Knowing when to apply N is key. Wait until soil temperatures at the 4-in. depth are below 50° F. Temperature impacts the activity of soil organisms that can mediate conversions of ammonium to nitrate.


Fernandez says it's critical to understand that although the rate of nitrification is significantly reduced when soil temperature is below 50°, microbial activity continues until temperatures are below freezing. In order to minimize risk, don't apply N before the third week of October in central Illinois – or the second week in northern Illinois, even if air temperatures are getting cooler.


In addition, do not use nitrogen or nitrogen with a nitrification inhibitor if you live south of Illinois Route 16 or if soils are prone to leaching, he adds.


Anhydrous ammonia is a preferred N source for fall application because it has a slower nitrification rate than other sources. Once it's applied in the soil, ammonia reacts quickly with soil water and is converted to ammonium. Use a nitrification inhibitor with anhydrous ammonia applications. The nitrification inhibitors are chemicals that inhibit the activity of bacteria responsible for the first step in the process of nitrification, Fernandez says.


"Proper use of these inhibitors will reduce the rate of nitrification, thus maintaining for a longer period a great proportion of the applied nitrogen in the ammonium form," he says.


Fernandez recommends ammonium sulfate because it works well for no-tillfields where broadcast applications are preferred. It can also be applied on frozen ground as long as the slope of the field is less than 5% and the potential for surface water runoff is very low.


Urea should not be used in the fall because it has been shown to be less effective than fall-applied anhydrous ammonia. Urea converts to ammonia and then to ammonium within a few days of application, causing greater risk of losses before nutrient uptake by the crop the following spring.