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.