One of the important questions that corn growers need to answer is; how much nitrogen should be applied to achieve a good corn yield? In our fertilizer applicator certification programs, participants learn that this is a tough question to answer. In addition to nitrogen supplied from commercial fertilizer sources nitrogen is provided through manure applications, mineralization from organic matter sources, and the atmosphere. Nitrogen can also be lost through leaching and denitrification. The addition of nitrogen from mineralization and the losses of nitrogen by leaching and denitrification are heavily influenced by the weather, especially the amount and timing of rainfall. Therefore, we don’t have good formulas or charts to tell a grower how much nitrogen to apply to their corn crop. However, there are some tools available to help manage nitrogen and provide growers with some guidelines that may be useful on their farms. One of these nitrogen management tools is the corn stalk nitrate test. This is an end of season test described as a “report card” or “post-mortem” evaluation of the adequacy of the nitrogen program that was used in the current season. The results of this test provide information that can be used to make nitrogen management decisions in future years.
The stalk nitrate sampling procedure involves cutting an 8-inch stalk segment out of at least 15 randomly sampled plants in a field. Samples are collected one to three weeks after corn reaches R6, physiological maturity, indicated by the formation of kernel black layer. Similar to soil sampling, if an area of the field has a different soil type, drainage pattern, or fertility treatment, consider those areas as a separate sample. The 8-inch segment begins at 6 inches above soil level so the sample segment represents the 6-14 inch height of the stalk. Remove leaves and leaf sheaths from the sample segment. Stalk segments should be kept as cool and clean as possible while sampling, if it is hot, a cooler can be used to store samples. Place each group of 15 stalk segments into a paper bag for shipment to the testing lab. Samples that will be delayed for more than a day before sending to a lab should be refrigerated.
Test results are reported in terms of parts per million (ppm) of nitrate-nitrogen (N). Purdue University uses the following categories to interpret results:
Low: less than 450 ppm, high probability nitrogen is deficient
Optimal: 450-2000 ppm, yields not limited by nitrogen
Excessive: greater than 2000 ppm, uptake exceeds requirements
Iowa, Pennsylvania and Michigan use 700 ppm as the low category, and optimal ranges from 700 to 2000 ppm.
The value of the stalk nitrate test is increased as the test is repeated over years for the same field, particularly when paired with notes or observations regarding weather patterns during the growing season. That trend information will allow a grower to make some informed decisions regarding nitrogen applications. The Purdue University publication on end of season stalk nitrate testing is available on-line at http://tiny.cc/Purduestalknitratetest, or contact the Wayne County Extension office at 330-264-8722 for more information.