Recent and continuing rains across parts of the Corn Belt have farmers wondering how much nitrogen is available in their cornfields. Is a supplemental application necessary at this point? Experts at the University of Minnesota and University of Nebraska-Lincoln offer insight into nitrogen concerns.

As the calendar has now turned to June, it's likely that anywhere from half to all of the applied nitrogen, regardless of the fertilizer form or application timing, is in the nitrate form. 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-inch soil temperature was 64° F. Research shows that soils that stay saturated for four days at this temperature will lose approximately 10% of the nitrogen present in the nitrate form, and the loss approaches 20% if the soil stays saturated for 10 days. 

Additionally, nitrate will leach as the water drains away. A rule of thumb for this is that nitrate will travel approximately 6 inches for every 1 inch 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. 

The best way to be determine if previously applied nitrogen is still in the root zone is to soil sample at different depths to get a sense of where the bulk of the applied nitrogen resides at the moment. If the bulk of the nitrogen is at the 3-4 foot depth, further rain on wet soils will likely lead to leaching nitrogen below your corn’s root zone. At this point in the season the plants have taken up very little nitrogen and you can use the UNL algorithm to offset your soil nitrogen credits and revise your nitrogen management plan.

If a significant amount of nitrogen has been lost, it is likely that a supplemental fertilizer application will be necessary to reach the corn crop's yield potential. The University of Minnesota has developed a worksheet to assist farmers with making supplemental nitrogen application decisions for corn. In addition, the worksheet is available as an app for smart phones.

Here are a few things to consider before applying supplemental fertilizer:

1.  Soils that have been saturated for a long time will in all likelihood have stunted plants with reduced yield potential, so will require less nitrogen.

2.  In instances where leaching has occurred, it is probable that the nitrogen has moved lower in the soil profile; however, it may not be lost, since increased root growth may put the nitrogen back in reach of the plant.

3.  There are several new technologies focused on making supplemental (and variable rate) nitrogen decisions, and while they hold promise, they are new, and the jury is still out with respect to how well they work.

To help manage your nitrogen this season, register on our site and get a FREE download of our special issue: Sidedress: Our best ideas, insight and tips to manage nitrogen application.