Lime-induced changes in indices of soil phosphate availability

Increases in soil P availability due to liming have been reported in a number of glasshouse and field trials, but the mechanism responsible for this effect has not been identified definitely. In a laboratory study, we examined the effects of lime on labile P fractions in six New Zealand soils that varied in P-retention capacity. The soils (5.1-5.5 initial pH in water) were incubated with four rates of CaCO3 to raise pH incrementally to a maximum of approximate to6.5. Snbsequently, P las KH2PO4) was applied to give three P levels in each soil. Liming generally decreased Olsen bicarbonate values, with the effect being largest at the highest rate of P addition. Averaged across P treatments, the decrease in Olsen P for a unit increase in pH ranged from 3 to 7 mg kg(-1). Liming also tended to depress water-extractable P, Decreases in extractable P suggest that liming increased phosphate adsorption. When data for the lime and P treatments were combined, water-extractable P and Olsen P were well correlated, although each soil showed a different relationship. Phosphate retention capacity appeared to have a strong influence on the relationship between water-extractable P and Olsen P, with the high P retention soils having relatively low proportions of water-extractable P. When exchangeable cations were replaced with Na, soils that had been limed released significantly more P to distilled water than their unlimed counterparts. The results confirm that the nature of the exchangeable cation suite has a major influence on the pa-dependence of the phosphate adsorption-desorption equilibrium. In limed soil, exchangeable Ca and pH increase simultaneously so that shifts in this equilibrium may be small and unpredictable.