ABSTRACT
Summary The Winchmore trial was commenced in 1952 to investigate the superphosphate requirements of irrigated ryegrass-white clover pasture under grazing. Treatments receiving 0, 188, and 376 kg/ha annually of single superphosphate have produced average annual yields of 4.3, 10.0, and 11.2 metric tons (t)/ha dry matter, respectively. The control production is being maintained by continuing weathering of soil P and by the input of S in irrigation water and rainfall. Topsoil organic C, N, and S reached plateaus on all treatments after 10-15 years. Organic P (Po) showed a linear increase on all treatments until the trial was limed (4 t/ha) in 1972, which increased the pH from 5.8 to 6.6. Following liming, accumulation of Po ceased. There was a subsequent positive change in Pi and Truog P but not in either Olsen P or in increased plant uptake. This finding can be explained partly by the short-term precipitation of phosphate on lime particles and partly by a decline in the quality of superphosphate used since the early 1970s. The very large accumulation of Po prior to liming above pH 6.0 has led to investigations to determine: (1) whether this accumulation is general to New Zealand's climate; (2) the mechanisms, and forms of Po, responsible for the accumulation, (3) the extent to which the accumulation can be reduced by various means, including liming; and (4) the consequent savings in P fertilizer.
