While many are understandably anxious to begin another growing season, wheat producers should be cautious not to jump the gun on their spring nitrogen applications.
As you are reading this, the weather conditions are likely more favorable for spring N applications, although wet field conditions in several parts of southern Illinois may still be keeping producers out of the field. However, a couple weeks ago, I received several calls and heard chatter from others in the southern Illinois ag community about spreading urea while the ground was frozen with an inch of snow on the ground. Furthermore, there were several inches of rain in the forecast.
While most people I spoke with were sure to say they would be using a urease inhibitor (hopefully with NBPT), this may not be enough to keep all or most of your applied nitrogen in the field. As the name implies, urease inhibitors target the naturally abundant urease enzyme in an effort slow the conversion of urea to ammonium carbonate, which is in turn converted to ammonium (NH4+) in the process of ammonification.
Reducing urease activity is indeed beneficial for reducing nitrogen losses from urea applications, as a major route of nitrogen loss from urea is volatilization of ammonia gas (NH3). The conversion of ammonium to ammonia gas occurs when ammonium reacts with free hydroxide (OH-) ions in the soil solution. If urea is incorporated into the soil by either tillage or rain prior to conversion to ammonium, nitrogen loss via volatility is reduced as more ammonium is bound by cation exchange sites and unavailable for reaction with hydroxide ions.
The opportunity for nitrogen loss from urea applied to frozen, and perhaps snow-covered ground is not only from volatilization. On the contrary, it is the solubility of urea itself that may contribute to large nitrogen losses in certain situations, namely when the ground is frozen and substantial rains are in the forecast. Urea spread into snow will dissolve as that snow melts, or before, if temps are higher than about 20-25 degrees F. However, if this happens quickly, perhaps with the aid of rain, most urea will likely dissolve into solution but would be unable to move into the soil profile if soils remain frozen at or near the soil surface. Depending on the slope and runoff potential of a given field, as well as precipitation volume and soil porosity at the time of rain events, this may result in movement of urea to low spots in the field or possibly, completely off the field into waterways, drainage ditches, and ultimately, other surface waters.
I understand that many producers are looking to do what they can to increase tiller count after a wet, cool, and likely delayed planting season last fall, which often resulted in wheat plants that were small with few tillers formed prior to dormancy. Some producers may have had the chance to get some starter nitrogen applied in the fall, which will likely help with tiller count. But, if tiller counts of representative areas of your fields are coming in below an average of 40-50/ft.2, getting a good dose of nitrogen on at green-up (around Feekes 3), or soon thereafter, is often recommended in an effort to boost tiller counts and maximize yield potential.
Depending on how late in spring it is before you are able to get your nitrogen on, opting for a single application rather than split makes more sense. Split applications may make sense if you are able to get nitrogen applied early in the year, when chances of loss are higher. But data from 35 site years of U of I research from 1997-2005 show a single nitrogen application yielding more than split applications 66 percent of the time.
More recommendations for total nitrogen rate depending on soil organic matter, price of wheat, and cost of fertilizer can be found in the Illinois Agronomy Handbook Ch. 9 (https://go.aces.illinois.edu/IAHCh9). The trick is making sure the applied nitrogen can be taken up by the plants, and not lost via runoff or volatilization.
Talon Becker is a University of Illinois Extension commercial agriculture educator .