Most growers and crop advisors are familiar with interpreting soil nitrate data when it is being used to determine a rate of nitrogen to apply at sidedress. The problem with PSNT interpretations is that they are only intended to be used in situations where the only form of nitrogen applied to the soil up to that point is manure and is done when the corn is V4 to V6. (For more information about PSNT, please visit our previous blog post.) With the ever-increasing access to late season nitrogen application options, it is necessary to utilize soil test data to better determine a rate to apply after commercial fertilizer has already been applied. Tissue testing is valuable in monitoring crop nitrogen levels, however has limitations. (For more information about tissue testing for nitrogen management, please visit our previous blog post.)
As with any nitrogen model or prediction, there will always be a level of uncertainty due to the highly variable nature of plant available nitrogen resulting from the influence of weather and soil microbiology. However, a few years ago Purdue University proposed an approach to assess soil test of nitrate and ammonium especially in seasons where there is a higher chance of nitrogen loss. (Please see the entire article here. http://www.kingcorn.org/news/timeless/AssessAvailableN.html).
The general idea is that you can add the ppm of NO3-N and NH4-N together and multiply the total by 4 to estimate the total available nitrogen in pounds per acre, assuming the data is from a 12-inch-deep sample. We advise to test for both nitrate and ammonium to have a more detailed nitrogen inventory and to help determine future nitrogen availability. (For more information about nitrate and ammonium soil testing, please visit our blog post.) For example, if a sample was collected near tasseling, and the soil test results showed that there is 10 ppm NO3-N and 5 ppm NH4-N, you can estimate that there is approximately 60 pounds of nitrogen still available to the crop. To make a decision with this data, you have to determine how much more nitrogen might be needed to achieve your yield potential. At the time of tasseling, a corn crop has taken up approximately 60% of its nitrogen requirement. If the field that has a potential of 250 bushels, that means it has already taken up about 150 pounds per acre (assuming 1 pound of nitrogen per bushel). This crop could be about 40 pounds short to reach the yield potential. Had the same soil test results come from a field with a 180-bushel potential, it would suggest that there is enough nitrogen to finish out the season.
If you need helping making sense of late season nitrogen samples, contact your ALGL agronomist.