Lime-Induced Changes in Indices of Soil Phosphate Availability
- D. Curtina and
- J.K. Syers *b
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 ≈6.5. Subsequently, P (as 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 pH-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.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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