Corn roots, wet soils, and nitrogen

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While much of the northern half of Illinois received less than normal amounts of rainfall through the middle of June, above normal rainfall amounts throughout Illinois during the second half of the month left root systems standing in saturated soils, in some cases in fields which had already been replanted after earlier floods, said University of Illinois crop sciences professor Emerson Nafziger.

Nafziger said rainfall amounts during the fourth week of June totaled as high as 3 to 6 inches in some areas. “While getting rainfall in June is certainly preferable to getting little or none as happened in Illinois in 2012, standing water and wet soils can badly damage a rapidly growing corn crop,” he said. “We saw this in parts of Illinois when heavy rain fell in late June of 2010. This year’s crop was planted later and is not nearly as far along as was the crop that year.”

Even with these rainfall amounts, the very good crop color present in June means that early nutrient uptake was good, which means that the crop recovered after being in earlier wet soils, Nafziger said. “While we expect the root system to recover after soils dry out, some plants after the latest round of rain may lose some of their green color before roots are fully functional again,” he added.

Although many worry that any stress during mid-vegetative growth will lower yield potential, Nafziger explained that there is not much evidence that reduced photosynthetic rates over a few days has much effect on yields, at least if it occurs more than one to two weeks before tasseling.

A head-high corn crop still needs to take up 110 to 120 pounds of nitrogen. “In years when June is wet, a common question is whether or not the crop might run out of nitrogen, leaving the crop short. Some producers have already applied more than normal amounts, thinking that some had been lost under wet conditions in April and May,” Nafziger said.

Warm, saturated soils lose nitrogen through the process of denitrification, which results in loss of nitrogen as a gas from the soil. “We do not think that such losses have been very large in most fields, given the temperatures and the fact that most flooding was temporary,” Nafziger said. “Better-drained soils would suffer less denitrification, but in those soils percolating water can move some of the nitrogen deeper, perhaps below the root system or into tiles lines.”

By the time corn accumulates 1,000 growing degree days (GDDs), reaching about stage V13 (13 visible leaf collars), it has accumulated about 20 percent of its dry weight and about 40 percent of its season-long nitrogen accumulation. During this period the crop will take up 3 to 3.5 pounds of nitrogen per acre per day. By the time of pollination, it will have taken up about 60 percent of its nitrogen and produced about 40 percent of its dry weight.

While the need for 20 or more pounds of nitrogen per week could raise the possibility of a shortage, the production of plant-available nitrogen from soil organic matter through the process of mineralization is also at its maximum rate in mid-season, Nafziger explained. “In a productive soil with a healthy crop, mineralization at mid-season likely provides at least half the nitrogen needed by the crop on a daily basis. This means that normal amounts of fertilizer nitrogen, even if there has been some loss, should still be adequate to supply the crop,” he said.

“Though we could measure soil nitrogen present or apply urea by air on the wetter fields or parts of fields where the crop shows deficiency, it would seem prudent to wait to see if the crop recovers its green color before going to this expense. The loss of crop color in wet soils is mostly due to loss of root function and less nutrient uptake, and roots will need to recover before the canopy does. Even without adding more nitrogen, odds are good that the crop will recover and thrive in the coming weeks, providing the weather remains favorable,” Nafziger said.

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