EFFECT OF REACTIVE PHOSPHATE ROCK ON THE PH OF SOIL UNDER PASTURE

Soil pH was measured over 6 years in 10 field trials in which superpho sphates (SPs) and reactive phosphate rocks (RPRs) were applied annuall y to clover/grass pastures at rates equivalent to 0, 0.5, 0.75, 1.0, a nd 2.0 times the estimated amounts of phosphorus (P) required for main taining near-maximum pasture production. Over the 6-year period, soil pH (0.75 mm soil depth) fell by an average of 0.16 units in control an d SP treatments with no significant effect from rate of SP application . RPRs reduced the fall in pH and this effect increased with increasin g RPR application rate, the fall in pH being virtually eliminated by t he highest RPR application rate. The effect of RPRs on reducing the fa ll in soil pH could be largely accounted for by the difference in phos phate protonation compared with SPs, and the carbonate content of the RPRs.