II On a Kokotau silt loam

Abstract

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 of forms 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 (1: 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 Al‐P and Fe‐P fractions but changed occluded‐P or Ca‐P little. In contrast Ca‐P accumulated and Al‐P and Fe‐P declined greatly in soils receiving the Nauru and Gafsa treatments. Triennial applications of the manufactured fertilisers increased Al‐P greatly in the years after application, with declining values in the intermediate years. Fe‐P, occluded‐P, and Ca‐P tended to increase with time for these treatments. For the rock phosphates, Al‐P and Fe‐P declined with time, occluded‐P tended to increase, and Ca‐P increased in the year immediately after application, with decreases in the intermediate years. Available phosphate by the Bray No. 1 and Olsen methods gave patterns similar to those of Al‐P. Available phosphate increased greatly immediately after triennial applications, but decreased to the level of the annual applications after 3 years.