Evaluate corn N management with stalk nitrate test
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EmailEditor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.
As corn grain has or soon will reach black layer, corn growers should consider using the stalk nitrate test to evaluate their nitrogen management program. Even though the stalk nitrate test is post-mortem (end-of-season) it can be useful for long-term adjustment of N management practices. The nitrate N concentration in the lower portion of the corn stalk at the time grain black layer is formed is a good indicator of the nitrogen status the crop experienced throughout the growing season. When used over a number of years, the stalk nitrate test can help identify N efficient fields and/or management systems, thus enabling fine tuning of N inputs.
How to do the test
The optimum time is one to three weeks after black layer has formed in 80% of the corn kernels. Cut an 8-inch segment of the stalk between 6 and 14 inches above the ground, from 15 stalks within the area of interest. Remove any portions of leaves that may remain attached. Splitting the stalk segments will facilitate drying. Refrigerate the stalk segments if the sample cannot be sent or delivered to a test lab within 1 day. Do not use plastic bags as this will prevent drying and may cause spoilage. The testing lab will oven dry and grind the stalks prior to analysis. This service is provided by the MSU Soil and Plant Nutrient Lab and various private labs. The fee at the MSU Lab is $12 per sample.
As corn approaches maturity, plants stressed for N will move nitrate from the lower cornstalk to the ear resulting in a low stalk nitrate concentration. When corn plants have sufficient N or more than sufficient N for maximum yield, nitrate accumulates in the corn stalk. Extensive studies done by Purdue and Iowa State universities have shown the usefulness of this test in distinguishing between sufficient and excess N situations. With the high cost of fertilizer N, elimination of excess N use improves the net return and provides a positive environmental situation. Interpretation guidelines are presented in Table 1. Stalk nitrate values from nitrogen rate studies (3 years) at Michigan State University support these guidelines.
Table 1. Interpretation of the corn stalk nitrate N test
|
Excessive |
> 2000 ppm. |
Excessive N available to the crop, or some other production factor limited crop growth and yield |
|
Optimum |
450 to 2000 ppm (Purdue) |
Grain yield was not limited by amount of N available to the crop. |
|
700 to 2000 ppm (ISU) |
||
|
Marginal |
250 to 700 ppm (ISU) |
Nitrogen supply may have limited yield |
|
Low |
< 450 ppm (Purdue) |
N was likely yield limiting during the growing season, especially < 250. |
|
< 250 ppm (ISU) |
Both sets of studies show that a stalk nitrate N concentration above 2000 ppm is indicative of excessive nitrogen having been available to the corn crop. Quite often this is associated with the application of animal manure, but may also be related to over application of fertilizer N. In the Purdue studies maximum yields were associated with stalk nitrate N concentrations above 450 ppm. Iowa State uses 700 ppm as the transition value. Above this value is considered the zone of “luxury” N consumption, ie., no response to applied N. Values below 450 ppm have been associated with inadequate N being applied for maximum yield.
