II On a Kokotau silt loam

Abstract

Trials on a Kokotau silt loam showed a 20% response in pasture dry matter yield to Ibex phosphate and a 4.5% response to gypsum over a 6-year period. In a comparison offorms of phosphatic fertilisers, yield, P concentration, and P uptake data placed the fertilisers in the following order of decreasing efficiency: Thermophos, superphosphate, Calcined "C"-grade Christmas Island rock phosphate (CCIP)-superphosphate (l :3), CCIP, Gafsa rock phosphate, Nauru rock phosphate. The trial design did not permit an accurate quantitative evaluation of relative effectiveness. Yields over the 6-year period were similar whether the fertilisers were applied annually or at 3 times the annual rate every 3 years. There was evidence that CCIP and Gafsa phosphates improved relative to superphosphate with time. Annual applications of the manufactured fertilisers increased soil AI-P and Fe-P fractions but changed occluded-P or CaP little. In contrast CaP accumulated and AI-P and Fe-P declined greatly in soils receiving the Nauru and Gafsa treatments. Triennial applications of the manufactured fertilisers increased AI-P greatly in the years after application, with declining values in the intermediate years. Fe-P, occluded-P, and CaP tended to increase with time for these treatments. For the rock phosphates, AI-P and Fe-P declined with time, occluded-P tended to increase, and CaP increased in the year immediately after application, with decreases in the intermediate years. Available phosphate by the Bray No. I and Olsen methods gave patterns similar to those of AI-P. Available phosphate increased greatly immediately after triennial applications, but decreased to the level of the annual applications after 3 years.