Soil pH was measured over 6 years in 10 field trials in which superphosphates (SPs) and reactive phosphate rocks (RPRs) were applied annually to clover/grass pastures at rates equivalent to 0, 0.5, 0,75, 1.0, and 2.0 times the estimated amounts of phosphorus (P) required for maintaining near-maximum pasture production. Over the 6-year period, soil pH (0#x2013;75 mm soil depth) fell by an average of 0.16 units in control and SP treatments with no significant effect from rate of SP application. RPRs reduced the fall in pH and this effect increased with increasing RPR application rate, the fall in pH being virtually eliminated by the highest RPR application rate. The effect of RPRs on reducing the fall in soil pH could be largely accounted for by the difference in phosphate protonation compared with SPs, and the carbonate content of the RPRs.
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