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

ABSTRACT

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.

Keywords:

• P concentrations in shoots
• rosette growth stage
• soil test P

ACKNOWLEDGMENTS

Funds were provided by the Grain Research and Development Corporation (DAW0075) and by the Western Australian Department of Agriculture and Food. Soil and plant chemical analyses were done by chemists of the Chemistry Centre (WA). Technical assistance was provided by M. J. Baker, T. D. Hilder, M. Kennedy, R. J. Lunt, and F. M. O'Donnell.

Notes

^a P extracted from soil by the 0.5 mol/L NaHCO₃ procedure (Colwell 1963).

^b P applied as single superphosphate (9.1 % P) at 0 and 40 kg P/ha at sowing.

^c Yield response of dried shoots (SR) [(yield for P₄₀) minus (yield for P₀)].

^d Grain yield response (GR) [(yield for P₄₀) minus (yield for P₀)].

^e OPR, required for optimal economic return, calculated by differentiating the exponential equation fitted to data for the relationship between grain yield and the amount of P applied, using 2006 cost for single superphosphate and price for canola grain in the region, with P as single superphosphate costing A$2.5/kg, and the price for canola grain A$0.4/kg (A$400/t).

^f Oil concentration response to applied P (OR) [(oil concentration for P₄₀) minus (oil concentration for P₀)].

^g ND, not determined.

^h NSR, no significant response to applied P, and when the non-significant responses to applied P were positive, the response was too small to calculate OPR values.