Economic impact of residual Nitrogen and preceding crops on wheat and canola

The application of nitrogen (N) fertilizer is known to significantly improve crop yield, but knowledge of its residual effect combined with the residual effects of legume and non-legume preceding crops (PCs) on crop yield and profits in future years is limited. A 6-year study was conducted in multiple locations across western Canada in order to determine the residual effects of various PCs (field pea, lentil, canola, wheat, faba bean harvested for grain, and faba bean harvested as a green manure) and nitrogen application on the profits of wheat and canola grown in subsequent years. In the first phase of the study, canola was seeded in 2010, barley in 2011, and canola again in 2012; N fertilizer was applied at varying rates for each crop. In the second phase of the study, spring wheat was grown in 2013 and canola in 2014; no N fertilizer was applied in these years. Legume PCs were found to have a positive effect on N supply and profits for the following two crop years, but these benefits decreased over time and were almost nonexistent in phase II. Residual N fertilizer for most rates was also found to be beneficial for about two years before its effect dissipated; residual N had a positive effect on the profits of wheat in 2013, but on canola in 2014 the economic benefit was mostly with the highest application rate of N (120 kg N ha-1). The results of this study showed that although production profits are greatest with an annual fertilization program based on the production capacity of the region, under the dry conditions of the Canadian prairies excess N remaining in the soil after crop production can be used by following crops. While this residual N is not generally sufficient to optimize crop production, it reduces the economic risk from the over-application of fertilizer if N is not utilized by the crop due to adverse growing conditions that reduce crop yield and N uptake in the year of fertilizer application.