Soybean legume credit (25lbs) + soil test nitrogen value (25lbs) + applied nitrogen (150lbs) / yield (200) or
25+25+150/200=1 lb N/ bu.
If your yield would have been 175bu/ac then the NUE in the above example would have been 1.14 or if yield was 225bu/ac then the NUE would be .88. we intuitively use this calculation every year when determining nitrogen fertilizer recommendations but we have never used this calculation to determine if management procedures should change. I think everyone can agree that having the lowest possible NUE is the desired outcome of a nitrogen program.
Why is this calculation potentially valuable to you? One reason is that it can tell you two things: first, you can identify the locations in your fields where you are applying the least amount of N fertilizer and realizing highest relative yield and, secondly, and conversely, you can identify those areas where you are applying the most nitrogen fertilizer and realizing the lowest relative yield. At first, this may seem like a fairly simply concept, but if this calculation was done only with applied nitrogen you would get a false view of the field – what if you had variable soybean yield, hence, variable soybean nitrogen credit? What if you had variable soil residual nitrogen?
I think a couple of the best uses of an analysis such as this is to evaluate yield and direct nitrogen applications to take the best advantage of the soil mineralization you may see. If you have a NUE of, let’s say, 1.3 or above we should maybe ask why. Was it because yields were low? If so, why was that. Was the same yield seen on a NUE of 1.3 as was on a NUE of .9? Why was that? Perhaps the nitrogen application plan was artificially high or the yield goal in that area not accurate. Another great use of this type of analysis is directing future sampling. I chose the above field as an example because this one illustrates perfectly how NUE and soil type correlate. In the above field the areas of most efficient NUE had about the same yield as the areas of less efficient NUE. A quick calculation shows that about .3 pounds of nitrogen less was needed to realize the same yield. On a 200 bu/ac yield that is approximately 60 pounds of nitrogen or approximately $30/ac. I think that is worth investigating.
Why the difference in the NUE in those areas? It was because they were sampled separately for nitrogen and differing soil residual N was found and taken advantage of in the nitrogen application plan. An anomaly? I don’t think so. So far this spring I have gotten back a handful of spring nitrate tests and the results of those tests are below. There is a range of nitrogen values in your fields and we should be taking full advantage of them.
24 inch soil nitrate results - Spring 2016
68 33 41 56
64 29 81 40
76 15 34 47
128 68 57 29
156 32 21 73
36 33 61 71
30 38 40 58
33 42134 44
59 130 37
42 105 60