Phosphate sources for pasture production on summer-dry soils in eastern New Zealand

In the period 1988-96, staff of Ravensdown Fertiliser Co-operative Ltd undertook a series of pastoral trials with the primary purpose of examining the performance of commercially and locally available fertiliser products containing various forms of phosphorus (P) and sulphur (S). Treatments included Superphosphate (SSP), Longlife Super, two partially acidulated phosphate rocks (PAPRs), and North Carolina reactive phosphate rock (NCPR), all containing where required additional sulphur (as elemental S, So). These were applied annually for 6 years to supply the current requirements for P and S at each site. The sites were predominantly in eastern New Zealand. This paper covers 12 pastoral sites representing the summer-dry Pallic soils (yellow-grey earths and their intergrades to yellow-brown earths). In mowing trials, 10 sites responded significantly in dry matter production to the commercial fertilisers; the remaining two sites had high initial Olsen P values or did not last 6 years. Superphosphate, Longlife Super, and the PAPR products Hyphos Supreme and Phospar/So produced significantly more dry matter than the control in all years, as did NCPR/So from year 2. Superphosphate gave the highest mean dry matter in all years, producing significantly more dry matter than NCPR/So in all years, Longlife Super in year 4, and the PAPR treatments in years 2-4. By year 5, the Longlife Super and PAPR treatments had reached 80-90% and the NCPR/So 60-70% of the relative effectiveness of SSP. There was some variation in the relative performance of individual fertiliser treatments with each site, however the causes could not be clearly isolated due to the interaction of climate on P and/or S plant availability. Results suggest NCPR/So treatments were not suitable for these soils, due to their inability to release P quickly, and the high requirement for S on these soils. Longlife Super and PAPR treatments performed well but only on some sites. Currently they are not cost effective compared with SSP. Rainfall, in particular summer rainfall, was considered the most important driver of the treatments in releasing P (and S) on these soils.