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A report card on nitrogen in corn in 2021

Tom J. Bechman corn leaves showing signs of nitrogen deficiency
PLANTS LACK NITROGEN: Firing on leaves beginning at the tip and coming down the midrib represents classic nitrogen deficiency symptoms. The question is why some fields were deficient.
Corn Illustrated: Some fields lost N and yields suffered; other fields fared better.

Conditions varied across Indiana during 2021, from dry early to very wet in late June and July to very dry in late July and August. It’s not surprising that farmers saw a variety of responses to nitrogen from corn. Some saw plants that were deficient before tasseling, with yield losses of 100 bushels per acre in those spots compared to other areas in the same field. Others with tile saw few problems with nitrogen.

Jim Camberato, a Purdue University Extension soil fertility specialist, and Betsy Bower, an agronomist with Ceres Solutions, based in west-central Indiana, attempt to add some clarity to what people saw. With nitrogen prices soaring, it’s critical to understand how nitrogen works in the soil.

Nitrogen deficiency on somewhat poorly drained soils without tile. Some farmers reported nitrogen deficiency symptoms and pale green plants before tasseling on light, silty clay loam soils with drainage issues. Yield was off 100 bushels per acre in some spots, even though 180 to 200 pounds of actual N was injected in late spring.

“Where soils are waterlogged, nitrogen can be lost by denitrification,” Bower explains. Nitrates are turned into nitrogen gas by bacteria working in waterlogged soils. Nitrogen gas escapes into the atmosphere. Here, it’s not a matter of nitrogen lost by leaching through the soil, it’s N loss into the atmosphere after denitrification occurs, Bower says.

Damaged roots or lost nitrogen? Some farmers suspected that nitrogen was there, but roots didn’t take it up. “That can happen if plants are damaged in some way,” Camberato says. “However, if plants resumed growing normally after soils were waterlogged and then showed signs of N deficiency, it’s more likely nitrogen was actually lost into the air through denitrification.”

Soil compaction can restrict root growth, but plants severely affected by soil compaction are often stunted. “It’s true that even roots of healthy plants don’t work as well when soils are saturated,” Camberato adds. “If there is less nitrogen left and roots aren’t working efficiently as well, you’re more likely to see deficiencies.”

Causes of less mineralization of nitrogen. If conditions are right for mineralization, you can get from 100 to 150 pounds of nitrogen from a soil with a few percent of organic matter in the topsoil each growing season, Camberato says. It’s one of the reasons it sometimes takes fewer pounds of applied N to produce a bushel of corn.

“Very wet or very dry conditions slow down mineralization,” he says. “So, in wet spots, you’re losing N to denitrification, roots aren’t working as well, and then you don’t get nearly as much mineralization from the soil either.”

Bacteria that pull nitrogen from organic matter require oxygen, so if they work at all in waterlogged soils, they drop oxygen levels even more, making denitrification more likely. It’s insult upon injury, Camberato says.

High yield in pattern-tiled fields. If you’re going to lose nitrogen, some farmers figure it would be in well-tiled fields. Yet on tiled, silty clay loam soils, most reported high yields.

“You’re not going to lose much nitrogen through tile because root development was good, and corn was able to find the nitrogen,” Bower says. “Tile prevents more of the soil from becoming waterlogged.

“Plants spend more time actively taking up nutrients because soils aren’t waterlogged. Nutrient uptake stops in waterlogged soils.”

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