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Corn+Soybean Digest

Monitor Corn Nitrogen Carefully

Nitrogen (N), particularly liquid nitrogen, applied within a few hours of a heavy rain may be gone faster than it took you to put it on.

Tracy Blackmer, research director for the Iowa Soybean Association, says checkoff funded studies suggest that heavy rains this past week over parts of Iowa and other Midwest states may have carried away as much as half of the N farmers applied.

“Studies from Iowa State University and the Integrated Farm and Livestock Management program conducted by the Iowa Soybean Association suggest that if more than 2 inches of rain falls on a field where nitrogen was applied in the 24 hour period preceding the rain, losses could be as much as 60 percent,” he says.

Fred Blackmer, an Iowa State University agronomist and soil fertility researcher, says the reasons for such losses are easily understood when you consider that most liquid N solutions are made up of equal parts of urea, ammonium and nitrate (UAN) in a water based solution.

“In the first 24 to 36 hours following application, both the urea and nitrate portions of the solution are vulnerable to leaching if a heavy rainfall flushes through the soil,” he explains. “The ammonium portion at this time adheres to the soil molecules, so resists leaching.”

After a few days, the urea is hydrolyzed to form ammonium. At that point, only the nitrate third of the solution is vulnerable to leaching. After a week or more, however, the ammonium, both the original and that formed from the urea, are converted into nitrate. At this point, all the N from a 28 percent or 32 percent UAN solution becomes vulnerable.

Blackmer says since heavy rainfalls are less likely as the season progresses, and because plants are taking up water and nitrate fertilizer, nitrogen is likely to be lost after corn plants are about a foot tall.

Iowa growers who experienced heavy rains within hours of applying liquid N this spring may want to consider using a late spring nitrate nitrogen soil test. Blackmer says if this is done properly, it can give growers a good basis for deciding whether to apply additional fertilizer.

“It doesn’t always pay to apply more, even if some has been lost,” Blackmer continues. “More often than not, too little N is lost to justify adding extra. The late spring test will tell you how much of the fertilizer remains in the soil, but works best with broadcast applications of liquid fertilizer and manure. You’ll need to follow special soil sampling procedures if you injected a nitrogen solution, since the fertilizer will be concentrated in a band.”

Instructions for using the late spring test can be found in ISU-Extension Pamphlet PM 1714, Nitrogen Fertilizer Recommendations for Corn. To access this on the Internet, go to

Blackmer says that if soil sampling instructions are not followed precisely, test results and recommendations will not be accurate.

Additionally, it’s important that you wait until the corn is at least 6 in. tall to gather soil samples. Blackmer cautions against getting in too big a hurry to sample soils for this test. But don’t wait until the corn is more than 12 in. in height, since you’ll need a few days to receive test results and you don’t want the corn to be too tall for normal sidedressing equipment.

Once you’ve received the results, you’ll be able to weigh the fertilizer and application costs against the potential yield enhancement to decide whether applying more N is economical.

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