How to Interpret Soil Nitrate Testing Information

The Soil Nitrate Test or the Pre-Sidedress Nitrate Test (PSNT) offers a timely in-season check on how much nitrate remains available in the soil profile before corn enters its period of rapid nitrogen uptake. Rather than relying solely on preseason nitrogen plans, the Soil Nitrate Test provides field-specific information that can help fine-tune sidedress nitrogen decisions.

University of Minnesota research found a critical Soil Nitrate Testing value of approximately 20 ppm nitrate-N statewide, increasing to 21.5 ppm during dry springs and decreasing to 17.4 ppm during wet springs.

One important finding from the University of Minnesota study, is that each 1 ppm increase in soil nitrate-N measured in the 0–12 inch sampling depth represents approximately 12.3 pounds of available nitrogen per acre.

University of Minnesota researchers also found that spring weather affects the critical threshold:

• During dry springs, the critical PSNT value increased to 21.5 ppm.

• During wet springs, the critical PSNT value decreased to 17.4 ppm.

• Across all environments, 20 ppm remained the best overall target.

Iowa State University researchers state that for every calibrated soil test has a critical test value, above which there is little expectation of response to applied nutrients. For the Late-Season Nitrate Test (LSNT), that value is 25 ppm nitrate-Nitrogen. That means that if a test result is more than the critical value, there is adequate plant available nitrogen in the soil system and no nitrogen application is needed.

What Does This Mean for Growers?

A simple way to interpret nitrate test results in 0-12 inch utilizing the University of Minnesota and Iowa State University information:

• Above 25 ppm: Additional sidedress nitrogen is less likely to produce an economic yield response; and in turn is not recommended.

• 20-25 ppm: Nitrogen availability is optimum, however based on yield environment supplemental nitrogen could be considered.  

• Below 20 ppm: Consider supplemental sidedress nitrogen, particularly if yield potential remains high.

  
  

How to convert my nitrate test results from lbs per acre to ppm?

The illustration below shows the mathematical process of converting the nitrate nitrogen results from 0-12 inch sample results from lbs per acre to ppm.

Nitrate Nitrogen Lbs Per Acre / by 4 = Nitrate ppm

The updated research reinforces the value of Nitrate Soil Testing as a practical tool for in-season nitrogen management. By measuring the nitrate already present in the soil, growers can make more informed sidedress decisions, improve nitrogen use efficiency, and potentially reduce unnecessary fertilizer applications while maintaining yield potential.

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Souza, E. J., and F. G. Fernandez. 2025. What's the Right PSNT Number? New Research Refines the Critical Threshold for Corn Nitrogen Decisions in Minnesota. University of Minnesota Extension. Available at: University of Minnesota Extension article

Sawyer, J. E., & Mallarino, A. P. (2017, May). Use of the late-spring soil nitrate test in Iowa corn production (CROP 3140). Iowa State University Extension and Outreach. Available at: https://www.agvise.com/wp-content/uploads/2017/06/Late-spring-soil-nitrate-test-ISU-2017.pdf