Mitscherlich‐Modeled, Semi‐Arid Canola Nitrogen Requirements Influenced by Soil Nitrogen and Water

Prospects for canola ( Brassica napus L.), as a rotational crop for agronomic and market diversification, have stimulated agronomic research to fit this crop into the unique environments and soils where wheat ( Triticum aestivum L.) production dominates. The objectives of the present study were to define N and water requirements of canola following wheat or fallow in a semi‐arid climate and to develop a predictive model for making N fertilizer recommendations. Field experiments were conducted at 12 site‐years with five N rates (0, 45, 90, 134, and 179 kg N ha −1 ). Pre‐plant soil and post‐harvest plant sampling provided an assessment of Mitscherlich‐modeled relationships among yield, soil N and water availability, economic optimal N supply, and fertilizer rates over the course of the study. Soil nitrogen supply (N s ) availability following wheat was lower than following fallow (77 vs. 205 kg N s ha −1 ) leading to higher N fertilizer requirements (47 vs. 0 kg N f ha −1 ) for canola following wheat, despite having lower water limited‐yield potentials. Unit nitrogen requirements (UNRs) at economic optimal yield levels ranged from 7 to 17 kg N s (kg grain) −1 across site‐years. Overall, these UNRs are generally higher than other reported canola recommendations from similar production areas. We attribute this to our inclusion of greater residual soil N depths and N mineralization contributions to N supply estimates. The N requirements were found to be completely or largely satisfied by non‐fertilizer N sources in situations when non‐fertilizer N supplies are high and water availability limits canola yield potential. Core Ideas Available water influences canola yields and N responses. Fertilizer N responses obeyed the Law of Diminishing Returns. Unit N requirements decreased with increasing water‐driven yield potentials.