With severe drought conditions in June, July and August of 2018, scattered throughout mid-Michigan, many corn growers may be wondering if their traditional nitrogen (N) fertilizer practices were effective this year. The end of season cornstalk nitrate test is a useful diagnostic tool to assess overall N management.
The nitrate N concentration in the lower portion of the corn stalk after the black layer formation is a good indicator of the N status the crop experienced during the season. As corn approaches maturity, plants stressed for N will remobilize N from the lower cornstalk to the kernels, resulting in a low stalk N concentration. Conversely, if plants had excess available N for maximum yield, this N tends to accumulate in the stalk.
Accordingly, the stalk nitrate N concentrations are divided in to three broad categories: low (less than 700 ppm), optimal (700 to 2,000 ppm) and excess (greater than 2000 ppm). The low range indicates an N shortage situation, and often associated with visual deficiency symptoms. The optimal indicates N availability that corresponded to maximum economic returns. The excess range indicates N availability greater than required for maximum economic returns. Quite often, the excess range is associated with over application of N fertilizer or animal manures.
The stalk nitrate test interpretation given here is for “normal” weather and production conditions. So what if you were in a drought zone? From past experience, we know that in wet years, generally the stalk nitrate test levels tend to be on the low side and in dry years, stalk nitrate test levels are on the high side, irrespective of the amount of fertilizer nitrogen applied.
In 2018, test results could be generally high due to the dry conditions and impact on plant growth and reduced grain yield. The kernels may not utilize all the N taken up by the plant. So the excess N will instead accumulate in the stalk even if the right amount of N fertilizer was used.
On the other hand, June and July were extremely dry and corn roots take up N with the water they take up from the soil. With so little moisture in the soil, roots may not have been capable of taking up all the nitrogen that was present in the soil. In this scenario, it will be interesting see how the stalk nitrate levels are affected depending on when the drought occurred. In general, the test may have to be interpreted with some caution due to the unprecedented drought this year.
The test does not provide any remedy for the current year, but familiarity with the data over a number of years, including drought years, should assist producers in fine-tune their N fertilizer practices to suit seasonal changes in weather Please refer to “Nutrient Recommendations in Field Crops in Michigan,” MSU Extension bulletin E2904, for the latest corn N recommendations.
Field areas with different soil types or management zones ideally should be sampled separately. The time of sampling is very critical. The correct time is two to three weeks after physiological maturity or black layers have formed on 80-90 percent of the kernels. At this stage, any further mobilization of N to the kernels has ceased and most leaves and stalk have turned to brown color.
Many Michigan soil testing laboratories will perform this test in a two- to three-day turnaround. The MSU Soil and Plant Nutrient Laboratory charges $12 per sample. Please refer the “End of Season Cornstalk Nitrate Test” fact sheet for further information.