By Brad Carlson, Extension educator
Recent, excessively wet weather has raised concerns over the possibility of nitrogen (N) loss in corn for some parts of Minnesota. As the calendar has now turned to June, it's likely that anywhere from half to all of the applied N, regardless of the fertilizer form or application timing, is in the nitrate form. This conversion is due to soil microbial activity and is fueled by heat and moisture. This is significant, since nitrate is susceptible to environmental loss.
Mechanics of N loss
Nitrate will "denitrify" when the soil is saturated. Denitrification is the conversion of nitrate into inert nitrogen gas, which is then lost to the atmosphere. This is also a microbial process that speeds up when the soil is warm. Most recently, the Southern Research and Outreach Center in Waseca reported the 4” soil temperature was at 64° F. Research shows that soils that stay saturated for four days at this temperature will lose approximately 10% of the N present in the nitrate form, and the loss approaches 20% if the soil stays saturated for 10 days.
Additionally, nitrate will leach (move downward) as the water drains away. A rule of thumb for this is that nitrate will travel approximately 6” for every 1” of water that leaves as drainage. It is important to note that these processes will be highly variable across most fields based on local soil and water conditions.
Is there a need for supplemental N?
If a significant amount of N has been lost, it is likely that a supplemental fertilizer application will be necessary to reach the crop's yield potential. The University of Minnesota has developed a worksheet to assist farmers with making supplemental N application decisions for corn. The worksheet can be found at: http://z.umn.edu/supplementaln. In addition, the worksheet is available as an app for smart phones at: http://z.umn.edu/cropcalc.
Considerations before applying supplemental N
Here are a few things to consider before applying supplemental fertilizer:
- Soils that have been saturated for a long time will in all likelihood have stunted plants with reduced yield potential, so will require less N.
- In instances where leaching has occurred, it is probable that the N has moved lower in the soil profile; however, it may not be lost, since increased root growth may put the N back in reach of the plant.
- There are several new technologies focused on making supplemental (and variable rate) N decisions, and while they hold promise, they are new, and the jury is still out with respect to how well they work.