The Illinois State Water Survey says that wet conditions in March, April, and May of 2009 were the "fifth wettest since statewide records began in 1895," leading many farmers to wonder if they should apply more nitrogen fertilizer to cornfields.



"I have received a lot of inquiries about nitrogen loss from corn fields. 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 nitrogen should be applied," said Fabian Fernandez, University of Illinois assistant professor of soil fertility.



The key factors in determining whether additional nitrogen is needed are the soil type, source of nitrogen 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 2 weeks after applying a urea and ammonium nitrate solution (UAN) or 4 or more weeks after applying anhydrous ammonia, you might consider applying 50 to 100 pounds of nitrogen per acre for the new corn crop. This situation occurs most often in low areas of a field," Fernandez said.



Fernandez says in sandy or light-textured soils that have had 7 or 8 inches of rain 2 weeks after applying UAN or 4 weeks after applying anhydrous ammonia, it is likely that a substantial part of that nitrogen was leached out of the root zone. In this situation you might also consider applying between 50 and 100 pounds of nitrogen per acre.
"In silt-loam or fine-textured soils with poor drainage where a large rain event caused ponding for 1 to 3 days and UAN was applied at least 2 weeks before or anhydrous ammonia at least 4 weeks before the time of waterlogged conditions, you might consider applying 30 to 50 pounds of nitrogen per acre," he said.



He says the situations where nitrogen loss potential is low are where excess soil water was present for 1 to 3 days within a week after applying UAN or urea; where anhydrous ammonia was applied less than 3 weeks before soils became waterlogged; or in light-textured soils where infiltrated rain was less than 4 inches and most of the applied nitrogen was not in nitrate form.



If a farmer side-dressed UAN or urea and had heavy rains the next day, nitrogen loss may be excessive only on sandy soils.



"On sandy soils, if more than 7 or 8 inches of rain fell, much of the nitrogen was likely leached out of the root zone. If rainfall was 4 to 7 inches, some of the nitrogen probably leached out, and you might consider applying 30 to 50 pounds of nitrogen per acre. If there was less than 4 inches of rain, most likely additional nitrogen is not needed," he said.



Fernandez says a farmer's experience with the particular field is important, but the best measure of whether enough nitrogen is available is the response of the crop.
"One simple way to test whether the crop has sufficient nitrogen is to establish a reference strip. If you are planning to apply additional nitrogen, 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 nitrogen is needed.



"If you determine that additional nitrogen is not necessary in your field, it might be worth your time to apply some additional nitrogen in a small area just to double check. If you don't see differences, it will indicate that you have made a correct decision."