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 nitrogen management. However, wet conditions accompanied by cooler temperatures reduce the potential for nitrogen loss. Nitrogen loss pathways are driven by water and are dependent on nitrogen 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 (Table 1). Due to last fall’s wet conditions and delayed harvest, there was very little nitrogen applied in the fall. For those that were able to apply anhydrous ammonia, if you waited until soil temperatures were 50ºF and getting cooler before applying, most of that nitrogen has not nitrified because of the cool temperatures, especially during the spring. One advantage of anhydrous ammonia is that it inhibits soil microbes from nitrifying the applied nitrogen for a while and buys more time in the fall for the soils to get cold and for nitrification to stop or become very slow.
Table 1. Monthly cumulative precipitation and monthly average air temperature departures from the 30-yr normal (1981-2010) for various locations in Minnesota. May values are partial (calculated May 23)
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, and recent data from southwest and west-central Minnesota show that wet spring conditions result in more nitrogen loss from fall-applied urea than in years past. That said, because of the cooler temperatures this spring, it is likely that less nitrogen loss has occurred at this point in the season from urea relative to past years.
It is important to recognize that what we have said so far is based on what we have observed up to this point. Whether substantial nitrogen loss from fall or early spring applications occurs is still unknown, as that will be dictated by what happens in June. Soils will continue to warm up, and with that nitrification will increase. If we have relatively dry conditions moving forward, the potential for loss will be low, but if we continue to experience the wet conditions we have seen so far, then nitrogen loss will become a larger concern. Once nitrogen 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 nitrogen 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 nitrogen early in the growing season, so as long as the application is done by V4 or so, there should not be concern of starving the crop. The only situation where application of nitrogen at planting is critical would be in continuous corn where there is substantial crop residue and nitrogen immobilization can result in low nitrogen availability early in the season. This problem could be exacerbated this season because the low temperatures have resulted in little nitrogen mineralization.