Rapid fixation of added phosphorus (P) into non-plant available forms means that P deficiency is commonly expressed in crop plants in Australia. Field trials conducted on the highly calcareous soils of Eyre Peninsula (South Australia) showed significant crop growth responses to fluid forms of P fertiliser, as compared to granular forms of P. A glasshouse trial was conducted to compare the responsiveness of wheat to fluid and granular forms of P fertilisers in 29 soils from southern Australia. Two fluid P fertilisers (phosphoric acid and ammonium polyphosphate) were compared to a granular P fertiliser (triple superphosphate) applied at 12 kg P ha^-1. In 86% of the soils tested wheat growth was P responsive, and in 62% of these P responsive soils wheat dry matter was significantly greater when fluid P fertilisers were used as compared to the granular form.

These soils were characterised using several chemical analyses with the aim to identify soil characteristics that influence the responsiveness of wheat to fluid fertilisers. Several traditional indicators used to predict P availability such as Colwell P, resin P and total P did not provide an indication of fertiliser type response. Soil calcium carbonate content had a dominant influence on the likelihood of differential responsiveness of a soil to fluid P fertiliser over granular. A bimodal relationship between plant responsiveness to fluid fertilisers and soil pH was observed where in acid and alkaline soils fluid fertilisers outperformed granular products. Therefore, crop response to fluid P fertilisers in various alkaline non-calcareous and acidic soil types requires further testing.