In fall of 2009 I announced that my program, with the support of the University of Missouri Soil Testing Lab, would pay for the analysis of up to 10 stalk nitrate samples per farm for Missouri farmers. The stalk nitrate test provides a "post mortem" analysis of nitrogen management for a field when a sample is obtained one to three weeks after black layer has formed on 80% of the kernels of a corn ear.
Caution is needed when interpreting trends in voluntarily submitted data such as this. Still some trends in the data were clear.
We received 73 samples from 14 central and northern Missouri counties under the program. A summary of the Information submitted by the growers showed that:
The stalk nitrate test divides samples into four categories: low rating (0-250 ppm) suggests nitrogen was limiting to yield; marginal (250-700 ppm) suggests that nitrogen availability to the crop bordered on insufficient; optimum (700-2000 ppm) suggest that nitrogen availability was likely within the range to maximize profits; excess (>2000 ppm nitrogen) indicates a high probability that more fertilizer N was applied then needed to maximize profits.
In 2009, a high proportion (53%) of the test results were below optimum likely reflecting the excellent conditions for nitrogen loss in many fields across Missouri this spring. Nitrogen loss inhibitors helped in this group of fields. Forty-five percent of the 53 samples that reported no use of nitrogen loss inhibitors were nitrogen deficient (low) and only 32% were marginal or optimum. In contrast 25% of the 20 samples from fields that used nitrogen loss inhibitors were low and 60% were marginal or optimum.
Other clear trends were difficult to define is this limited data set. There was no clear benefit or cost to fall applied nitrogen. Median stalk nitrate value for the 41 samples from fields where anhydrous ammonia was the primary nitrogen source was 815 ppm compared to 514 ppm for the 20 fields using predominantly urea or UAN solution. Thanks to the growers who submitted their samples to the stalk nitrate challenge program and filled out the one-page questionnaire on nitrogen management. We hope to have a similar program in fall 2010.
With or without the program the corn stalk nitrate test can provide you good information about nitrogen management on your field. A sample result outside of optimum does not necessarily mean nitrogen management on the field was inappropriate. Results need to be interpreted in the context what else you know about the growing season. For example, this year we had an extremely wet spring. Samples rating marginal likely would be optimal in a more typical year. On the other hand, a farmer should look for opportunities to reduce nitrogen fertilizer on a field rating excessive in a high nitrogen loss year like 2009.
Iowa State University has an excellent publication outlining how to use the corn stalk nitrate test (available on the web at http://nmplanner.missouri.edu/resources/IA_Stalk_NO3_Test_ PM1584.pdf ). Stalk nitrate samples can be submitted to the University of Missouri Soil Testing Laboratory for analysis.
REVISED: September 30, 2013