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Issue No. 6, Article 3/May 13, 2011

How Much Nitrogen Do I Still Have?

Wet soil conditions in the spring always create concerns that nitrogen (N) applied last fall for the new corn crop might be lost. The most important thing to remember is that 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 or intensively tiled soils.

Most fall-applied N is either ammonium (NH4+) or a form that transforms rapidly into ammonium. Nitrification, 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 50°F. The bacteria also need aerobic conditions (when soil is not water-saturated) to nitrify ammonium. The amount of nitrification that occurs in the soil thus depends largely on soil temperature and the time elapsed between application and when the soil becomes saturated with water. Further, the nitrification process can be reduced with the use of nitrification inhibitors that lower the activity of the bacteria and allow N to stay in the ammonium form for a longer period. 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 warm well into the end of October and first part of November, with soil temperatures at the 4-inch depth dropping below 50°F 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. Another reason for having concern would be if nonrecommended sources of fall N, such as urea, ammonium nitrate, or urea-ammonium nitrate solutions (UAN), were used.

While air temperatures were warm for a long time in the fall, once soil temperatures dropped below 50°F to allow N applications, the air temperatures remained low. That leads me to speculate that where N was applied correctly last fall, there was very little chance for nitrification. I suspect that any nitrate that was present before soils froze was probably retained in the soil until spring, because much of winter's precipitation did not move through the soil because it was frozen. Since soils have thawed, however, we have had a lot of precipitation, and I suspect some nitrate in soil has been leached out. But as I mentioned, I suspect there was not much nitrate to begin with. Since soils have been cool and wet until just recently, 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 to nitrate or soil N mineralized at this point.

Last week we collected soil samples from depth increments of 0 to 6 inches and 6 to 12 inches to measure ammonium, nitrate, and total inorganic N (the sum of ammonium and nitrate). This was done from the zero N (control) plot and the plots receiving 160 lb N/acre as anhydrous ammonia, with N-Serve applied 6 inches below the soil surface the first week of November last year. These plots are located near Urbana, and the soils are silty clay loam.

Figure 2. Percent ammonium, nitrate, and total inorganic N (ammonium plus nitrate) recovered in soil at the beginning of May from fall-applied N.

I estimated percent N recovery by subtracting soil N in the control plot from that in the plot treated with 160 lb N/acre and dividing by the application rate of 160 lb N/acre. I present these data to provide some general information, but the values are not absolute and should be considered so, because many factors impact N recovery. Figure 2 shows that 26% of the total N was recovered within the top 6 inches of the soil and 28% within the top 12 inches. The figure also shows that 17% of the applied N is still in the ammonium form and that most N is still present in the top 6 inches of the soil, with very little movement of nitrate into the 6- to 12-inch increment. These values represent typical recovery values observed in other recent studies at a similar sampling time. For instance, percent recovery from fall-applied N was 22% and 23% in two very wet years and 54% in a year with a dry spring with low potential for N loss. Across all three years of that study, N recovery was 33%, and average loss of corn yield relative to the economical optimum N rate was less than 2%.--Fabián G. Fernández

Fabián Fernández

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