Not all fall-applied nitrogen sources are equal. Fall-applied anhydrous ammonia behaves very differently than fall-applied urea. Anhydrous ammonia is as harmful to microorganisms as it is to people, so the microbes that convert ammonium to nitrate in the soil are typically killed in the area right around the injection band.
These microorganisms will eventually recolonize the injection zone and ammonium will begin converting to nitrate. Urea does not inhibit the activity of these microorganisms. As a result, anhydrous ammonia delays the conversion to nitrate by at least 2 weeks longer than urea and other fertilizer forms.
These two or more weeks can be critical. Consider the following scenarios:
- I applied my N as anhydrous in the fall after the soil temperature hit 50° F. The remainder of the fall was excessively wet, but stayed cold. Did I lose much N? How would this be different if I had used urea instead?
- I applied my N as anhydrous after the soil hit 50° F. The remainder of the fall was excessively wet, with soil temperatures climbing above 50° F for several days before falling back below 50° F until freeze-up. Did I lose much N? How would this be different if I had used urea instead?
- I applied my fall N on November 2nd, when soil temperatures were around 44° F. The weather remained dry and cool during the fall, and rainfall during the following spring was also lower than usual. What sort of N losses should I expect?
Let’s look at scenario three first. Under this scenario, very little nitrogen is likely to be lost regardless of source. Nitrogen was applied when soils were cool, so little conversion to nitrate is likely until soils have warmed the following spring. Most importantly, there hasn’t been enough rainfall to move any nitrate out of the soil.
Now, let’s compare scenarios one and two. These scenarios are similar except for the fact that in number two, the soil temperature goes up again for a number of days.
Nitrifying microorganisms will convert ammonium to nitrate down until roughly freezing temperatures, about twice as fast for each 10° F rise in temperatures. The 50° F temperature mark is often recommended because, while nitrate is still being produced, it happens slowly below 50° F.
Meanwhile, temperatures are falling from 50° F in scenario one, meaning that for a number of days, more than 10 times as much nitrate is being produced in number two as number one. With the wet fall, a significant proportion of that nitrate could be moved down or out of the soil.
Since anhydrous ammonia temporarily sterilizes the application band, you will always have quicker conversion to the nitrate form with urea than with anhydrous. From there, soil temperatures after application, which determines how much converts to nitrate, and precipitation, which determines N movement, will affect nitrogen loss.
In scenario two, the soil stays relatively warm and gets a lot of precipitation. You would likely see much greater N loss with urea than with anhydrous.
These scenarios show that climate conditions interact with nitrogen source to determine how much nitrogen loss will occur. In dry years, there might not be a noticeable difference in losses between anhydrous ammonia and urea, but in a wet year these differences could be quite large.
The frequency of adverse temperature and moisture conditions varies across the state. In South Central MN, weather conditions are consistently wet enough that urea application is not recommended at all.
In the Southwest, West Central, and Northwest regions of the state, fall and spring conditions are dry enough that urea is still an acceptable practice.
However, from an agronomic perspective, anhydrous ammonia is always the preferred source for fall nitrogen applications. It simply sticks around longer in the soil than urea.