Taking time this fall to conduct a stalk nitrate test can help you "grade" your current nitrogen application rate and provides key information on which to base any changes for next spring.
Taking cornstalk samples now can help determine if the corn was under, adequately, or over fertilized with nitrogen. If the nitrogen applied this year was greater than that recommended by the University of Nebraska-Lincoln and this fall's stalk nitrate samples indicated excess nitrogen, consider reducing nitrogen rates for next season, says Charles Shapiro, UNL Extension soils specialist in Concord.
Use the cornstalk nitrate test in irrigated fields where moisture was not limiting. Fields that tend to have high stalk nitrate tests are those where manure or excess nitrogen was applied and fields following alfalfa. Iowa State University developed the corn stalk nitrate test, and its usefulness has been verified in other states.
The results of the cornstalk nitrate test indicate whether the corn was over fertilized during the season, according to Shapiro. The test shows low, optimal and excess stalk nitrate values (Table 1). Low values indicate nitrogen may have been deficient. Excess values indicate that there was more nitrogen than the plant needed to produce grain.
The scientific basis for this test is the fact that corn will continue to accumulate nitrogen past the level at which grain yield is increased, Shapiro says.
"Since corn does not show visible symptoms of excess nitrogen, analysis of the stalk tissue can determine when this occurs," he says. "This test is probably best used for finding excess nitrogen since deficiencies can be spotted visually by leaf yellowing."
This season, if the test comes back in the "excess" range, that indicates that reductions in nitrogen may be possible next season.
For more information on recommended rates, get Extension NebGuide, Fertilizer Suggestions for Corn (EC117) or visit cropwatch.unl.edu/web/soils/home in CropWatch where there are worksheets and an on-line calculator to help determine recommended nitrogen rates.
Take cornstalk samples up to three weeks after black layer formation in 80% of the kernels. Newly published information indicates that the stalk test can be taken as early as when the milk line is one-fourth of the way down the kernel. To take the test, remove an 8-inch segment from 6 inches to 14 inches above the ground.
Remove the sheaths. Don't take diseased stalks or stalks damaged by hail or insects. Take 15 stalks per sample, keep them cool and send to the laboratory.