Corn Illustrated Weekly: Low N Rates Produce Classic Nitrogen Firing

Symptoms disappear at higher rates.

When you don't apply nitrogen on corn, you expect that corn to show signs of nitrogen deficiency sooner or later. When it happens by pollination time, the effects will obviously impact yield.

That's what's likely going on at the Corn Illustrated Plots near Edinburgh, Ind., this week. Corn is tasseling and generally holding on well so far, even after only about an inch of rain spread itself over the first six weeks after planting. An inch at that point and a few showers since leaves corn looking decent at tasseling, even though it's low organic matter soil with low CEC and gravel at three foot or so below the surface. The saving grace is that it is not a sandy soil in the top three feet, but instead a loamy–textured soil.

Rates of zero, 50, 100, 150,.200 and 250 were applied sidedress with liquid 28% injected just below the surface when corn was roughly knee-high. About 20 pounds per acre was applied in dry fertilizer spread over the field before planting. The field was in soybeans a year ago. So to figure true N rate applied, add 20 to the sidedress rate. These are approximate rates. Actual rates applied by the applicator will be determined and provided when data for this replicated plot is available alter this year.

What was obvious this week, however, was that there were textbook examples of firing on lower leaves, primarily in the zero and 50 pound per acre sidedress plots. "You don't get much more classic than this," says Dave Nanda, a Corn Illustrated plot consultant and long-time plant breeder, holding up a leaf pulled from the bottom of a plant. "Yellowing and then browning begins down the middle. If it was potassium deficiency instead, the firing and browning would be on the outer edges of the corn leaves first."

Daniel Dorney, Director of Research for Bird Hybrids, Tiffin, Ohio, also inspected the plots this week. Dorney conducts much of his own small plot research near his home near Paris, Ill. He observed the same symptoms as Nanda.

"These corn plants look sick," he noted, referring to the plot that received zero N sidedress. What he didn't know was how much worse the zero plot looked before the mid-June salvation rain. At that point, plants were much more yellow and lagging in height behind plots receiving N. Still a very pale green, with plenty of firing going on and somewhat shorter stature, at least the zero plot is pollinating and setting ears.

The 50 pound rate was included because in other tests in the area a year ago, there were no improvements in yield after 100 pounds per acre sidedress was applied. So Nanda wondered if maybe they didn't go low enough to find the breaking point. Hence the 50-pound rate was included this year. He notes, however, that every year is different. Last year moisture came at just about the right time on that plot almost every time it needed it. That certainly hasn't been the case so far this year.

Even if dry weather continues and yields are subpar, Nanda believes data from the nitrogen plot will be valuable. "It will set an extreme- it's what you can expect in a very dry season if you don't get enough water at the right time," he says. "From the farmer's standpoint it's a disaster. But from a researcher's standpoint it could be valuable information, because you get a look at what happens under one of the possible extremes in the set of theoretically possible situations that could occur."

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