Soil phosphorus-crop response calibration relationships and criteria for oilseeds grain legumes and summer cereal crops grown in Australia

Australian cropping systems are dominated by winter cereals; however, grain legumes, oilseeds and summer cereals play an important role as break crops. Inputs of phosphorus (P) fertiliser account for a significant proportion of farm expenditure on crop nutrition, so effective fertiliser-use guidelines are essential. A national database (BFDC National Database) of field experiments examining yield responses to P fertiliser application has been established. This paper reports the results of interrogating that database using a web application (BFDC Interrogator) to develop calibration relationships between soil P test (0-10 cm depth; Colwell NaHCO3 extraction) and relative grain yield. Relationships have been developed for all available data for each crop species, as well as for subsets of those data derived by filtering processes based on experiment quality, presence of abiotic or biotic stressors, P fertiliser placement strategy and subsurface P status. The available dataset contains >730 entries but is dominated by data for lupin (Lupinus angustifolius; 62% of all P experiments) from the south-west of Western Australia. The number of treatment series able to be analysed for other crop species was quite small (<50-60 treatment series) and available data were sometimes from geographic regions or soil types no longer reflective of current production. There is a need for research to improve information on P fertiliser use for key species of grain legumes [faba bean (Vicia faba), lentil (Lens culinaris), chickpea (Cicer arietinum)], oilseeds [canola (Brassica napus), soybean (Glycine max)] and summer cereals [sorghum (Sorghum bicolor), maize (Zea mays)] in soils and farming systems reflecting current production. Interrogations highlighted the importance of quantifying subsurface P reserves to predict P fertiliser response, with consistently higher 0-10 cm soil test values required to achieve 90% maximum yield (CV90) when subsurface P was low (<5 mg P/kg). This was recorded for lupin, canola and wheat (Triticum aestivum). Crops grown on soils with subsurface P >5 mg/kg consistently produced higher relative yields than expected on the basis of a 0-10 cm soil test. The lupin dataset illustrated the impact of improving crop yield potentials (through more effective P-fertiliser placement) on critical soil test values. The higher yield potentials arising from placement of P-fertiliser bands deeper in the soil profile resulted in significantly higher CV 90 values than for crops grown on the same sites but using less effective (shallower) P placement. This is consistent with deeper bands providing an increased and more accessible volume of profile P enrichment and supports the observation of the importance of crop P supply from soil layers deeper than 0-10 cm. Soil P requirements for different species were benchmarked against values determined for wheat or barley (Hordeum vulgare) grown in the same regions and/or soil types as a way of extrapolating available data for less researched species. This approach suggested most species had CV90 values and ranges similar to winter cereals, with evidence of different soil P requirements in only peanut (Arachis hypogaea - much lower) and field pea (Pisum sativum - slightly higher). Unfortunately, sorghum data were so limited that benchmarking against wheat was inconclusive.