What This Spring's Wet Conditions Mean for Nitrogen Loss

Wondering how this spring's prolific rainfall is impacting nitrogen (N) loss? Fabian Fernandez and Dan Kaiser, University of Minnesota Extension specialists, share some insights on what farmers can expect.

Over the last several years, we have noticed that wet springs are becoming the norm. In addition to keeping us out of the field at a critical time, these wet conditions also create anxiety about nitrogen loss.

Wet conditions in the spring are normally bad news for N management. However, wet conditions accompanied by cooler temperatures reduce the potential for N loss. N loss pathways are driven by water and are dependent on N being in the nitrate form.

Nitrate is formed in the soil from ammonium by bacteria through the nitrification process. Nitrification slows when soils are cold, keeping much of applied fertilizer N in the ammonium form. Since ammonium is a positively charged molecule, it is held by the soil and will not readily leach with excess water. On the other hand, nitrate has a negative charge and is repelled by soil particles, so it moves freely with water.

In general terms, most of the agricultural land in Minnesota has been wetter and cooler than normal since last September. Due to last fall's wet conditions and delayed harvest, there was very little fall N applied. Farmers who did anhydrous ammonia and waited to apply until soil temperatures were 50ºF. and cooling before applying will find most of that N has not nitrified due to cool spring temperatures. One advantage of anhydrous ammonia is that it inhibits soil microbes from nitrifying the applied N for a while. This buys more time in the fall for soils to cool and for nitrification to stop or become very slow.

Urea is a Different Story

Urea, on the other hand, nitrifies quickly after application and is more susceptible to loss. For this reason, we do not recommend fall urea in south-central Minnesota. Recent data from southwest and west-central Minnesota show that wet spring conditions result in more N loss from fall-applied urea than in years past. That said, cool spring temperatures have likely keyed less N loss at this point in the season from urea relative to past years.

What's Ahead

Whether substantial N loss from fall or early spring applications occurs is still unknown, as that will be dictated in June. Soils will continue to warm up and increase nitrification. If we have relatively dry conditions moving forward, the potential for loss will be low, but if we continue to experience wet conditions, N loss will become a larger concern. Once N is in the nitrate form, it can be lost through denitrification in waterlogged fields or leach as water moves down below the root-zone or into tile drains.

Having applied little or no N yet is probably a good thing this year because it provides more flexibility to manage N as conditions change. For example, if corn has not been planted yet, switching to a different crop may be an easier decision. Corn does not require much N early in the growing season. As long as the application is done by V4 or so, there should not be concern of starving the crop.

The only situation where N application at planting is critical would be in continuous corn where there is substantial crop residue. N immobilization can result in low N availability early in the season. This problem could be exacerbated this season, as low temperatures have resulted in little N mineralization.