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Is Spring Nitrogen Application for Winter Canola Necessary?

Posted by Blythe Howell | April 25, 2019

Winter canola requires 5-7 lbs N/acre for every 100 lbs/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.


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


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. 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.

Haiying Tao

For questions or comments, contact Haiying Tao via email at with the Department of Crop and Soil Sciences at Washington State University.

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