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Interpretation of Results


Less than 450 ppm nitrate-N (Purdue), less than 250 ppm (ISU) Indicates high probability that greater availability of N would have resulted in higher yields. Visual signs of N deficiency are usually observed in this range.

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 decreasedCornstalk-Testing-Image.jpg 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

For laboratory specific information regarding plant tissue analysis, procedures, guidelines, test packages/supplies or analysis kits, please contact one of our laboratories nearest you listed below:  

Laboratory Locations:

AgSource Laboratories - Bonduel, WI715-758-2178 bonduel@agsource.com 
AgSource Laboratories - Ellsworth, IA 515-836-4444ellsworth@agsource.com 
AgSource Laboratories - Lincoln, NE 402-476-0300 lincoln@agsource.com 
AgSource Laboratories - Umatilla, OR541-922-4894 umatilla@agsource.com