Greg Klinger and Anne Struffert, Extension Educators 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.