The importance of exchangeable cations and resin‐sink characteristics in the release of soil phosphorus

Abstract

SUMMARYThe significance of exchangeable cations in the release of phosphorus by sequential extraction with water was evaluated in 11 acid (pH 5.0–6.3) New Zealand soils contrasting in P status and P retention. The release of P from Na‐saturated soil exceeded that from the original Ca‐dominated soils by up to four‐fold. Possible explanations for the larger P release in the Na system include: (i) desorption of P induced by increased surface negative potential associated with the exchange of Na for Ca/Mg, and/or (ii) accelerated dissolution of Ca phosphate compounds or complexes resulting from the creation of a sink for Ca.The potential of a series of anion‐ and cation‐exchange resin systems (AER and CER, respectively) as sinks for labile soil P was also examined. For all soils studied, P extracted by AER‐HCO3 <AER‐OH/CER‐Na< AER‐OH/CER‐H. The success of the AER‐OH/ CER‐H system in extracting P is attributed to its ability to remove exchangeable Ca and Mg, and to reduce the ionic strength of the equilibrating solution. A high correlation (r= 0.99**) was obtained between P extracted by AER‐HCO3 and that removed by sequential extraction with water. Also, the amounts of P extracted by AER‐OH/CER‐H and NaCl/ H2O were closely correlated (r= 0.95**), suggesting similar release mechanisms. The results obtained indicate that charge‐balancing cations, particularly Ca which is the predominant exchangeable cation in the majority of soils, exerts a more significant control on soil P equilibria in acid soils than is commonly recognized.