Phosphate balances in grazed, hill-country pasture: Implications for fertiliser recommendations

Abstract

The effect of different rates of phosphate (P) fertiliser (10, 20, 30, 50, or 100 kg P/ha) on various P pools was monitored on four slope groups (0–10°, 11–20°, 21–30°, and 31°+) in a large-scale, hill-country grazing trial. The P pools involved were plant P, faecal P, and soil P (inorganic, organic, nonoccluded, occluded, calcium-bound, and Olsen). Plant availability of the soil P pools was investigated in an exhaustive cropping pot trial. Results from these trials were used to test some of the assumptions and predictions within the Computerised Fertiliser Advisory Scheme (CFAS) used by Ministry of Agriculture and Fisheries advisors to make fertiliser recommendations for many New Zealand farmers. Animal losses of P per stock unit were found to be slightly lower than predicted by the CFAS; estimates of soil losses were similar when based on increases in non-available inorganic P. The net effect was a slightly lower P requirement than suggested by the CFAS. Difficulties with making comparisons between trial results and CFAS data are discussed as are the problems associated with defining what constitutes a loss of P in the soil.

Keywords:

• fertiliser
• Olsen P
• phosphate