Agronomic performance of RPR fertiIiser in a grazed pasture

The effectiveness of Sechura RPR (SPR) plus sulphur (S) as a development and maintenance fertiliser was compared with superphosphate (SSP) on an irrigated grazed pasture. In 1980, SSP or SPR plus S was applied at 71 kg/ha phosphorus (P) to pastures that had not received fertiliser for 22 years, but had been recently over-drilled with ryegrass and white clover. Annual maintenance rates of 250 kg SSP/ha and 175 kg SPRjha plus S, (supplying 22 kg P/ha and 27 kg S/ha), were applied from 198 1 to 1992. In the first year the SSP treatment restored pasture production to near optimal level and this level was maintained over subsequent years with annual topdressings. In contrast, the SPR plus S treatment did not restore pasture production until the third year. In the first two years production from this treatment was only 69% and 88%. respectively, of the 250 kg/ha/year SSP treatment. From year 3, annual applications of SPR plus S maintained pasture production. In the first 3 years P concentrations of pasture topdressed with SPR were lower than those of pastures topdressed with SSP. Herbage S concentrations were adequate on both fertiliser treatments. Soil Olsen P levels rose quickly in the first year after the initial application of SSP then decreased to the same level as the SPR plus S treatment. Olsen P levels have been rising steadily since 1983, from both forms of P, indicating that 22 kg P/ha/year is above maintenance for this site. Keywords development fertiliser, irrigated pasture, maintenance fertiliser, Sechura reactive phosphate rock, superphosphate