Here’s a puzzle. Two farmers in southern Manitoba seeded on the same day last spring (around June 10), but applied different rates of nitrogen with their canola. One farmer applied 60 pounds per acre, based on crop insurance data and seeding dates. The second farmer figured there was potential for higher yields with the right weather, and applied a more common rate of 120 lbs./ac. At the end of the season, both of these farmers had roughly 30 bushels per acre of canola in the bin and 20 lbs./ac. of residual nitrogen in the ground (at a 24-foot depth).
Where did the second farmer’s nitrogen go?
In this case, I believe denitrification was responsible for a significant portion of the loss. Simplified, denitrification occurs under anaerobic conditions. Soil microbes need oxygen to breathe. They can strip oxygen from nitrate molecules, leaving the nitrogen molecules in a form susceptible to atmospheric loss. In anaerobic conditions with a soil temperature of 5 C, two to four pounds of N can be lost every day.
Last year in southern Manitoba, the season started wet and quickly turned dry, which was likely the biggest yield limiting factor. In southern Manitoba, our heavy clay soils had a three to four inch rainfall event at the end of June, when soil temperatures were already in the mid to high teens. At that point, the crop was barely three weeks old and unable to draw any significant amount of moisture from the ground.
Optimal nitrogen levels
With the price of nitrogen creeping up again, use efficiency is becoming more important. Unfortunately, so many variables affect nitrogen uptake that the optimal application rate is a moving target from year to year. Some of the factors I believe have a big influence on nitrogen use efficiency are moisture, crop staging and temperature.
The first variable to consider is the amount of nitrogen required to grow the crop. These amounts vary by geographical area as well as from field to field, since nitrogen use relies heavily on water and many other factors like soil texture, topography, drainage and temperature. When we look at the optimal ranges in the chart adapted from the Canadian Fertilizer Institute we can see that requirements can vary substantially. As agronomists and farmers, we usually start with a base number of pounds of N per bushel, then fine tune that number for the local environment. A good agronomy team with local experience should have the tools required to help you figure out potential gains and losses.
Once you know how much nitrogen you need to grow the crop, the next step is to see what’s already available. Soil tests are an excellent way to learn how much residual nitrogen is in the ground from previous seasons. Testing organic matter levels help us understand the nutrient supplying capacity of the soil. Complete soil tests can also give us a good idea of the nutrient and moisture holding capacity of soils and help estimate potential yields.
It is important to note that high residual nitrogen levels can indicate a high degree of variability in a field. If a composite test comes up with levels of 60-80 lbs./ac. or more, there is a good possibility that some areas of the field are testing 20-30 lbs./ac. and other areas could be testing more than 100 lbs./ac. A strategy for these fields might be to zone-test based on topography, or use satellite imagery to determine if a variable rate nitrogen application would be a fit.
If residual nitrogen levels are low (20-30 lbs./ac.), chances are slim that some areas of the field have significant residual N levels. However, a low-testing field does not mean there is no variability — in some areas the nitrogen may have gone into yield, while in other areas it may have been lost to the environment.
Different crops require nitrogen at different times of the year. If we know when the plant needs nitrogen and how much, we can do a better job of having that nitrogen readily available. In many areas of the prairies, rainfall is most abundant early in the season. If we apply all the required nitrogen for the season upfront, it will be more susceptible to loss. If our fields are really prone to losses from leaching or denitrification, we may want to look at a slow release fertilizer or a split application to get us through the spring rains.
If you’re considering a top-up treatment later in the season, it’s important to consider at what stage a crop sets its yield. Cereals generally set their yield early in the season. Using wheat as an example, the three- to six-leaf stage is when tiller numbers and head size are determined. This stage is critical in setting up for big yields. If the crop is stressed during this crucial period, yield may be capped at a lower level and additional fertilizer may not provide a benefit. If conditions have been very good, it might pay to add a little more and target a higher yield.
Canola, on the other hand, has a very “elastic” yield. With the right conditions and fertility it can make up some yield later in the season. Knowing this, even though the plants may go through a stress period, there can still be a good chance of a payback from additional fertilizer if losses are suspected.
Another crucial component of big yields is water. Too little moisture and the crop can’t take up any nutrients. Too much and the crops won’t access required nutrients either. Moisture availability will vary between different geographical regions, and also within a field. Within a field, yield variability is strongly influenced by topography. Ridges usually have less topsoil and a coarser soil texture. Rainfall either runs through ridges or runs off right away, often carrying valuable topsoil with it. On the other end of the scale are the bottoms that receive the rain and topsoil from the higher ground.
Different areas of a field react differently to different weather environments. It’s important to address this variability by either fertilizing for the average or splitting a field into separate management zones.
So to pull everything together, we need to recognize that variability within most fields makes predicting exact yield and nitrogen requirements a big challenge. A lot of this variability is influenced by weather and out of our control. Applying all of our estimated nitrogen requirements up-front at seeding is risky if yield potential is reduced, or excess moisture makes the nitrogen unavailable to the crop. The more flexible we can be with nitrogen applications, the better we can match the requirements of the crop and minimize potential losses. Once a season in underway we usually have a better idea of a crop’s potential. If conditions are poor, we can minimize losses. If they are good we can raise our yield goals and add a little more nitrogen. The end result should be higher yields and better nutrient use efficiency. †