AN APPRAISAL OF SOIL-PHOSPHORUS TESTING DATA FOR CROPS AND PASTURES IN SOUTH AUSTRALIA

Data from more than 580 field experiments conducted in South Australia over the past 30 years have been re-examined to estimate extractable soil phosphorus (P) levels related to 90% maximum yield (C-90) for 7 c rop species (wheat, barley, oilseed rape, sunflower, field peas, faba beans, potato) and 3 types of legume-based pasture (subterranean clove r, strawberry clover, annual medics). Data from both single-year and l onger term experiments were evaluated. The C-90 value for each species was derived from the relationship between proportional yield responsi veness to applied P fertiliser rates (determined as grain yield in cro ps and herbage yield in ungrazed pastures) and extractable P concentra tions in surface soils sampled before sowing. Most data assessments in volved the Colwell soil P test and soils sampled in autumn to 10 cm de pth. When all data for a species were considered together, the relatio nship between proportional yield response to applied P and soil P stat us was typically variable, particularly where Colwell soil P concentra tion was around C-90. When data could be grouped according to common s oil types, soil surface texture, or P sorption indices (selected sites ), better relationships were discerned. From such segregated data sets , different C-90 estimates were derived for either different soil type s or soil properties. We recommend that site descriptors associated wi th the supply of soil P to plant roots be determined as a matter of co urse in future P fertiliser experiments in South Australia. Given the above, we also contend that the Colwell soil P test is reasonably robu st for estimating P fertiliser requirements for the diverse range of s oils in the agricultural regions of the State. In medium- and longer t erm experiments, changes in Colwell soil P concentration were measured in the absence or presence of newly applied P fertiliser. The rate of change (mg soil P/kg per kg applied P/ha) appeared to vary with soil type (or soil properties) and, perhaps, cropping frequency. Relatively minor changes in soil P status were observed due to different tillage practices. In developing P fertiliser budgets, we conclude that a maj or knowledge gap exists for estimating the residual effectiveness of P fertiliser applied to diverse soil types under a wide range of South Australian farming systems.