North Carolina corn farmers should plan on a sidedress application of nitrogen and consider applying other nutrients such as sulfur in order to avoid reductions in yields following heavy rains that inundated the state April 23 to 25.

This advice comes from North Carolina State University Extension Corn Specialist Ron Heiniger who said the heavy rains will have the largest impact on growers who have already applied most of their fertilizer.

“They will need to supplement what they already have applied,” Heiniger counselled. “For growers who were planning sidedress applications these recommendations should just remind them to consider changes in soil nutrient levels caused by these recent storms.”

In his Spring 2017 Extension publication “Corn Kernels,” Heiniger noted that excessive water can result in poor nutrient uptake and can lead to root diseases such as Pythium or crown rot. “In general, plants cannot tolerate more than 4 to 5 days of conditions where the soil is covered by water without showing symptoms of severe nutrient stress,” Heiniger wrote.

The April 23 to April 25 storms delivered more than nine inches of rains to parts of North Carolina and Heiniger noted that the most common issue following the event is saturated soil conditions but not standing water or flooding in most corn acreage.

“The key problem in these cases is the leaching of mobile nutrients such as nitrogen, sulfur, magnesium or boron and/or denitrification of nitrogen. The question is how much of these essential nutrients have I lost and what should be done to replace them?” Heiniger wrote.

“It is difficult to determine the loss of nutrients such as nitrogen from leaching. In general, deep sandy soils will have the greatest loss due to leaching (up to 80 percent of the nitrogen  applied) and clay or heavy organic soils will have the least (30 percent  or less of the nitrogen  applied),” according to Heiniger.

“Corn is usually planted on heavier soils with less leaching potential. Even so leaching losses from 30 to 50 percent of applied nitrogen or sulfur can occur on Norfolk, Goldsboro, or even Lynchburg soils. Given the nature of this storm event the best guess in most cases is that around 30 percent of the applied nitrogen has been lost.”

Heiniger explained that denitrification is difficult to estimate.

“It depends on how long the soil remains saturated and soil temperature. Information from the University of Nebraska suggests that at soil temperatures above 75 degrees Fahrenheit about 20 percent of the nitrogen applied will undergo denitrification for each day the soil remains saturated. Denitrification will be worse on poorly drained clay or organic soils and least on sandy loams,” Heiniger said.

For corn that has not been flooded but where nutrient loss is the primary concern, Heiniger estimates that nutrient losses from the event are in the range of 30 to 50 percent of the applied nitrogen with similar losses of sulfur, magnesium and boron.

And how much fertilizer should a farmer apply?

“The most economical approach is to tissue test,” Heiniger advised. “Take a tissue sample from V3 to V4. If the sample shows that nitrogen is deficient this would indicate that nitrogen losses up to 50 percent had occurred and growers should replace 50 percent of the nitrogen they had previously applied. This same strategy would hold true for sulfur, magnesium or boron.”

If the tissue test shows that nitrogen is not deficient, Heiniger still recommends that growers still replace at least 30 percent of the nitrogen that was previously applied. “This would ensure that enough nitrogen was available for the corn to carry it through into the grain fill period,” Heiniger explained.

“For growers who have not applied all of their nitrogen, sulfur or other nutrients, this approach will help them determine if they need to adjust their nutrient application rates based on changes in soil nutrient levels. In some cases these growers may not need to change the amount of nutrients they had planned to apply and can save the cost of adding fertilizer they don’t need.”