Phosphorus requirements and use for pastoral agriculture in New Zealand

Abstract

Pasture phosphorus requirements to replace on farm losses through animals and soil were calculated for a range of soil sets, and summarised for major soil groups economic farm classes and provinces. Predicted responses to P fertiliser on South Island "unimproved" soils were similar for 33 major soil groups out of 401Five further groups were almost similar in response. Olsen P data collected recently were similar to an earlier set (1977-79), and corresponded to relative pasture yields of top farmers within each group of soils. There were differences between the MWD estimates of stocking rates and census data of 578,000 stock units for the South Island and 100,000 for the North Island with however more variability between provincial totals (average of 1,031,000 +- 600,000 for the North Island and 519,000 +- 400,000 for the South Island). For the South Island, if Olsen P levels were below 10.2 for low producing pastures and 11.0 for high producing pastures, more P was needed for first year maintenance than for long-term at average farmer stocking rates. At top farmer stocking rates below an Olsen P of 12.4 more P was needed in the first year than for long-term on low producing pastures, while on soils with high producing pastures only one major soil group needed more P in the first year than for long-term. At potential farmer stocking rates all major soil groups with high producing pastures needed more P in the first year than for long-term maintenance, while all soils with high producing pastures with an Olsen P < 19.5 needed more P in the first year. Less P is needed for first year maintenance than for long-term at average and top farmer stocking rates for almost all the economic farm classes of the South Island. Unimproved soils of the North Island appear to be low in Olsen P and there are only two major soil groups where less P is needed for the first year than for long-term at average and top farmer stocking rates Olsen P levels of improved soils in the North Island were not available. Olsen P levels of "unimproved" soils in the South Island were correlated with P retention levels, demonstrating an important relationship (effect) of P retention on their P status.

Pasture utilisation (and relative yields) for North Island low and high producing grasslands were similar, but there were lower levels on low producing than on high producing pastures for the South Island. Although efficiency of P use (kgP/s.u.) decreases rapidly at relative yields approaching the potential farmer stocking rates, on some soils, e.g. South Island high country yellow grey earths, efficiency of P use is better than for some North Island soils at average farmer stocking rates where soil P losses are high.