By Haiying Tao, Washington State University 

Winter canola requires 5-7 pounds N/acre for every 100 pounds/acre seed yield (this is called unit N requirement) depending on N use efficiency. The high rainfall area where winter canola has higher yield potential, the unit N requirement is lower and vice versa. Winter canola is a deep rooted crop that can reach water and nutrients to 6 feet below the surface. 

Measuring available N in 6-foot soil depth is important to avoid excessive N applications. A recent study conducted in the Pacific Northwest found that there was no yield benefit when soil test N, including ammonium-N and nitrate-N, in the 6-foot soil depth was higher than 100 ppm at planting in fall regardless rainfall zone.

However, seed quality could be affected by N applications. The higher the N application rate, the higher the seed protein and lower oil contents were. Farmers only need to apply N when soil test N in the 6-foot soil depth is lower than 100 ppm.The timing of N application makes a difference in canola seed quality and nitrogen use efficiency.

Spring application results in the greatest reduction in oil content (Figure 1). For example, the oil concentration reduced by 10% when N was applied where soil test N in 6-foot was higher than 100 ppm in spring in a field located in intermediate rainfall zone. This was because N was not a limiting factor for reaching its yield potential. 

The additional available N only increased protein concentration. Oil and protein concentrations are linearly and negatively related (Figure 2). This is due to competition for carbohydrate skeletons during carbohydrate metabolism, therefore, increased N supply enhances the synthesis of proteins at the expense of fatty acid synthesis.

Figure 1. Relationship between seed oil concentration and timing of N application.

Fall-applied N in the high rainfall zone leads to high N loss. Spring application or split application between fall and spring is a better choice when N fertilization is needed where soil test N in the 6-foot depth is lower than 100 ppm. In this area, spring soil test in combination with pre-winter tissue test for determining N applications is the best practice and more important than fall soil test.

Figure 2. Relationship between protein and oil concentrations in winter canola seed.

Pre-winter tissue test tells us how much N has taken up by the plants, and spring soil test tells us N sufficiency level in spring. The spring N application rate than can be determined by using mass balance approach (i.e. N rate=(yield goal÷100)×6-N uptake in fall-spring soil test N-estimated N mineralization from soil organic matter). This is also true in the intermediate rainfall zone when winter rainfall is higher than normal. This practice results in the highest N use efficiency. In the low rainfall zone where N leaching loss is minimum, deep injected N in fall or during fallow is the best practice.