The influence of soil properties on the effectiveness of phosphate rock fertilisers

Eighteen soils from south-western Australia were used to measure the effectiveness of 2 reactive phosphate rocks (RPR) [North Carolina and Sechura (Bayovar) RPRs] relative to superphosphate (relative effectiveness, or RE) using (i) yield of dried shoots of 30-day-old wheat plants (RE yield), (ii) P content of the dried shoots (REPcontent), and (iii) bicarbonate soil test P (REbicP) as measures of effectiveness. The RE values were positively related to PR dissolution in soil, titratable acidity, and to P and Ca retention by soil, and were negatively related to soil pH.No single soil property adequately predicted REyield, REPcontent, or REbicP. Stepwise multiple regression indicated that: (1) PR dissolution, soil pH, and organic carbon together accounted for about half the variation in REyield; (2) organic C, soil pH, and titratable acidity together accounted for about 67% of the variation in REPcontent; (3) PR dissolution, P retention capacity, and titratable acidity together accounted for about 71% of the variation in REbicP. We conclude that the agronomic effectiveness of phosphate rock fertilisers is a consequence of complex interactions of phosphate rock with soil that cannot be adequately predicted by measurements of a single soil property.