What’s the right PSNT number? New research refines the critical threshold for corn nitrogen decisions in Minnesota

Fig. 1. The recommended timing for the Pre-Sidedress Nitrate Test (PSNT) is between V4 and V6, when most roots are concentrated in the top 12 inches of soil. Samples should be collected to a depth of 12 inches to accurately assess nitrate-N availability for sidedress decisions.

By: Emerson Souza, Researcher, soil fertility & nutrient management & Fabian Fernandez, Extension nitrogen management specialist

If you're using the Pre-Sidedress Nitrate Test (PSNT) to guide your nitrogen (N) decisions in corn, here’s the good news: We’ve updated the critical number you should aim for, based on field trials conducted across the major corn production regions of Minnesota.

Even more important, PSNT becomes especially valuable when weather disrupts your normal N management. If wet soils prevented a pre-plant N application, or if you applied early but then experienced heavy rainfall, PSNT offers a timely, in-season check on how much nitrate remains in the root zone, and whether a sidedress application might pay off.

Based on 34 field experiments, we found that a PSNT value of 20 ppm of nitrate-N in the top 12 inches of soil at the V4-V6 stage reliably delivers 97% of maximum corn yield. That’s your target.

Here’s what else we learned:

• 20 ppm is a strong statewide target, but weather matters
• We saw variation in the critical PSNT depending on early-season rainfall:
• In dry springs, the critical PSNT rose to 21.5 ppm
• In wet springs, it dropped to 17.4 ppm

Still, when we looked at all the data together, using 20 ppm ± 2.5 ppm gave the best overall fit. It’s a solid benchmark that holds up across most conditions.

Takeaway: If your PSNT is close to 20 ppm, you’re in good shape. But if it’s below that, especially in a dry year, additional sidedress N might pay off.

What to do if PSNT is low? Use this simple guide.

If your PSNT result is below the 20 ppm mark, how much N do you need to get back on track?

Our data show that you need about 12.3 lbs N/acre for every 1 ppm increase in soil nitrate-N in the 0–12 inch layer. Of course, there is some variability around that number, but it provides a good way to determine how much might be needed.

Fig. 2. Estimated additional N needed (lbs N/ac) to reach the critical PSNT threshold of 20 ppm, based on measured in-season PSNT values. The calculation is based on a conversion factor of 13.8 lbs N/ac per 1 ppm NO3--N below the threshold.

 

Example: If your PSNT is 16 ppm, you’re 4 ppm short. Multiply that by 12.3 lbs → you’d need around 49 lbs N/acre at sidedress to reach the critical threshold. Of course, before applying additional N, you should make sure to evaluate other factors, such as crop yield potential, to make a more informed decision on whether or not the application is warranted.

The PSNT is a useful tool but always use it along with local context

This study reinforces that the PSNT remains a valuable tool for in-season N management in Minnesota. Just remember:

• Sample between V4 and V6.
• Go to a 12-inch depth.
• Use recent weather to guide your interpretation.

And most importantly, use the test alongside your best judgment, using your knowledge of the field and crop history.

Want to dig deeper?

This information is based on a recently published study, Precipitation Influences Pre-Sidedress Soil Nitrate Thresholds for Corn Production (https://doi.org/10.1002/saj2.70085). Reach out to nutmgmt@umn.edu if you’d like help interpreting PSNT results on your farm.

Funding for this project was provided by the Minnesota Department of Agriculture and the Clean Water Fund.

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