NITRATE CONCENTRATION CATEGORIES
Marginal: 250 - 700 ppm nitrate-N (ISU) Producers should not be concerned when samples test in this range. N availability was close to the minimum amount needed for maximum yields but should not be the target for good nitrogen management.
Optimal: 450-2000 ppm nitrate-N (Purdue), 700-2000 ppm nitrate-N (ISU) Indicates that N supplies were sufficient for maximum yields. Note: The high end of this category is appropriate when N costs are low and corn prices are high. The low end of this category is appropriate when N costs are high and corn prices are low.
Excess: Greater than 2000 ppm nitrate-N Indicates that N supplies were above levels needed to maximize profits.
The concentration of nitrate in the stalk at the end of the season reflects all actors that influenced N availability and N needs during the growing season. High rainfall seasons will likely result in lower concentrations, while low rainfall seasons will likely result in higher concentrations.
After consideration is given for weather conditions, fertilization rates should be increased for areas that usually test in the low range and decreased on areas that usually test in the excess range. When concentrations are consistently in the excess category; most producers profit by using the late spring nitrate soil test to guide N fertilizer needs.
The "End of Season" corn stalk test does not directly indicate how much rates should be increased or decreased but continued use for several years enables producers to fine tune their N management. The underlying reason for over fertilization is that corn plants show no visual symptoms that enable producer's to recognize when above-optimal rates of N have been applied. The end-of-season cornstalk test makes it possible for producers to avoid the "zone of luxury uptake".Producers who grow corn on manured soils, who grow corn after alfalfa and those comparing alternative management practices such as fertilizer materials or methods of application will benefit greatly from observing cornstalk nitrate levels on a yearly basis. Thoughtful use of the test for a few years will help producers optimize rates for their fields.
Citation: Iowa State University Extension and Outreach, PM 1584, Cornstalk Testing to Evaluate Nitrogen Management A.M. Blackmer , A.P. Mallarino, 2000. Permission to use instructions and images was granted by ISU. For more information, visit: http://www.extension.iastate.edu/Publications/PM1584.pdf