As we indicated in a recent Bulletin article, nitrogen applied as anhydrous ammonia in the fall is an effective source of N for the corn crop, but is also a little more subject to loss compared to N applied in the spring. One of the main factors that determines how much of the fall-applied N is subject to loss is how much of the ammonia converts to nitrate (is nitrified) in the fall and early spring, before the next planting season.
 
One way to slow nitrification of fall-applied ammonia (which converts quickly to ammonium in the soil) is to add a nitrification inhibitor. The other way is to wait to apply ammonia until soil temperatures are low enough to limit the activity of the bacteria responsible for the conversion to nitrate. Scientists have studied the effect of soil temperature on nitrification rates for decades. Most of this work has been done on soil brought indoors and kept at different temperatures, then adding ammonium and measuring the amount of nitrate that forms over time.
 
Estimating how quickly ammonium is converted to nitrate in the soil turns out to be complicated, but like most biological processes, it takes temperatures close to freezing to stop the nitrification process completely. Adding the ammonium as anhydrous ammonia also increases the pH and kills a large number of bacteria, effectively delaying the start of nitrification by days or weeks, with longer delays when soil temperatures are lower. Once nitrification begins, soil temperatures in the upper 70s produce maximum rates of nitrification; rates drop to about 50 percent of maximum at 60 degrees, to 10 percent of maximum at 50 degrees, and to about 5 percent of maximum at 40 degrees.
 
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