soil dirt

Now is the time to update the formula you use to determine corn sidedress rates.

The pre-sidedress soil nitrate test was developed in the 1990s to estimate how much nitrogen in the soil will be changed to a plant-available form during the growing season, and to adjust nitrogen fertilizer application rates accordingly.

The theory behind the test is still good, but soil management in our region has changed significantly, including the shift to no-till, the widespread use of cover crops, and the increase in manure injection.

These management changes affect how nitrogen behaves in the soil, so the test methodology and calibration were updated.

The pre-sidedress soil nitrate test is done by taking a soil sample to 12 inches deep when corn is 12 inches tall (about V5), drying immediately, then sending the sample to a lab for analysis of the soil nitrate concentration.

Charlie White, a Penn State nutrient management specialist, explains that his research group has calculated a new PSNT formula, shown below.

The new calibration is based on 47 site years of experiments between 2012 and 2022. It includes both on-farm and research station experiments with diverse crop, cover crop and manure histories.

All sites were no-till during the year corn was grown. Less than 50 pounds of nitrogen per acre were applied at planting, then a gradient of nitrogen rates was applied at sidedress to determine the amount required to maximize corn yield.

The calibration fits well following single-species cover crops. But the results are unreliable following mixes, so we do not recommend using this formula following grass-legume plantings.

The following formula is only for fields where manure was spread two or more out of the last five years, not including the current season.

Sidedress N fertilizer recommendation in pounds per acre = (0.71 × corn yield goal in bushels per acre) – (4.9 × PSNT NO3-N ppm)

Make sure the corn yield goal you enter in the formula reflects a realistic yield goal for the particular site and growing season.

The soil sample result should be entered in parts per million nitrate-N (ppm NO3-N), which is how the lab should report it.

An example calculation for a corn yield goal of 180 bushels per acre and a PSNT nitrate level of 15 ppm NO3-N would be as follows:

(0.71 x 180 bu/ac) – (4.9 x 15 ppm NO3-N) = 54 lbs/ac nitrogen fertilizer to sidedress

There are three main differences between the new and old calibration.

First, the new calibration gives increased credit to nitrate in a no-till system.

The research showed that a unit of soil nitrate measured under no-till contributes more to reducing the fertilizer requirement than in the older dataset, which was calibrated in tilled systems.

Second, the constant multiplied by the corn yield is reduced from 1.0 to 0.71.

This reduction in the nitrogen needed per bushel of corn yield goal is from improvements in nitrogen use efficiency, likely from better corn genetics and improved fertilizer management practices.

Lastly, in the new calibration, there is no soil nitrate level at which zero sidedress nitrogen fertilizer is recommended.

In the old calibration, a result above 21 ppm NO3-N meant no sidedressing was recommended.

In the new dataset, there were several sites where corn yield increased with nitrogen fertilizer even when the soil nitrate was above 21 ppm. So, sidedressing is still recommended by the new equation.

Manure injection complicates sampling, as manure and elevated nitrogen mineralization rates are concentrated in the injection bands. Contact your local Penn State Extension educator for more details on sampling PSNT in injected fields.

We encourage you to use the new calibration formula in the 2023 growing season.


Heidi Reed is a Penn State Extension educator in York County.