With a still fresh memory of the drought conditions during last year, recent rains have reduced concerns over water availability for the start of the 2013 growing season, but at the same time, concerns over nitrogen (N) loss have increased. Nitrogen loss is difficult to predict because it depends in many factors such as time of N application, type of N source, soil type and temperature and the amount of precipitation received. While it is difficult to know how much N is lost without a direct analysis of soil N, you can determine what to do about N applications this growing season.
Most of the fall-applied N is either ammonium (NH4+) or a form that transforms rapidly into ammonium. Nitrification, or the conversion of ammonium to nitrate (NO3-), is a bacteria-mediated transformation. The bacterium Nitrosomonas converts NH4+ into nitrite (NO2-) while the bacterium Nitrobacter converts NO2- to NO3-. The activity of these bacteria is minimal at temperatures below 50º F. These bacteria also need aerobic conditions (unsaturated soil-water conditions) to nitrify ammonium. Thus, the amount of nitrification that occurs in the soil is largely dependent on soil temperature and the time elapsed from application until the soil becomes saturated with water. Further, the nitrification process can be reduced with the use of nitrification inhibitors that reduces the activity of Nitrosomonas and allow N to stay in the ammonium form for a longer period of time.
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When soils become saturated, the potential for N losses is directly related to the amount of N present in the nitrate (NO3-) form. While wet soil conditions this spring may be a reason for concern that some of the N applied last fall may be lost, the cold temperatures we had until recently likely substantially reduced nitrification. From now on, as temperatures increase, nitrification will also increase. When nitrate is present and soils warm up, N loss will start under saturated water conditions mostly through denitrification in fine-textured soils and through leaching in coarse-textured soils or intensively drained soils.
An important point to keep in mind is that the portion of the applied N that is in nitrate form is only subject to denitrification or leaching. However, the fact that N is in the nitrate form does not mean that N is lost; it simply means that it is susceptible to loss.
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