Effects of lime on the residual effectiveness of phosphate in acid soils

Abstract

The effectiveness of phosphate, applied 9 months previously, was compared with that of freshly applied phosphate in relation to lime-induced changes in several phosphate characteristics of 14 phosphate-deficient, acidic soils. Effectiveness was measured in terms of the yield and phosphorus uptake of white clover herbage in a glasshouse experiment. Lime increased yields on the majority of soils, whether phosphate was applied or not. These were accompanied by increases in uptake of soil phosphate, which were usually associated with higher levels of exchangeable and soluble phosphate but not of sodium bicarbonate-extractable phosphate. On the other hand, lime had much more variable effects on the apparent uptake of fertilizer phosphate which were unrelated to lime-induced changes in soil phosphate sorptivity. Lime increased the uptake of previously applied phosphate in all but two soils, and it had a more beneficial or less detrimental effect on the uptake of old phosphate than of fresh phosphate in 9 of the 14 soils. Because lime usually increases the uptake of soil phosphate but often depresses the uptake and effectiveness of fertilizer phosphate, the more beneficial effect of lime on phosphate residues is consistent with their partial transformation into compositions resembling native soil phosphates.