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Corn Stalk Nitrate Testing Can Help Assess This Year’s Nitrogen Management Decisions

By Charles White
In a wet year like 2019, there are many questions about how nitrogen fertility programs held up.
Corn stalk nitrate testing can give you a glimpse into whether your corn ran out of N, had just enough N, or whether N applications were excessive.
Figure 1. Maize kernels at approximately 1 Corn /3 milk line. stalk nitrate samples can be collected anytime between ¼ milk line stage and 3 weeks after the black layer forms 
Nitrogen (N) management is an adaptive process- you start out each season with a game plan for N applications in your crops, but as the season develops you have to decide whether to make adjustments or stay the course. At the end of the season, it can be helpful to know, did the N management decisions you made this year result in just the right amount of N supply to corn, or did you apply too little or too much? The late-season corn stalk nitrate test can provide this type of feedback on your N management program each year.
The corn stalk nitrate test works by measuring the nitrate concentration in the lower part of the corn stalk. This is an indication of how much of the nitrate taken up by the plant was used to build useful molecules for the plant, such as proteins or chlorophyll. High levels of nitrate in the stalk at the end of the season indicate that the N supply to the crop throughout the season was excessive and the plant had access to more nitrogen than was actually needed. Very low levels of stalk nitrate indicate that the crop may have run out of N, causing a yield loss. Optimal levels of nitrate remaining in the stalk, which Penn State interprets as 700 to 2,000 ppm N, indicate that the crop had just the right amount of N available to maintain optimal growth and yield.
Detailed instructions for collecting a sample from the field for the corn stalk nitrate test are described in this fact sheet and video . Briefly, samples can be collected anytime between the ¼ milkline stage and 3 weeks after black layer formation. From each plant, cut out the section of stalk starting at 6 inches above the ground and extending another 8 inches up (in other words, the section from 6 to 14 inches above the ground). Collect stalk samples from 10 randomly selected representative plants per field. Stalk segments should be further cut into 1- to 2-inch long segments to facilitate drying. Place the samples in a paper bag and send to the lab for analysis as soon as possible.
The current cost of a stalk nitrate test at the Penn State Agricultural Analytical Services lab is $12. If a stalk nitrate test was collected from a 6 acre field, the analytical cost would only be $2/ac. It is easy to see how the information gained, such as knowing whether you could potentially cut back on N fertilizer applications, or whether yield could be increased by increasing N fertilizer rates, would quickly pay for the cost of the analysis. If the results show you achieved optimal N management for the year, it can build your confidence in continuing with the N management decisions that led to this success. Without any feedback on how your N management decisions performed this year, N management will continue to remain a guessing game, potentially resulting in costly mistakes such as applying more N than is needed or losing yield due to N shortages.