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Corn+Soybean Digest

Keep Corn Green

Was your corn a little yellow last year? That was a problem for a lot of growers.

Steve Commerford, a New Ulm, MN, crop consultant, says nitrogen deficiency in corn appears to be increasing. Growers either aren't applying enough nitrogen fertilizer, or more of the applied nitrogen is being lost to leaching and denitrification.

With that in mind, Commerford and other experts say it's time to improve nitrogen management.

"The days of basing nitrogen needs on yield goals are over," he says.

Alfred Blackmer, Iowa State University soil specialist, agrees. "Using general yield goals ignores the important year-to-year variability due to weather," says Blackmer. "Even realistic yield goals tend to be very poor predictors of actual fertilizer needs."

Several new tools are available to help improve nitrogen management. They include the preplant soil nitrate test, presidedress soil nitrate test and end-of-season stalk nitrate test.

The preplant soil nitrate test traditionally has been used west of the Missouri River, where soil nitrate losses due to leaching are unlikely. Minnesota and Wisconsin fertilizer and agronomy specialists refined testing methods to make it the soil test of choice in those states. Fertilizer suppliers, extension crop specialists or crop consultants can provide more information on this and other tests discussed here.

Iowa's Blackmer is a proponent of the presidedress (or late spring) soil nitrate test, which he helped perfect for use in the Corn Belt.

"It can be used to make site-specific evaluations that lead to improved nitrogen management," he says. "Our current recommendations are that farmers take a few samples from each field each year."

Blackmer says growers can apply minimal amounts of nitrogen preplant or at planting and then use the test to determine whether a sidedress application is needed. Even growers who can't work sidedressing into their programs can benefit.

"We're finding that many farmers are losing a lot of their fertilizer. This test can check to see how much of the nitrogen they applied is still there."

Heavy rain led to a lot of leaching last spring. If growers had used the presidedress test, they would have found out the nitrogen was no longer there and could have avoided deficiencies, Blackmer says. The test is extremely valuable when manure is used.

The amount of nitrate in the stalk after grain matures is a good indicator of nitrogen availability during the growing season.

Blackmer suggests using the end-of-season stalk nitrate test to find out whether the amount of nitrogen applied was sufficient or excessive. "This test may be difficult for farmers to work into their routines, since the stalks need to be collected one to three weeks after black layer development in the grain."

He adds, though, that stalk samples can be collected at or immediately after harvest, provided the combine head is set to leave the lower portion of the stalk unbroken, and harvest falls within the specified time frame.

A total of 15 stalk samples should be submitted for each area to be tested. Areas differing in soil type, fertilizer application rate or other factors should be sampled and submitted separately for testing. For best results, submit the bottom 6-14" of stalk.

Blackmer says to avoid severely damaged stalks. Remove leaf sheaths from the segments, then put them in paper bags and ship them to a lab for analysis as soon as possible. Refrigerate samples if stored more than a day before being shipped.

Results are quoted in parts per million nitrate nitrogen. "We've found that optimum yields are produced when the nitrate nitrogen is in a range of from 700 to 2,000 ppm," says Blackmer. "If stalks run higher than that, there was probably an excess of nitrogen available to the crop."

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