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Corn and Nitrogen as Rain Continues

Rain has fallen in Illinois nearly every day for the past three or four weeks, and rainfall totals for this period are two to three times above normal for more than half of the state. This has a lot of people wondering if enough nitrogen remains in the soil to supply the corn crop, said a University of Illinois crop scientist.

Daily high temperatures have averaged close to normal over the past three weeks whereas night temperatures have been 3 to 4 degrees above normal, so growing degree accumulation rates remain high, said Emerson Nafziger. “Sunshine amounts have been marginal, but growing conditions have been good enough to keep the crop coming along rapidly,” he added.

Fields that were planted in April in central Illinois are rapidly approaching tasseling, Nafziger noted. “With warm temperatures during June and plenty of water, we can expect corn plants to be tall this year,” he said.

“Except where roots have been in water for a week or more, fields and parts of fields where crop roots are still supplied with oxygen continue to show good canopy color. Much of the early-planted crop is in the rapid nitrogen uptake period, from about V9 through tasseling, during which the crop takes up as much as 6 or 7 lb of nitrogen per acre per day,” he added.

Nafziger is continuing to monitor soil nitrogen in an ongoing nitrogen-tracking study that is described here. The most recent samples, taken before the end of the recent deluge, did not show the large losses in soil nitrogen that might have been expected with such high rainfall.

“We know these numbers are variable and that we need to be cautious in using them, but the fact that soil nitrogen didn’t decrease sharply, especially at Urbana where so much rain fell, provides some confidence that losses have not been very high,” he said. “The crop had also taken up some nitrogen by the time of the last sampling, but not enough to draw down soil nitrogen by very much.”

The percentage of nitrogen found as ammonium increased some form early to mid-June, reflecting an increase in mineralization as soils warmed up, and perhaps some loss of nitrate, Nafziger explained.

“As plants begin to take up nitrogen at a rapid rate, we can expect soil nitrogen to drop, though nitrogen losses and mineralization will both affect the rate of change,” he said. “Over the next few weeks, we expect that the plants (canopy color) will be a better gauge of nitrogen availability to the plant than will amounts of nitrogen we measure in the soil.”

The question that remains, without a solid answer, is whether nitrogen levels might slip below those needed to maintain crop growth before uptake starts to slow. “From what we’re seeing so far that seems unlikely, at least if rains slow before too many more days,” Nafziger said.

“A more immediate question is whether the pale green or yellow spots in fields need nitrogen applied now in order to prevent serious yield loss. Rapid loss of color in places where water stands comes from loss of root ability to take up nitrogen, not from loss of nitrogen from the soil,” he said, adding that this is because parts of fields where roots are in aerated soil are not showing deficiency.

The extent to which roots of plants in wet or flooded soils will recover will not be known until soils dry and re-aerate enough for root function to return. Adding nitrogen before then will do nothing for the plants, and if soils remain wet or flooded, some of this nitrogen will be lost before the plant has a chance to take it up, Nafziger said.

“Aerial application of urea is not inexpensive, and while it can test our patience, waiting until soils dry out for a week or more before deciding that more nitrogen is needed is the best course of action. Our hope is that a lot of acres will return to green once soils dry out.

“Even if that happens, we expect such areas to have lost yield potential, and the larger the plants were when first flooded and the longer soils stay wet, the larger will be the loss. Adding nitrogen now will do nothing to fix this,” he added.

As plant size and leaf area increase, the ability of the crop to help move water out of the will increase as well so the crop itself will help to dry the soils and will speed progress toward aerated soil conditions.

“Water loss in yellow, root-damaged corn that is standing in wet soils is very slow, so we won’t see much help there. We even see drought stress symptoms such as curled leaves where plants are standing in water, because such roots are so disabled,” Nafziger explained.

“Our best hope is to get two weeks of weather without much rain and with average or below-average temperatures to help get the crop back on track. Even then, plants in some places will be so damaged that they can’t recover,” he added.

Source: Emerson Nafziger and Stephanie Henry, University of Illinois 

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