Relative efficiency of phosphatic fertilisers in pasture topdressing

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Abstract
Pastures grown on Rosedale silt loam, a central yellow-brown earth yielded 22-54% more dry matter (DM) after addition of "Ibex" mono-calcium phosphate (IMCP) and 12% more DM after gypsum addition over a 2.5 year period with regular mowing cuts. In a comparison of phosphatic fertiliser forms, fertilisers were placed in the following order of decreasing effectiveness in terms of DM yield, P concentration, and P uptake data: Thermophos > superphosphate > 1:3 mixture of calcined “C” grade Christmas Island phosphate rock (CCIP) and superphosphate > CCIP = Gafsa phosphate rock > Nauru phosphate rock. CCIP and Gafsa improved relatively to Thermophos and superphosphate in the third year of the trial. Annual application of Thermophos, CCIP-superphosphate, and CCIP increased levels of Al-P and Fe-P in the soil. Superphosphate increased Al-P but not Fe-P. These manufactured fertilisers led to little change or a decrease in occluded-P and Ca-P levels. In contrast there was a large accumulation of Ca-P in the Gafsa and Nauru treatments which showed no change or a decline in Al-P and Fe-P levels. A single heavy application of the manufactured fertilisers increased Al-P and Fe-P in the first year after application but values declined in the second and third years. Heavy applications of Gafsa and Nauru phosphate rocks could not maintain the initial levels of AI-P and FeP. These treatments resulted in large accumulations of Ca-P in the first year followed by decreases in the second and third years. Bray No.1 and Olsen values for available P gave patterns of change similar to Al-P.

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