Early Firing Symptoms and End-of-season Stalk Nitrate Test for Corn
With above normal rainfall favoring early season nitrogen (N) losses, we are starting to see corn fields with typical N deficiency symptoms. When N deficiencies occur early and mid-season, corn leaves on the lower part of the plant begin to turn yellow prematurely starting from the leaf tip and progressing towards the stalk (Photo 1). Lower leaves will be affected first as N is reallocated to the upper leaves. The yellow color will turn brown followed by
The end-of-season cornstalk nitrate test is a useful diagnostic tool to assess the adequacy of N during the season (Photo 3). The methodology and interpretation of this test were discussed in a previous Michigan State University Extension article “End of season corn stalk nitrate test.” The portion sampled is the 8-inch segment of stalk between 6 and 14 inches above the soil.
One obvious advantage is that this test does not require in-field N reference strips such as the ones needed for chlorophyll meter test. The broad range of nitrate-N in each category is meant to accommodate some of the differences due to hybrids, soil types and weather. The test has a few limitations. The test is performed at the end of the season, so it does not provide any remedy for the current year, nor does it accurately tell you how many pounds of N were over applied or under applied. The time of sampling also is very critical. It is two to three weeks after physiological maturity or black layers have formed on 80-90 percent of the kernels. At this stage, most leaves and stalks have turned to brown color and mobilization of N to the kernels has ceased. Sampling should avoid planting skips and stalk rots.
Familiarity with the data over a number of years, covering a wide range of weather
Corn stalks from field areas with different soil types or management zones ideally should be sampled separately. Many Michigan private soil testing laboratories will perform this test. The MSU Soil and Plant Nutrient Laboratory charges $12 per sample.
Michigan State University Extension’s recommendation is to split the N so that about one-third is applied at planting and two-thirds is
Source: George Silva, Michigan State University