Nitrogen Losses in Soil During Wet Weather

The steady flow of wet weather in April has caused many farm managers to worry about nitrogen losses in their fields, especially in fields where nitrogen was applied prior to the rains. Fortunately, nitrogen losses might not be as bad as many producers expect, said Lloyd Murdock, PhD, extension soils specialist with the University of Kentucky College of Agriculture.

"Producers always expect high nitrogen losses during extremely wet conditions," Murdock said. "In most cases, they aren't going to have the losses that they anticipated. Not knowing this could cause them to spend an excessive amount of money on additional fertilizers."

Producers who farm areas that flood annually, such as along the Mississippi and Ohio rivers, typically haven't applied nitrogen yet. Those who farm along creeks and secondary rivers, however, might have already applied nitrogen. These smaller tributaries could be submerged for some time due to recent major flooding of the larger rivers.

The main cause of nitrogen loss in wet soils is denitrification. This occurs when bacteria convert the nitrate nitrogen in the soil into nitrogen gas. Denitrification is triggered when the soil remains saturated for two to three days. Thus, poorly drained, low-lying areas are the most susceptible. During denitrification an average of 3 to 4% of nitrate nitrogen is lost per day for each day of saturation.

Fertilizers are comprised of varying rates of nitrate nitrogen. One variable in determining the amount of nitrate nitrogen in the soil is the length of time between the fertilizer application and the time of the soil saturation. Over longer periods of time ammonia, ammonium, and urea are converted to nitrate. Murdock developed a chart to help producers determine how much nitrate nitrogen is in their soil. That chart, along with sample calculations, is available online.

Soil testing specifically for nitrate nitrogen levels is another way to determine the amount of nitrate nitrogen in the soil. Producers should make sure soil samples are a foot deep. They will also need to take soil samples of low-lying and upland areas for comparisons. Soil test results lower than 11 parts per million (ppm) mean the soil has insufficient nitrate nitrogen. Producers in this situation will likely want to side-dress nitrogen at a rate of 100 to 150 pounds of nitrogen per acre. If the soil test is between 11 and 25 ppm, producers will want to side-dress at a reduced rate--up to 100 pounds of nitrogen per acre, with the lower soil test numbers receiving the higher application rates. If the test reveals a number greater than 25 ppm, sufficient nitrogen exists in the soil and side-dressing is not needed.

Nitrogen losses will likely be less in well-drained, upland areas that have experienced heavy rainfall. Heavy rainfall results in quicker runoff, which means less water infiltrates the soil and less nutrient removal occurs.

Katie Pratt is an agricultural communications specialist within UK's College of Agriculture.

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