Current guidelines for fall application of anhydrous ammonia are based on the assumption that soil temperature at the time of application is the only factor that can be used to estimate potential for losses of this N during spring rainfall. This assumption needs to be questioned, however, because recent studies have shown that soil pH is an important factor affecting losses of fall-applied anhydrous ammonia (Blackmer et al., 2000). In this paper we provide a review and update on research concerning the effects of soil pH on rates of nitrification in soils and potential for losses of fall-applied N during spring rainfall.