Does your corn N strategy deserve an A or a D?
Research Supporting Use of The Stalk Nitrate Test
An article (Fox et al., July 2001) in the Agronomy Journal compared the stalk test, late season chlorophyll meter, and green leaf count techniques.
click image to zoom Based on this article, I have summarized their analysis of the results in Table 2. The authors used experimental data to determine the error rate of using different critical levels to interpret the test results. Because the tests were conducted on corn grown in replicated experiments, they could determine if the diagnostic test level accurately matched the plant response. Their criteria for whether the test was valid was whether the yield was at 93% of maximum yield. For example, with the chlorophyll readings taken at one-fourth milk line they used a critical value meter reading of 52. They derived the 52 reading from their previous research.
Once the criteria was set, they determined if the treatment correctly predicted sufficient nitrogen. They also divided the errors into two groups: one predicted the plant was nitrogen deficient when it wasn’t while the other predicted the plant had adequate nitrogen when it was deficient. Using the chlorophyll meter reading to determine if the plant had adequate nitrogen wrongly predicted the crop was deficient 13.4% of the time. The plant actually had adequate nitrogen even though the meter suggested it was low. Using the same meter reading criteria, 1.7% of the time it falsely suggested the plant had adequate nitrogen when it was low.
When the authors lowered the criteria from 52 to 48, the total error rate actually decreased from 15.1% to 7.3% because the percent the meter falsely predicted deficiency decreased from 13.4% to 2.7%. There was not a corresponding increase in the false prediction of adequate nitrogen.
The data on the stalk nitrates also shows the change of error rates when the criteria for predicting deficiency changes. The Fox et al. data indicates that using 250 ppm would keep prediction errors to 7.2%. Using the 700 ppm critical value used by Iowa had a 0% error rate for falsely predicting nitrogen sufficiency and a 12% overall error rate.
The Fox et al. data provide more evidence that corn stalk nitrate tests are a useful tool in nitrogen management. They are best used to determine if adequate nitrogen was available. They would be especially useful in fields with manure history where the producer needs reassurance that reducing fertilizer nitrogen will not affect yields. This year they may also help producers determine if reducing nitrogen rates decreased yields.