Effectiveness of rock phosphate, coastal superphosphate and single superphosphate for pasture on deep sandy soils

The effectiveness of coastal superphosphate and two rock phosphate fertilizers was compared with the effectiveness of single superphosphate for pasture production on deep, humic, sandy podzols in high rainfall (> 800 mm annual average) areas of south-western Australia. The pastures were subterranean clover (Trifolium subterraneum) or mixed subterraneum clover and serradella (Ornithopus compressus). Coastal superphosphate was made by adding rock phosphate and elemental sulphur to superphosphate during manufacture, as it came out of the den before granulation. One rock phosphate was a 50% mixture of apatite rock phosphate from Nauru and Christmas Islands, and which was also used to make the single and coastal superphosphate used in this study, and superphosphate made in Western Australia at the time these experiments started. The other rock phosphate was Calciphos, the fertilizer produced by heating (calcining), at about 500 °C, Christmas Island C-grade ore, a calcium iron aluminium rock phosphate. There were two types of experiments. In the three Type 1 experiments, levels of each fertilizer were applied annually. In the two Type 2 experiments, levels of fertilizer were applied once only to new plots in different years. Coastal superphosphate was the most effective fertilizer in the Type 1 experiments, with both rock phosphates and single superphosphate being equally effective. All fertilizers were equally effective in the Type 2 experiments. There were large variations in fertiliser effectiveness values between yield measurements in the same or different years. It is known that P leaches from freshly-applied superphosphate in these soils. The extent of this leaching probably varies between yield measurements affecting effectiveness values determined for all fertilizers because the effectiveness values were calculated relative to the effectiveness of single superphosphate. The humic, sandy podzols remain wet during the growing season, are acidic, and are known from laboratory studies to possess adequate hydrogen ions to cause extensive dissolution of North Carolina rock phosphate so that rock phosphates are equally or more effective than single superphosphate in these soils. When elemental sulphur in coastal superphosphate is oxidized to SO₄ hydrogen ions are produced which in previous studies has been shown to enhance dissolution of rock phosphate in biosuper, a mixture of rock phosphate and elemental sulphur.