The steady flow of wet weather in April caused many Kentucky producers to worry about nitrogen losses in their fields, especially in fields where nitrogen applications were made prior to the rains.
Fortunately, nitrogen losses may not be as bad as many producers expect, said Lloyd Murdock, 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 they anticipated. Not knowing this could cause them to spend an excessive amount of money on additional fertilizers.”
Producers who farm areas that annually flood, such as along the Mississippi and Ohio rivers, typically haven’t applied nitrogen yet. Those who farm along creeks and secondary rivers, however, may have already applied nitrogen. These areas may be submerged for some time as the smaller tributaries will be backed up due to 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. Poorly drained, low-lying areas are the most susceptible.
Daily losses calculated
During denitrification, an average of 3 to 4 percent of nitrate nitrogen is lost per day for each day of saturation. Fertilizers are comprised of varying rates of nitrate nitrogen.
Another 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 at http://graincrops.blogspot.com/2011/04/estimating-nitrogen-losses-from-wet.html.
Another way to determine the amount of nitrate nitrogen in the soil is by soil testing to look specifically at nitrate nitrogen levels. Producers should take soil samples a foot deep. They will need to take soil samples of low-lying and upland areas for comparisons.
Soil test results lower than 11 parts per million means the soil has an insufficient amount of 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. In this situation, producers can apply amounts of 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, a sufficient amount of 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.
If corn develops a nitrogen deficiency during the growing season, a yellow line will appear along the midrib of the leaves. Bottom leaves will show the deficiency first as available nitrogen is transported up the plant to newer leaves. The entire plant will be a light green.
Planting in soils that are too wet can also cause problems. For a look at that situation see http://southeastfarmpress.com/grains/mudding-corn-could-cause-sidewall-compaction.