Corn stalk nitrate research update
click image to zoom Cornstalk NO3-N concentrations between 250 and 2,000 ppm were associated with relative yields greater than 80% of maximum yield (Figure 1) and an average relative yield of 98% for each category in this range of cornstalk NO3-N (Table 1). Nitrogen rate deficits for cornstalk NO3-N categories between 250 and 1,500 ppm ranged from 21-31 pounds of N per acre (Table 1). Adequate N (<2 pounds per acre excess) was associated with cornstalk NO3-N of 1,501-2,000 ppm (Table 1).
One-hundred percent of maximum yield and an excess N application of 25-31 pounds of N per acre were associated with cornstalk NO3-N concentrations between 2,001-4,000 ppm (Table 1). Only 7 cornstalk samples had NO3-N greater than 4,000 ppm. The average excess N application was 74 pounds of N per acre for this category.
Using the end-of-season cornstalk nitrate test to adjust fertilizer-based N management programs
Multiple seasons of CSNT evaluation are warranted before altering an N management program because the optimum N rate varies from season to season. Many factors affect the optimum N rate; including differences in soil N supply, loss of N from the rootzone, hybrid differences in N use, pest and weed impacts on N use, and the interaction of these and other factors. The average correct N rate for maximizing profit over the long term3 is almost certainly wrong in any one season – either too much or too little. Thus the evaluation of a N management system with the CSNT (or any other N assessment tool) on any given field in a single season is interesting, but not particularly useful in making management decisions for future years. Unfortunately, there is no concrete guidance on how many years the CSNT should be conducted, but I would suggest three or more seasons to be reasonable.
Although it would be great to have applied the optimum N rate every year it is not likely to be possible. Unfortunately there is no concrete guidance on what level of N excess should trigger a reduction in N application rate. In my opinion plus or minus 20-30 lb N/acre is normal variation in optimum N rate from year to year for a particular cropping system. Based on the most recent Indiana research a NO3-N concentrations in excess of 4,000 ppm might represent excessive N application rates (74 lb N/acre excess). Unfortunately this estimate of excess N is based on only 7 samples. Ongoing research will hopefully add to this analysis after harvest this season.
If end-of-season cornstalk NO3-N concentrations are consistently less than 250 ppm or more than 4,000 ppm one might consider conducting N response strip trials, rather than rely solely on the CSNT to evaluate the current N management program. Guidelines on conducting suitable N response trials can be found at: <http://www.agry.purdue.edu/ext/ofr/protocols/PurdueNTrialProtocol.pdf>.
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