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Determine how much nitrogen is left

How much nitrogen do you lose after big rains?

Tom J. Bechman, Midwest Crops Editor

May 6, 2024

2 Min Read
 A flooded cornfield
RIPE FOR N LOSSES: Saturated soils set the table for nitrogen loss. In heavier soils like these, denitrification would be the major culprit. Tom J. Bechman

You took advantage of good fieldwork days and applied nitrogen before planting. Then rains came. In some areas, far too much rain fell — as much as 6 inches in a three-week period. Will that nitrogen still be there when corn needs it?

“There are a whole host of factors to consider before you can answer that question,” says Dan Quinn, Purdue Extension corn specialist. “It also matters whether you used a nitrification inhibitor to delay conversion of ammonium to nitrate.”

Quinn answers the most pertinent questions:

If nitrogen was lost, was it through leaching? Leaching below the root zone is one of the two major ways nitrogen could be lost. Nitrate is negatively charged and can move with soil drainage following big rains. This is typically common in more coarse-textured, sandier soils.

How else can N disappear? In fine-textured, poorly drained or ponded soils, denitrification occurs when nitrate is converted to nitrogen gas by soil bacteria. Oxygen is depleted in saturated soils. Two to three days of soil saturation is typically required for soil bacteria to begin denitrification.

How much N can be lost through leaching? It depends upon the amount of nitrogen applied that was in the nitrate form, as noted in the table (below). For example, if 150 pounds of N per acre was sidedressed as UAN containing a nitrification inhibitor one week ago, only 25% of the UAN is likely in the nitrate form. So, approximately 37.5 pounds of N has the potential to leach following a heavy rainfall.

That doesn’t mean that much will be lost. Soil drainage patterns matter, and a growing crop takes up some N. And just because N moves down the profile doesn’t mean it is out of the root zone.

A graphic table showing how long it takes fertilizer nitrogen to convert into nitrate

How much N could be lost through denitrification? First, remember that only nitrate is subject to denitrification. The higher the soil temperature, the faster nitrogen is converted from ammonium to nitrate. A nitrification inhibitor can help delay the conversion by about two weeks later in the growing season. A urease inhibitor can extend the conversion of urea to nitrate by seven to 10 days.

Saturated soils become anaerobic in one to two days. Then, at soil temperatures between 55 to 65 degrees F, approximately 2% to 3% of soil nitrate is lost per day. Above 65 degrees, approximately 4% to 5% is lost daily.

Suppose 150 pounds of N per acre as 28% UAN was sidedressed three weeks ago. Soils remained saturated for six days above 65 degrees. Calculations show you could expect 20 or 30 pounds per acre to denitrify and be lost. Note that this is only an estimate, but it puts you in the ballpark.

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About the Author(s)

Tom J. Bechman

Midwest Crops Editor, Farm Progress

Tom J. Bechman became the Midwest Crops editor at Farm Progress in 2024 after serving as editor of Indiana Prairie Farmer for 23 years. He joined Farm Progress in 1981 as a field editor, first writing stories to help farmers adjust to a difficult harvest after a tough weather year. His goal today is the same — writing stories that help farmers adjust to a changing environment in a profitable manner.

Bechman knows about Indiana agriculture because he grew up on a small dairy farm and worked with young farmers as a vocational agriculture teacher and FFA advisor before joining Farm Progress. He works closely with Purdue University specialists, Indiana Farm Bureau and commodity groups to cover cutting-edge issues affecting farmers. He specializes in writing crop stories with a focus on obtaining the highest and most economical yields possible.

Tom and his wife, Carla, have four children: Allison, Ashley, Daniel and Kayla, plus eight grandchildren. They raise produce for the food pantry and house 4-H animals for the grandkids on their small acreage near Franklin, Ind.

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