How Does Nitrogen Loss Occur During Wet, Saturated Conditions?

Wet, saturated and/or flooded conditions raise numerous questions concerning nitrogen loss and management recommendations for corn, according to an article posted on Mississippi State University Extension Service’s Website.

Nitrogen loss resulting from saturated conditions and flooding occurs primarily through denitrification, particularly in heavy soils. Denitrification occurs when nitrate nitrogen is converted into nitrogen gas by microorganisms and escapes into the air. Warm soil temperatures accelerate this process. Research indicates denitrification rates range from 2-3% per day at soil temperatures from 55-65 F or 4-5% per day if soil temperatures exceed 65 F. (Hoeft, 2002 – Illinios).

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Timing of nitrogen application and nitrogen source will affect the amount of applied nitrogen that will be in the nitrate form. Growers who utilize UAN-solution (n-sol) can expect about 50% of the nitrogen to be converted to nitrate one week after application, 70% after two weeks, and 90% or more after 3 weeks.

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Therefore, you can estimate nitrogen loss by multiplying the nitrogen applied by the percentage converted to nitrate, then by multiplying this result by the number of saturated days times the denitrification rate.

Example:

A grower applied 200 lbs. of N (UAN) two weeks prior to the saturated conditions. The field has been saturated for 10 days. Soil temperature has been about 65 F. How many pounds of nitrogen have been lost?

200 lbs. of N * 70% converted to nitrate = 140 lbs. of nitrate nitrogen

140 lbs. of nitrate nitrogen * (10 days * 4%) = 56 lbs. of nitrogen loss

Growers can use these estimates to help make additional nitrogen application rate decisions. However, growers should realize the yield potential of this crop will likely be reduced because flooding/saturation inhibited root growth, stunted vegetative development, and promoted crown rot, and other diseases which may appear later, including crazy top. Thus, supplemental nitrogen application may not be warranted if growers have already applied their seasonal allotment. In the example noted, this field still has enough nitrogen available to produce a corn yield of around 110-140 bushels per acre. Thus, this grower may not need additional nitrogen because this is a realistic yield goal.

Growers who have not applied their intended nitrogen allotment may be forced to apply nitrogen by air because the corn may be too tall to permit ground equipment. However, nitrogen should not be applied to saturated soils. Ammonium nitrate is the preferred nitrogen source for aerial application because it is not subject to volatilize, compared to urea. Urea volatility would be substantial when temperatures are warm and rapid evaporation occurs, which will likely occur when rains cease. Granular nitrogen fertilizer sources will burn plant leaf tissue where granules lodge in the whorl. Thus, application should be limited to around 200 lbs. of fertilizer per acre if the corn is less than three feet tall or 100-150 lbs. per acre if the corn exceeds three feet tall.

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