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 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.
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.