Mechanisms of phosphorus solubilisation in a limed soil as a function of pH

Phosphorus (P) quantity-intensity relationships are central to the solubility and release of P from soil to water. Relationships between P extractable by 0.5 M NaHCO extractable P (Olsen P; quantity, Q) and P extractable by 0.01 M CaCl2 (CaCl2-P; possible predictor of soil solution or drainage water P; intensity, 1) are curvilinear: above a certain Olsen P concentration, CaCl2-P becomes much more soluble than when below it. Aluminium-, Fe- and Ca-P forms (extractable by Olsen's reagent) are thought to control P solubility. Thus, our objectives were to identify P forms in equilibrium with CaCl2-P via solubility equilibrium experiments, and the behaviour of CaCl2-P in relation to Al, Fe and Ca associated P, determined with P-31 high power decoupling magic angle spinning nuclear magnetic resonance spectroscopy (P-31 HPDec/MAS NMR). Results indicated that two Q-I relationships occurred, one for soils above pH 5.8, and the other for soils below pH 5.8. Above pH 51, soils were saturated with respect to hydroxyapatite (Ca-5(PO4)(3)OH) and undersaturated with respect to beta-tricalcium phosphate (beta-Ca-3(PO4)(2)), while log ion-activity products showed that all soils and pHs were either saturated or in equilibrium with variscite (AlPO4.2H(2)O) or its amorphous analogue. Using P-31 HPDec/MAS NMR, Ca-P was best correlated with CaCl2-P in soils above pH 5.8, and with Al-P in soils below this pH. This study demonstrates the value of solid-state NMR in conjunction with wet chemical techniques for the study of labile P and P loss from pasture soils with a wide range of managements. (C) 2003 Elsevier Science Ltd. All rights reserved.