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Issue No. 11, Article 5/June 6, 2008

Nitrogen Applications for This Spring

We continue to stay excessively wet this spring, with several additional inches of water falling in the first days of June. The big questions for many people now are whether they will have a crop worth harvesting this year and whether to replant corn or plant an alternative crop; these topics have been addressed in recent articles in the Bulletin. Another question people are asking is whether they have sufficient nitrogen (N) in their fields and what to do if they find they do not.

The first point to be remembered is that whether planting was delayed or not, the cool and wet conditions prevailing this spring have caused, and continue to cause, reduced yield potential. At this time it is difficult to know exactly what the reduction may be, because much will be dictated by weather conditions later during the summer. What is certain is that we should be very cautious about application of additional N this spring, for two main reasons: if the later part of the season turns out to be limiting for yield, there is very little likelihood that N in addition to that already applied in fall or preplant will increase yield; and the risk of not recovering the investment on expensive nitrogen is high, not only because of the possibility of low yields but also because, depending on weather conditions in the next two weeks, important decisions on replanting or replacing with a different crop altogether will have to be made in many cases.

It is challenging to expect patience at this time, but the best advice is to wait to apply N until weather conditions allow a more informed decision. Research in Illinois has shown that there is no yield reduction due to N if applications are done before the 5-leaf stage. This is because most soils in Illinois can provide sufficient N to satisfy the demands of young corn plants. After about the 7- to 8-leaf stage, N uptake is rapid until after pollination. So if supplemental N cannot be applied before the 5-leaf stage, it is critical to apply it as soon as possible before the 10-leaf stage and definitely not later than tasseling.

At present there are no soil tests that provide very reliable information to determine how much N will be available and whether additional N is needed at side-dress time. The pre-side-dress soil nitrate test (PSNT) can help determine the amount of organic N that will be available to the plant through mineralization. However, accurate collection of samples is critical to obtaining reliable information from the test. Soil samples should be collected when corn is in the 4- to 6-leaf stage to a soil depth of 12 inches, and each sample should be a composite of at least 10 cores (15 to 25 are recommended). Due to the difficulty in obtaining an adequate sample, this test is not being used extensively. If nitrogen results are greater than 25 ppm, the chance of increasing yield with additional N under typical growing conditions is very small. This interpretation is especially true for 2008. Due to the excessive rain, even if test values are below 10 ppm I would not apply a large amount of N. A more practical approach to determining whether additional N is needed is to perform strip N applications in a field to see if there is a response in growth or level of greenness.

Another possible method to determine the need for additional N is estimating the amount of nitrate N from N applications that have been lost. In an earlier article in the Bulletin I mentioned that the wet conditions in the early spring were not a cause to be overly concerned about N loss since the soils were still cool and not much conversion of ammonium to nitrate had likely taken place. By now, warm soils (temperatures well above 70°F) allow bacterial activity and conversion of ammonium to nitrate. Likely more than three quarters of the ammonium from fall anhydrous ammonia applications has been converted to nitrate by now. For early spring preplant N applications, probably half of the ammonia has been converted to nitrate. Of course, bear in mind that if an inhibitor was used these values will be lower.

Between the short window for application and limited supply of anhydrous ammonia last fall and the slow start to the growing season this spring, many have applied N in the last two weeks. This application, especially if anhydrous ammonia was used, would still be largely in the ammonium form and not subject to loss at this time. On the other hand, if the application used sources that contain nitrate (such as urea-ammonium nitrate), 25% of the N is in the nitrate form at the time of application.

Typically for Illinois, N losses occur mainly via denitrification in fine-textured soils. But the substantial rainfall (several inches in some areas) occurring in the first days of June on soils that were already moist could also result in substantial amounts of nitrate being leached out of the root zone. Still, denitrification is a great concern due to the warm soil temperatures combined with continuing excessive rainfall, causing fields to be saturated. Denitrification of nitrate in fine-textured soils can be significant under saturated conditions at this time of the spring. Research conducted in Illinois has shown that when soil temperatures are above 65°F, 4% to 5% of the nitrogen in the nitrate form (not the ammonium form) can be lost for each day that soils are saturated. Having an estimate of how much nitrogen is in the nitrate form and knowing how many days the soils are saturated can be used to estimate N loss via denitrification. The following is an example:

If 50% of a 140 lb N application is in the nitrate form, and soils are then saturated for 5 days, the N loss estimate would be (140 lb N per acre X 50% nitrate/100) X (5% per day/100) X (5 days) = 17.5 lb N per acre.

Having said all of this about N loss, I would like to reiterate the need to be extremely cautious about going out and applying additional N at this time. It is important to remember that some yield potential has been lost, and important planting decisions will have to be made in the next two weeks. Under present conditions, I doubt that supplemental N above 60 to 80 lb N acre-1 would be necessary to maximize yields.

If additional N applications are deemed necessary in your field, reduce the chance of crop injury from free ammonia by making sure soil conditions are adequate for applying anhydrous ammonia. Other alternative N sources (listed from most to least desirable) include application of UAN solution between rows, broadcast of solid ammonium-containing fertilizers, broadcast urea, dribble UAN solution between rows, and broadcast UAN solution.--Fabián G. Fernández

Fabián Fernández

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