Soil and Tissue Tests to Predict the Phosphorus Requirements of Canola in Southwestern Australia

Canola (oil seed rape, Brassica napus L.) is now a major crop grown on the predominantly sandy soils in southwestern Australia and knowledge about the phosphorus (P) requirements of the crop in the region is limited. The results of 22 experiments done in the region are reported to determine the relationships between absolute seed (grain) yield response to applied P and (1) soil test P (Colwell sodium bicarbonate procedure) and (2) concentration of P measured in dried shoots at the rosette growth stage. Large grain yield responses to applied P occurred when Colwell soil test P values for the top 10 cm of soil were < 20 mg/kg soil and when concentrations of P in dried shoots were < 3.6 g/kg. The fertilizer P requirements for optimal economic return for canola grain production in the region varied from 10–35 kg P/ha. The 9 different canola cultivars used in the experiments from 1987–2005 had no major effect on the relationship between absolute grain yield response to applied P and soil test P. Application of fertilizer P mostly had no significant effects on either oil or protein concentrations in grain.