Corn growers are concerned about the amount of fall-applied nitrogen that might have been lost through the winter and how this might change nitrogen management this spring.
The first question that needs to be asked on nitrogen management is simple says University of Illinois Extension Agronomist Emerson Nafziger. You need to know how much nitrogen the crop will need, then how much is naturally available, and finally, how much should be applied.
“Our best estimate, and this is a bit of a floppy number, is the crop will take up about a pound of nitrogen for each bushel it produces. About two-thirds of that is going to be in the grain and removed by harvest of the crop. The other third will be in the residue. Some of this will get back into the soil, some won’t”, says Emerson Nafziger.
The amount of nitrogen needed then is about 1 pound for every bushel expected. If the expected yield is 200 bushels to the acre, then it will need 200 pounds of nitrogen.
Pay attention to this part.
Nafziger wrote in an article for the U of I’s pest management bulletin on April 18th that the more productive soils in Illinois contain about 3.5% organic matter. A rule of thumb calculation, read it online in The Bulletin, puts the N from this organic matter at 140 pounds. In some years this is apparently all available to the crop, and in others it isn’t.
The N Rate Calculator, which you may find online, tries to average out the low and high organic N years. N added as fertilizer for corn following soybeans in southern and central Illinois should be about 170 pounds, 20 pounds less in northern Illinois.
As for nitrogen loss, Nafziger has this to say in The Bulletin as it relates to his recent nitrogen treatment studies, “ these results show both the risk of N loss and the benefit from delaying some of the N or using inhibitors may be a little less than we’ve thought. Getting data from another year or two will help paint the picture more fully, but these results give some reason to be confident that the N management systems in common use all have good potential to provide the crop with N. Adding costs by changing N management, for example by making another trip over the field to apply late N, may not provide a positive return compared to applying all of the N in one or two earlier trips.”
Nafziger says the corn crop takes up most of its nitrogen in June.