Issue No. 6, Article 5/May 14, 2010
Applying Nitrogen After Planting
This growing season has been busy, to say the least! Typically a good portion of the nitrogen required by corn is applied in the fall. Last year, due to the late harvest and prevailing wet weather conditions, no more than 15% of the fall nitrogen was applied, leaving the bulk of the application for this spring. Good weather conditions have allowed many to plant their crops early, and some producers were able to get quite a bit of their nitrogen applications done as well. Still, there are many acres with corn already growing or starting to spike out of the soil that need to be fertilized with nitrogen. What are the options at this time?
Since all the typical nitrogen fertilizers can be used for side-dress applications, the important thing now is to determine your options. Some nitrogen sources might not be a choice simply because they are not available or the application equipment is not available. Once you know your options, next most important is to determine what would make most sense in terms of protecting the crop from injury, reducing nitrogen loss, and minimizing cost-both per pound of nitrogen and in time spent doing the application. From the standpoint of avoiding crop injury, protecting nitrogen from losses, and increasing the speed of application, if I had my choice I would go first with injected anhydrous ammonia or UAN solution between rows. My second choice would be broadcast of solid ammonium-containing fertilizers, such as ammonium sulfate or ammonium nitrate. Third, I would apply urea with Agrotain as a broadcast application. My fourth choice would be dribble UAN solution between rows. As a last resort, I would use a broadcast UAN solution.
Why these rankings?
First choice: Anhydrous ammonia converts quickly to ammonium in the soil and is less prone to loss compared to other sources if weather conditions become too wet for a while after the application, as happened last year. Applications between rows keep nitrogen at a safe distance from the crop to avoid injury, and they also put nitrogen where the roots will be growing. Such applications have become even easier than in the past with RTK guidance systems. Keep in mind that there is no advantage to trying to apply N close to the row since roots will grow into the rows' middles by around the 4th-leaf stage. Another option to increase speed, reduce horsepower, or both is to apply N in every other row instead of every one. Research has shown that yield will not be negatively affected by this placement because every row will have N applied on one side or the other. Of course, the outside injectors should deliver half the rate since the injector will pass between those rows twice.
Second and third choices: Surface application of ammonium-containing fertilizers is a good idea because they are not subject to volatilization. When you use urea, make sure to include Agrotain to protect it from volatilization. Since you won't be able to incorporate these broadcast applications with tillage after the crop is planted, try to make them before it rains at least half an inch so the fertilizer gets incorporated into the root zone. To avoid volatilization losses, the sooner it rains, the better, but normally not much is lost if it rains within 10 days or so. Broadcast urea applications work fine until about the time the crop is knee-high, but the sooner they are done, the better. Earlier applications can help reduce canopy injury and increase the chance of a good rain to work the urea into the ground. While you would likely see some leaf burn with urea applications at the early stages of corn growth, the damage is aesthetic and will not likely result in yield reduction.
Fourth and fifth choices: Dribble UAN solution is slow, part of the nitrogen is subject to volatilization, and just like with dry nitrogen fertilizers, it needs a good rain to be incorporated into the root zone. Broadcast UAN application is the least desirable practice because in addition to some of the same concerns mentioned for dribble UAN solution applications, it can cause severe leaf burn.--Fabián G. Fernández