Phosphorus released by incremental acidification of alkaline Vertosols

Phosphorus (P) in the soil solution of alkaline, vertic soils is considered to be buffered by calcium phosphate minerals (CaP), influenced by pH and the solution concentration of P and/or Ca. We measured the P released to solution as we incrementally acidified 3 alkaline Vertosols containing 300-6000 mg kg-1 of acid soluble P in the presence or absence of an anion exchange membrane (AEM). The AEM kept the concentration of P in solution below 1 μM. After removal of labile forms, the concentration of P extracted remained low as pH decreased until there was a large increase as the pH passed a threshold. The thresholds varied little between soils, and were at pH 6.0-6.3 with the AEM, and were 0.7-1.0 pH units lower without the AEM. Soil pH buffer capacity (pHBC) affected the amount of acid required to release the P. The results indicate that rhizosphere acidification may increase the availability of CaP minerals to plants, though pHBC may restrict the availability of acid soluble P.