Shoot Growth, Phosphorus–Nitrogen Relationships, and Yield of Canola in Response to Mineral Phosphorus Fertilization

Crop responses to increasing P fertilization are often variable and poorly related to soil P test. Our objectives were to determine the influence of P fertilizer on the growth and seed yield of canola ( Brassica napus L.) and to develop a first approximation of its critical phosphorus concentration (P c ) in shoot biomass. An experiment with four rates of P fertilizer (0, 20, 40, and 80 kg P ha −1 ) was conducted on soils with a low available P content at 5 site‐years [Normandin (QC; 2010, 2011, 2012) and Brandon (MB; 2010, 2012)] in Canada. Dry matter (DM) yield, and N and P concentrations were measured weekly on 5 d from early bolting to late flowering, and seed yield was measured at harvest. Seed yield was significantly increased by P fertilization at only 1 of the 5 site‐years, even though a positive response was expected at all site‐years. Phosphorus fertilization did not affect shoot growth and P uptake at any of the 5 site‐years. However, the estimated shoot biomass on the first sampling day was reduced with no P application at 3 of the 5 site‐years. A reduction in early crop growth of more than 30%, however, was required to affect seed yield. The whole‐plant canola P c was expressed as function of N concentration (P c = 1.74 + 0.024N, R 2 = 0.24, P = 0.05) but this first approximation requires further validation.