COMPARISON OF SINGLE AND COASTAL SUPERPHOSPHATE FOR SUBTERRANEAN CLOVER ON PHOSPHORUS LEACHING SOILS

''Coastal superphosphate'', a partially acidulated rock phosphate (PAR P), is being considered as an alternative fertilizer to single superph osphate for pastures in high rainfall (> 800 mm annual average) areas of south-western Australia. The effectiveness of single and coastal su perphosphate, as P fertilizers, was measured in two field experiments using dry herbage yield of subterranean clover (Trifolium subterraneum ). The experiments were started in April 1990 and were terminated at t he end of 1993. In the years after P applications, soil samples were c ollected each January to measure Colwell soil-test P, which was relate d to plant yields measured later on that year, to provide soil P test calibrations. Relative to freshly-applied single superphosphate, the e ffectiveness of freshly-applied coastal superphosphate and the residue s of previously-applied single and coastal superphosphate were less ef fective in some years (from 3% as effective to equally effective), and up to 100% more effective in other years. This large range in effecti veness values in different years is attributed to different climatic c onditions. Soil P test calibrations were different for soils treated w ith single or coastal superphosphate. The calibrations were also diffe rent for different yield assessments (harvests) in the same year, and in different years. Consequently soil P testing can only provide a ver y crude estimate of the current P status of the soils.