Managing Nitrogen for Canola

At a recent virtual speaker series on canola topics, John Heard, Soil Specialist with Manitoba Ag, joined Ontario growers to answer questions on fertilizing canola. A canola crop needs 3 to 3.5 lbs of nitrogen (N) per bushel of grain produced. In spring canola production in Ontario, granular fertilizer is typically broadcast ahead of planting. The following are some of the questions and answers from the meeting.

In 2021 in Northeastern Ontario, there were heavy (5”) rains in July. How much nitrogen is lost in this situation?

Denitrification occurs on warm, saturated soils and is driven by microbial activity. The general number John uses is that for each day in spring when soils are cool (5° C) and saturated you lose about 2 to 4 lbs N/ac. The losses double with every 10° C increase in soil temperature, so later in spring when soils are about 15° C you lose 4 to 8 lbs N/ac per day. During July rains when soils are 25° C and saturated you have doubled the losses per day again, resulting in catastrophic N loss. Some producers noted those who added more nitrogen to canola in Northeastern Ontario after heavy July rains had higher yields than those who did not.

What are the ideal moisture conditions for mitigating N losses when broadcasting fertilizer?

When urea is not incorporated, up to 25% of the N can be lost to ammonia volatilization within a week if soils are warm and moist – this is the worst-case scenario. Soil moisture allows for urease enzyme activity (converting urea to ammonium/ammonia) while volatilization losses increase with temperature. If rain is not expected within 3-4 days, surface application without a protectant or incorporation is not advisable. Volatilization is a greater risk on sandy loams soils compared to clay loam or clay, but regardless of soil texture at least 4/10 of an inch of rain is needed to dissolve the urea and move it into the soil. If urea is applied to moist soil or 1/10 an inch of rain falls after application the urea melts and volatilization will continue until the soil dries or there is more rain to move it into the soil. As a rule of thumb, if there’s enough moisture that soil sticks to tires rather than throwing up dust, use a protectant like Agrotain. Where soils are persistently moist in the spring, it’s best to surface apply right before rain. Volatilization of nitrogen can also occur with ammonium sulphate (AMS) or Amidas if it sits on the soil for numerous days. This is especially true on calcareous soils and eroded knolls where high soil pH increases ammonia loss. There are no protectant products like Agrotain for AMS. John suggested that there may be lower losses with Amidas on acidic soils.