Winter canola response to soil and fertilizer nitrogen in semiarid Mediterranean conditions

Abstract

In the semiarid dryland wheat (Triticum aestivum L.) region of the U.S. inland Pacific Northwest, winter canola (WC) (Brassica napus L.) is an economically viable rotation crop. Winter canola produces marketable end-products while improving soil health and disrupting pest and disease cycles. Although annual production of WC in Washington State has increased in the recent decade, little regional fertility research has been conducted. As a result, WC is commonly fertilized in a manner similar to hard red spring wheat. Compared with wheat, WC has a deep and aggressive tap root system that can grow to depths of 180 cm to reach nutrients and water. Thus, WC requires a different N management strategy than wheat. Field experiments were conducted to evaluate the influence of soil residual N and fertilizer N application rate (range, 0–240 kg N ha−1) and timing (fall, spring, or split fall/spring) on WC yield and oil and protein concentrations. The study took place over a 2-yr period at seven locations across four agroecological classes. There was no yield response to N rate at six of the seven sites due to canola's high N uptake efficiency and the soils’ high residual N (92–224 kg inorganic N ha−1) after wheat–fallow. Increasing N rates and split or spring application resulted in lower oil/protein ratios. In addition, maximum yields correlated with total available water. Therefore, N management for WC should be based on soil test residual + mineralizable N, total available water, and end-use quality.