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Dec 17, 2023

Evaluating Nitrogen Management in Late Season Corn

Now is the time to use the late season cornstalk nitrate test to evaluate your nitrogen management. Nitrogen is a challenging nutrient to manage because of its many forms, transformations and loss

Now is the time to use the late season cornstalk nitrate test to evaluate your nitrogen management.

Nitrogen is a challenging nutrient to manage because of its many forms, transformations and loss pathways.

Despite a number of tools designed to improve nitrogen fertilizer recommendations in corn, such as the recalibrated pre-sidedress soil nitrate test or a new experimental tool to credit nitrogen supply from soil organic matter and cover crops, significant uncertainty remains in determining optimal nitrogen application rates.

Extension soil fertility specialist Charlie White said this uncertainty often leads growers to apply extra nitrogen fertilizer as an insurance against yield losses due to insufficient nitrogen.

The late-season cornstalk nitrate test is a useful diagnostic tool to use at the end of the growing season to determine whether excess nitrogen was applied to a corn crop, which can inform management decisions for future years.

Conducting a test allows producers to identify cornfields that were low (less than 700 ppm nitrate-N), optimal (700-2,000 ppm nitrate-N), or excessive (greater than 2,000 ppm nitrate-N) in the availability of nitrogen to the corn crop over the growing season. The test is especially accurate at identifying fields where nitrogen was excessive.

If your field falls into this category, there is a strong likelihood that nitrogen applications could have been reduced without any yield losses, which should improve the economic and environmental performance of your system. This information could give you increased confidence in making incremental reductions in nitrogen applications in the next growing season.

The test is less accurate at identifying fields with below optimum nitrogen availability. Many fields that test low may still have had sufficient nitrogen to realize optimal yields. Therefore, deciding to increase nitrogen application rates solely on the results of this test carries some risk that the crop will not have an economically positive response.

When choosing to increase nitrogen application rates in future years based on these test results, it is worthwhile to keep some strips of your previous management practice in each field to compare and assess whether increased application rates provided a positive return on investment.

It is also important to consider general weather patterns throughout the growing season, and if there were conditions that may have promoted nitrogen losses such as volatilization, leaching or denitrification.

Considering this information along with the test results allows for best management practices to be implemented in future growing seasons.

Again, if your test result is low, the correct management response may not necessarily be to apply higher rates of nitrogen fertilizer. You may need to adjust management to improve the fertilizer efficiency, such as applying the fertilizer closer to the time of crop uptake, incorporating manure and urea containing fertilizers, or using nitrogen stabilizers such as urease inhibitors, nitrification inhibitors or slow-release polymer coated fertilizer.

Adjusting management to improve efficiency helps the same amount of nitrogen fertilizer usage go further to improve crop yields and the overall economics of the crop production system.

The earliest that a producer can conduct a test is at one-quarter milk line, which can be determined when the ear of corn is broken in two. The milk line is the border between the milk and starch layers that can be seen when looking at the broken half containing the ear tip. One-quarter milk line means that the outer one-quarter of the kernel has converted to starch.

The latest that a producer can conduct the test is three weeks after black layer (physiological maturity). A minimum of 10 representative plant samples needs to be taken within the cornfield, or one stalk per acre if the field is greater than 10 acres. Choose healthy disease-free samples to include in your test. An 8-inch stalk sample needs to be taken beginning 6 inches above the soil surface. A set of pruning shears works well to assist you in cutting your samples.

You can make a homemade measuring stick 14 inches long with a mark at 6 inches, which will be a useful tool to quickly determine the cutting heights for the corn stalk nitrate test.

Using this measuring stick, an 8-inch section of stalk can be cut starting at 6 inches above the ground and extending to 14 inches above the ground. When cutting the stalk, it is easiest to make the top cut first, then the bottom cut.

Once samples have been collected, the stalks should be cut into 2-inch sections.

An alternative to cutting the stalks into 2-inch segments is to quarter each stalk lengthwise with a machete or kitchen knife and retain one quarter strip from each stalk to submit for analysis. Quartering stalk segments allows the sample to dry faster and is easier for the lab to process. Place samples in a paper bag to be mailed to the Penn State Agricultural Analytical Services Laboratory.

If samples are being held over the weekend, place them in a paper bag and put in a refrigerator. Do not freeze the samples. For more instructions on how to collect and submit a CSNT sample, visit the Agricultural Analytical Services Laboratory website.

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Leon Ressler is a Penn State Extension educator based in Lancaster County, Pennsylvania.