Effects of reactive phosphate rock and superphosphate on sheep and pasture production in hill country

Abstract

Arad reactive phosphate rock (RPR) and single superphosphate(SSP) were compared as phosphate fertilisers in a sheep farmlet grazing trial and in two mowing trials over 3 years. The trials were on a volcanic ash soil at the MAF Te Kuiti Research Area on a site with limited fertiliser addition in the previous 2 years (Olsen P soil test averaged 10 in the grazing trial). Soil sulphur levels were high and there was no requirement for added sulphur over the -3 years of the trial. Pasture and animal measurements showed responses to SSP in all years, with RPR being initially less effective than SSP. By year 3, responses from RPR were near those from SSP. Economicassessmentof thegrazing trial corresponded with production data, except in year 1 when there was little benefit from either fertiliser. SSP was more cost-effective than RPR in year2 whereas they were similarin year 3. Over the 3 years, both fertilisers were profitable and there was an apparent cumulative net benefit from SSP over RPR. It was estimated that by year 4 the cumulative net benefit would be similar for both fertilisers or would be reversed in favour of RPR. Keywords reactive phosphate rock, hill country, grazing trial, superphosphate, mowing trials