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Issue No. 3, Article 6/April 11, 2008

How Much Nitrogen Have I Lost?

Wet soil conditions for much of the winter extending through the present are creating concerns that much of the nitrogen (N) applied last fall for the 2008 corn crop might be lost. When soils become saturated, the potential for N losses is directly related to the amount of N present in the nitrate (NO3-) form. Under water-saturated conditions, nitrate is most likely to be lost through denitrification in fine-textured soils and through leaching below the root zone in coarse-textured soils. Most fall-applied N is either ammonium (NH4+) or a form that transforms rapidly into ammonium. Nitrification, or the conversion of ammonium to nitrate, 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 50F. The bacteria also need aerobic conditions (unsaturated soil) to nitrify ammonium. Thus, the amount of nitrification that occurs in the soil depends largely on soil temperature and the time elapsed from application until the soil becomes saturated with water. Further, the process can be reduced with inhibitors that lower the activity of the nitrifying bacteria and let N stay in the ammonium form for a longer period.

Although it has been a wet winter, soil temperatures have been cool and similar to previous years. In fact, it is not uncommon to have higher temperatures than what we have seen this winter. Soil temperatures at the 4-inch depth for selected Illinois locations (Source: Water and Atmospheric Resources Monitoring) are shown in Table 2. Since soils are still cool and wet, and nitrifying bacteria need warm temperatures and aerobic conditions to transform ammonium to nitrate, it is likely that not much fertilizer N has been transformed into nitrate or soil N mineralized at this point. If the spring stays wet during April and May as soil temperatures start to climb, it is likely that some of the fertilizer N will be lost. If conditions are conducive to N losses I will address what can be done. However, for the time being I believe we need not feel too concerned about N losses.

The greatest reason for concern about N losses at this time, or for most any given year, would be if fall N application guidelines were not followed. Last fall was dry and warm well into the end of October and the first part of November, with soil temperatures at the 4-inch depth dropping below 50F later than normal. If fall application recommendations were not followed, there is a greater chance that some of the N might have been transformed to nitrate and potentially lost due to the wet conditions experienced thus far.

Another reason for concern would be if fall N-sources that are not recommended, including ammonium sulfate, urea, ammonium nitrate, and urea-ammonium nitrate solutions (UAN), were used last fall. These forms are not recommended for fall application because they do not build ammonia concentrations sufficiently high to inhibit nitrification. Also, 25% and 50% of the N in UAN and ammonium nitrate, respectively, is subject to loss at the time of application since it is already in the nitrate form.

Finally, intensive rain events in some parts of the state have caused water to move out of fields, quickly carrying with it soil containing fertilizer N. Areas where heavy rains occurred after surface applications of polymer-coated urea or surface applications of N for wheat this spring could have experienced N losses if there was water runoff from the field. However, these situations are likely the exception rather than the rule. I suspect that if such a situation did occur, it was likely in small fractions of a field and should not be reason for excessive concern.--Fabián G. Fernández

Fabián Fernández

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