Low effectiveness of Duchess rock phosphate on pastures in northern Queensland

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Abstract
Rock phosphate from Duchess, northeastern Australia, and superphosphate were compared as phosphorus (P) sources for pasture in glasshouse and field experiments in northern Queensland. Duchess rock phosphate was an inferior source of P for pastures, compared with superphosphate. The yield response to rock phosphate, however, varied between soils. On strongly acidic soils with high P sorption capacity, rock phosphate increased yield and bicarbonate-extractable P in the soil. On near neutral soils of low P sorption capacity, rock phosphate had little effect on yield. When the rate of rock phosphate was expressed as citrate-soluble P, its performance improved relative to superphosphate. Rates of coarse Duchess rock phosphate need to be at least 10 times those of superphosphate to produce a similar effect on pasture growth. In a glasshouse experiment, the fine rock phosphate was 1.6 times more effective than the coarse material.