the experiment. Samples that produced high supernatant pH values (>6.8) generally were more soluble than hydroxyapatite and less soluble than octacalcium phosphate and were slow in attaining constant ion activity products relative to the Ca phosphate minerals considered. Samples that produced low supernatant pH values (<6.8) indicated an equilibrium with the amorphous analog of variscite throughout the experiment.