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Fall Applications of Nitrogen to Winter Canola

Posted by Blythe Howell | October 14, 2021

With winter canola heading into dormancy, it’s time to finalize your fertility plans for winter canola. Most research from the University of Idaho, Oregon State University, and Washington State University has shown that most of the nitrogen (N) should be applied later in the fall or early in the spring. If you are wondering how much N should be applied in the fall here are a few tips.

Timing

Timing is perhaps the most important factor when considering N management for winter canola. To optimize N uptake, we want the N to be leached into but not below the root zone. Fertilizer applied in the fall is more subject to leaching than spring applied fertilizer. However, as the spring of 2021 proved there is no guarantee of timely spring rains to transport spring-applied N into the root zone. One method for distributing N within the soil profile is to split apply fertilizer. Winter canola has been shown to perform best when nitrogen is split applied with 50% of the N applied in the fall and 50% of the N applied in the spring, and worst when 100% of the N is applied at planting in the fall.

Rate

A variety of experiments have been conducted in the Pacific Northwest with varying results. Some results indicate that if the total soil test N is equal to or greater than 100 lbs N/A there is no benefit from additional N applications. However, other studies indicate that the best method for determining the N rate is to use a unit nitrogen requirement (UNR) multiplier. The UNR is the pounds of N required to produce 100 lbs of canola seed. The UNR for winter canola in the Pacific Northwest has been shown to be between 6.5-7 lbs N / 100 lbs of canola seed produced. It is important to note that the UNR does not refer to the total pounds of N fertilizer, but rather the total amount of available N. Available N includes the soil test mineral N (nitrate + ammonium), the mineralizable N (contributions from organic matter), and the fertilizer N. Soil tests should always be taken prior to developing a nitrogen management plan. The following equations can be used to calculate the N fertilizer requirements using the UNR method.

N fertilizer req. = 6.5 x (yield goal/100) – (soil test mineral N + estimated mineralization)

Soil test mineral N = Nitrate Nitrogen + Ammonium Nitrogen

Estimated mineralization = % soil organic matter x 17 for no-till or 20 for conventional till 

Source

Surface applications of N in the fall are vulnerable to two types of losses. The first potential loss pathway is volatilization as ammonia gas. Ammonia volatilization frequently occurs in high pH and dry conditions. Urea is especially prone to volatilization as the initial dissolution reactions result in an increased pH which causes ammonium to convert to ammonia and volatilize as a gas. Urease inhibitors and timing fertilizer applications with rainfall can reduce the potential for ammonia volatilization. The second loss pathway is nitrate leaching. Nitrate leaching is most likely in high rainfall environments and can be reduced by using slow-release nitrogen fertilizers and nitrification inhibitors. Liquid or solid fertilizers can be used in split applications.


For questions or comments, contact Isaac Madsen via email at isaac_madsen@wsu.edu or via phone at (360) 448-9081.

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