The Illinois State Water Survey  says that wet conditions in March, April and May 2009 were the “fifth wettest since statewide records began in 1895,” leading many farmers to wonder if they should apply more nitrogen (N) fertilizer to cornfields.
“I have received a lot of inquiries about N loss from cornfields. The question in everybody’s mind is: Do I need more? The answer is not a simple yes or no. Even more complicated is deciding how much replacement N should be applied,” says Fabián Fernández, University of Illinois  assistant professor of soil fertility.
The key factors in determining whether additional N is needed are the soil type, source of N used, time of application and amount of precipitation since fertilization.
“In silt-loam or fine-textured fields with poor drainage, if you had excessive rain (and water sat on the field long enough to kill the crop) about two weeks after applying a urea and ammonium nitrate solution  (UAN) or four or more weeks after applying anhydrous ammonia , you might consider applying 50-100 lbs. N/acre for the new corn crop. This situation occurs most often in low areas of a field,” Fernández says.
Fernández says in sandy or light-textured soils that have had 7 or 8 in. of rain two weeks after applying UAN or four weeks after applying anhydrous ammonia, it is likely that a substantial part of that N was leached out of the root zone. In this situation you might also consider applying between 50 and 100 lbs. N/acre.
“In silt-loam or fine-textured soils with poor drainage where a large rain event caused ponding for one to three days and UAN was applied at least two weeks before or anhydrous ammonia at least four weeks before the time of waterlogged conditions, you might consider applying 30-50 lbs. N/acre,” he says.
He says the situations where N loss potential is low are where excess soil water was present for one to three days within a week after applying UAN or urea; where anhydrous ammonia was applied less than three weeks before soils became waterlogged; or in light-textured soils where infiltrated rain was less than 4 in. and most of the applied N was not in nitrate form.
If a farmer side-dressed UAN or urea and had heavy rains the next day, N loss may be excessive only on sandy soils.
“On sandy soils, if more than 7 or 8 in. of rain fell, much of the N was likely leached out of the root zone. If rainfall was 4-7 in., some of the N probably leached out, and you might consider applying 30-50 lbs. N/acre. If there was less than 4 in. of rain, most likely additional N is not needed,” he says.
Fernández says a farmer’s experience with the particular field is important, but the best measure of whether enough N is available is the response of the crop.
“One simple way to test whether the crop has sufficient N is to establish a reference strip. If you are planning to apply additional N, an easy way to do this is to apply a higher rate in one strip in each field. If you can see differences between the strip and the rest of the field, it likely indicates that more N is needed.
“If you determine that additional N is not necessary in your field, it might be worth your time to apply some additional N in a small area just to double check. If you don't see differences, it will indicate that you have made a correct decision.”