Improving the precision of phosphorus fertiliser recommendations for sugarcane

Phosphorus (P) fertiliser recommendations for growing sugar cane are currently made without taking account of either the rate or magnitude of the reaction between P and the soil. As a consequence, the recommendations are imprecise. This paper describes field and laboratory experiments aimed at improving the precision of P fertiliser recommendations. The response to fertiliser P differed for plant cane grown on two contrasting soils in the Herbert River district. For a sandy loam at Upper Stone, the estimated amount of P required to achieve 95% of maximum yield was 47.5 kg P ha-1• The response for a loamy clay at Hamleigh near Ingham was less well defined but differed significantly (p<0.025) from the Upper Stone result. Thus, the results indicate that different P fertiliser recommendations should be made for these two sites. However, using the current soil test and interpretation criteria, similar recommendations would be made for these two sites. In a laboratory experiment, soil samples from the same sites as the fertiliser trials were amended with P in solution. After a period of incubation, these soils were assayed using the BSES test, the Colwell test, which is widely used in many Australian agricultural industries, and the Pi test, a new test which mimics the action of plant roots in taking up P. Results suggest that all three tests reflect important differences between the soils, especially the more sensitive Pi test. It is concluded that fertiliser P recommendations for sugar cane could be improved if information on the reactions between P and different soil types were available.