Pasture responses to phosphatic fertilisers

The relative effectiveness of superphosphate and equal weights of alternative phosphate fertilisers has been determined on two Waikato soil types over a period of 4 years. Nauru rock phosphate gave approximately half the yield increases obtained with superphosphate on Hamilton clay loam and one-third on Horotiu sandy loam. (The superphosphate responses were of the order of 16% on Hamilton clay loam and 32% on Horotiu sandy loam). North African phosphate gave yields similar to superphosphate on unlimed ground, but was less effective on limed ground, markedly so in the case of Horotiu sandy loam. Oxalic acid superphosphate gave inferior results on Hamilton clay loam, yielding in eight seasons significantly less than superphosphate. Serpentine superphosphate yielded slightly but consistently less than superphosphate. "Super compound" (75% superphosphate, 15% Nauru rock phosphate, 10% serpentine rock) gave similar results to superphosphate, except in one season on each of the soils when yields were significantly lower. Carbonate of lime had no detectable effect on pasture production on the Nauru rock phosphate plots on either soil type. The addition of lime to superphosphate increased pasture production on Hamilton clay loam (initial pH 5.7), but had a negligible effect on Horotiu sandy loam (initial pH 5.9). Nauru rock phosphate greatly, and North African phosphate to a lesser extent, increased the soil phosphorus (Truog method). On Horotiu sandy loam the soil potassium was significantly decreased by the phosphate treatments.