Long-term availability of phosphorus from calcined rock phosphate compared with superphosphate

Abstract

The phosphorus supply from single dressings of three rates of superphosphate and Christmas Island C-grade ore calcined at either (a) 500�C or (b) 900�C were compared in a long-term field trial by measuring (i) bicarbonate-extractable phosphorus and (ii) dry weight of tops of a subterranean clover based pasture in spring each year. For superphosphate, the bicarbonate-extractable phosphorus decreased with time, whereas for the two calcined products, the amount of bicarbonateextractable phosphorous, although initially low, increased slightly with time. As measured by herbage yield, the effectiveness of superphosphate decreased from the first to the second year and thereafter changed little with time. The effectiveness of the two calcined products tended to increase with time. Superphosphate was much more effective than the two calcined products throughout the trial. The effectiveness of both calcined products were similar, despite the fact that the fertilizers differed in their proportions of total phosphorus which was soluble in neutral ammonium citrate. The bicarbonate soil test did not always predict the same future pasture production for the three phosphorus sources, indicating that different soil test calibration curves are needed for each source.