No. 10 Article 4/May 29, 2009

Nitrogen Management in a Challenging Spring

Substantial rainfall that creates ponded soils this late in the growing season results in loss of nitrogen (N). Nitrogen loss is difficult to predict because it depends on many factors, including time of application, type of N source, soil type and temperature, and amount of precipitation received. While it is difficult to know how much N is lost, I'd like to provide some information to help you estimate N loss for your situation, and I'd like to share some thoughts on how to apply additional N this late in the planting season.

Significant N losses occur only when N is in the nitrate (NO3-) form. The exception to this is urea, which is as leachable as nitrate. Urea transforms rapidly to ammonium (NH4+) once in the soil, but if rain comes within about a day after application, some of the N from urea or urea-containing fertilizers (like UAN) can be leached out of the root zone, especially in light-textured (sandy) soils.

Even though anhydrous ammonia and urea do not contain nitrate at the time of application, and 28% contains only one quarter of the N in nitrate form, of all these fertilizers are subject to nitrification. Nitrification is a process in which soil bacteria converts ammonium to nitrate. So before we can start talking about N loss, we need to estimate how much of the fertilizer applied is in the nitrate form.

For urea and UAN solutions, the conversion to nitrate starts fairly quickly after application at this time of the year because soil temperatures and moisture are close to optimal for the activity of nitrification bacteria. In the case of anhydrous ammonia, nitrification can be delayed as much as two weeks because this source kills the bacteria in the area surrounding the application. However, after that period nitrification will proceed quickly. For a while now, soil temperatures have been above 60 to 70°F. At such temperatures it takes between one and two weeks for all nitrogen to be fully converted to nitrate. In other words, regardless of the source of N that you use for your preplant application, if you did the application at the beginning of May, most or all of that N would now be in the nitrate form and subject to leaching and/or denitrification.

In a recent article in the Bulletin (issue 7, May 8), I discussed some of the ways you can estimate how much N is still available in the soil. There is no doubt that since that article was written the first week of May, N has been lost in most fields. With some of the heavy rains and high temperatures we had recently, if you applied urea or UAN at the end of April or anhydrous ammonia before the middle of April, it is possible that you will need between 40 and 100 pounds of N per acre if your field was saturated for several days and you are replanting or have not yet planted. If you find that only about 30 or 40 pounds of N per acre are lost, it is likely not worth your money to apply additional N.

If you have not yet applied N, you have several options. If you are still thinking of planting corn, very soon you will have to look into shorter-season hybrids; even for full-season hybrids you will want to consider the reduced yield potential and plan your N application accordingly, with the aim of getting a better return on your N investment. Optimum planting dates for Illinois are around April 10 to 15 in southern Illinois, April 20 to May 1 in central Illinois, and May 1 to 10 in northern Illinois. For each week that planting is delayed from the optimum for your area, it is recommended that you reduce N rate by 20 pounds per acre down to a minimum of 80 pounds per acre (for very late planting).

Due to the reduced yield potential with delayed planting, priority should be given to planting as soon as conditions are adequate; you can apply N later as a sidedress application. All of the typical N fertilizers can be used for sidedress applications. But in order of preference I would apply injected anhydrous ammonia or UAN solution between rows; broadcast of solid ammonium-containing fertilizers, such as ammonium nitrate or ammonium sulfate; broadcast urea; dribble UAN solution between rows; and broadcast UAN solution.

If you do a surface application, you must incorporate the fertilizer by rain or irrigation to move N to the root zone. With injection applications, place N between rows to reduce the potential for plant injury. Remember that there is no advantage to trying to apply N close to the row since roots will grow into the row-middles by the 4th-leaf stage.

Another option is to apply N in every other row instead of every row. Research has shown that this does not negatively affect yield 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. If you are concerned that corn might be too big by the time you can make N applications, it is possible, though not desirable, to apply N with high-clearance equipment or by aerial application. When doing aerial application it is important to keep rates below 125 lb N acre-1 and to avoid doing the application when the canopy is wet to reduce plant injury. If liquid applications are to be used, the rate should not exceed 10 lb N acre-1. For the time being, though, let's hope we can get all corn planted soon and prepare to sidedress N sometime before the 5th-leaf stage.--Fabián G. Fernández

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