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One of the critical factors to consider with each corn crop is the amount of nitrogen available to the crop and the efficiency of its usage. While there are several soil and tissue tests that can verify nutrient use during the season, looking back at the corn crop’s use of nitrogen from the past season can be valuable in determining future nitrogen application rates.
The nitrogen status of a corn crop can be assessed by measuring the nitrate at the end of the growing season. Corn plants that don’t have enough nitrogen remove it from the lower stalks and leaves during the grain-filling period, whereas those with excessive nitrogen accumulate nitrate in their lower stalks at the end of the season.1,3
Research has shown that using the late season cornstalk nitrate test has proven it to be a reliable end-of-season indicator of crop nitrogen status: too much, too little, or the right amount.2 Using test results combined with nitrogen management records can guide future management decisions. While this test will not tell you which management practices need to be changed, it can help you determine what is working and what is not. The true value of the test can be assessed from using it for repeated observations over multiple years for a given field.3
Table 1: Interpretation of late season cornstalk nitrate test.Table adapted from: Beegle, D. and Rotz, J. 2017. Late season cornstalk nitrate test. Penn State Extension. Agronomy Facts 70. https://extension.psu.edu
The timing and accurate representation of sampling is critical in order to provide the most valuable information to evaluate the corn crop’s nitrogen usage. Sampling should be completed any time between one and three weeks after black layer has formed on about 80 percent of the kernels of most ears. Cut 8-inch long sections at 6 inches above soil level from 10-15 randomly selected representative plants to form a single sample. Do not sample diseased, damaged, or stunted plants. Remove any leaves or leaf sheaths from sample segments and cut samples into 1 to 2-inch long segments to promote drying. Samples should be kept and sent in a paper bag to help prevent mold growth. It is best to send samples immediately after collection, but they may be refrigerated (not frozen) if there is more than a day delay before the sample can be sent. Complete necessary testing form(s) and submit samples as quickly as possible.
When interpreting results of the test, the first thing to remember is that the late season stalk nitrate test looks back at how things went this season; it alone does not indicate what needs to be done in the future. In order to capture the full potential of this test, its results should be combined with records for nutrient management, cultural practices, and growing conditions for the season prior to testing.
If results are low, you should first look at rate of nitrogen applied. Total nitrogen for corn should be around 1-1.2 pounds of nitrogen per bushel of yield.2,4 If the rate applied appears correct, the materials used and application method and timing should be examined. Nitrogen applied too early and/or subjected to intense precipitation may suffer significant losses. Urea and manure fertilizers that are not incorporated may be subject to serious losses. Low pH, poor weed control, and compaction conditions may limit a crop’s ability to take up nitrogen.
When rates are too high, the rate should be the first thing examined. If the rate appears to be correct, the previous crop and nutrient source should be examined. Fields that were previously planted to legumes would have residual nitrogen left in the soil which may not have been accounted for. Fields that had been regularly fertilized with manure would also likely have residual nitrogen remaining that may not have been accounted for. Finally, if the current corn crop suffered significant yield reduction from a pest or disease, this could also result in excess nitrogen in the crop.
Sources: 1 Blackmer, A.M., and Mallarino, A.P. 2000. Cornstalk testing to evaluate nitrogen management. Iowa State University Extension. PM 1594. http://extension.iastate.edu/; 2 Beegle, D. and Rotz, J. 2017. Late season cornstalk nitrate test. Penn State Extension. Agronomy Facts 70. https://extension.psu.edu; 3 Nielsen, R. 2003. End-of-season corn stalk nitrate test. Purdue Extension. Corny News Network. http://www.kingcorn.org/news/ 4 Camberato, J. and Nielsen, R.L. 2017. Nitrogen management guidelines for corn in Indiana. Purdue University Department of Agronomy Applied Crop Research Update.
http://www.kingcorn.org/news/. Web sources verified 10/16/17. 171023124431