Wheat and canola response to concentrations of phosphorus and cadmium in a sandy soil

An old phosphate rock experiment was used to determine critical Colwell soil test phosphorus values for spring wheat (Triticum aestivum L.) and canola (rape, Brassica napus L.). Different amounts of phosphorus, applied to the soil 16 years previously as triple superphosphate and phosphate rock fertilisers, and different amounts of triple superphosphate applied in the current year, were used to generate soil with different P status. The phosphorus fertilisers contained different concentrations of cadmium as an impurity. The experiment was thus used to relate soil test cadmium, measured using 0.005 mol CaNO3/L, to cadmium concentration in grain. Colwell soil test phosphorus, related to 90% of the maximum grain yield (critical value), was 58 mg phosphorus/kg soil for wheat and 19 mg phosphorus/kg soil for canola. In soil with low Colwell phosphorus concentrations, canola efficiently used phosphorus that was banded with the seed while sowing (drilled phosphorus), requiring 15 kg phosphorus/ha as triple superphosphate to achieve 90% of the maximum yield, compared to 65–70 kg phosphorus/ha for wheat. Soil test cadmium was highly correlated with grain cadmium in both wheat (R2 = 0.89) and canola (R2 = 0.96), suggesting soil testing for cadmium may be used to predict the likelihood of grain cadmium contamination.