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New PSNT test ready to use

The recalibrated PSNT test is backed up by 10 years of research.

June 6, 2024

7 Min Read
Emerging corn plants in a field
GROWING CORN: This emerged cornfield in Lebanon, Pa., would be ideal for a PSNT test once the crop reaches 12 inches in height, about V5. The PSNT is done by taking a soil sample up to 12 inches deep, then sending it to a lab to analyze soil nitrate concentration. Chris Torres

by Charlie White

Research aimed at updating the pre-sidedress nitrate test calibration has been underway for the past several years at Penn State. 

Two years ago, we published a provisional recalibration of the PSNT formula. After incorporating results from the 2022 growing season and finalizing the statistical analysis, we have a new formula that we expect to remain relatively stable in the coming years.

We allowed the calibration process to select from a variety of different management practices that could serve as history factors, such as the corn yield goal, type of winter cover, previous crop type, and the short- and long-term manure history.

Ultimately, the best-fitting calibration equation included a component that sets the maximum N fertilizer requirement based on the corn yield goal, then subtracting a quantity of fertilizer that is based on the PSNT result multiplied by a history factor if the field had a long-term manure history — defined as manure applied in two or more out of the past five years, not including the current season.

The calibration fit well for previous grass and legume cover crops, but the results had a high level of uncertainty when the previous cover crop was a grass-legume mixture. So, we do not recommend using this PSNT formula following grass-legume mixtures:

  • (0.72 x corn yield goal in bushels per acre) – (5.1 x PSNT NO3-N ppm)*

*If the field has a long-term manure history (manure applied in two or more out of the past five years, not including the current year).

In the equation above, the corn yield goal should be entered as bushels per acre and should reflect a realistic yield goal for the site and growing season. The PSNT result should be entered in parts per million nitrate-N (ppm NO3-N). An example calculation for a corn yield goal of 180 bushels an acre and a PSNT nitrate level of 15 ppm NO3-N would be as follows:

  • (0.72 x 180 bushels an acre) – (5.1 x 15 ppm NO3-N) = 53 pounds per acre N fertilizer.

How PSNT is taken

The PSNT is done by taking a soil sample up to 12 inches deep, then sending it to a lab to analyze soil nitrate concentration. The test should be taken when the crop is 12 inches tall, or about V5 stage. For normal planting dates, the sampling window usually occurs in mid-June.

A key assumption of the PSNT is that soil nitrate concentrations this time of year will serve as a relative indicator of the amount of N that will be mineralized the remainder of the season. However, there are several management factors that can affect this.

Soil disturbance from tilling creates a burst of decomposition that temporarily increases N mineralization rates and raises the soil nitrate concentrations early in the season. In no-till systems, mineralization rates increase at a slower pace because the soil stays cooler and undisturbed. However, mineralization rates in no-till can remain higher later in the season because of soil moisture conservation by the crop residue.

Cover crops also affect the temporal dynamics of soil nitrate over a growing season. Grass cover crops tend to scavenge most of the soil nitrate while growing in spring. Fallow fields have soil nitrate levels that are gradually building prior to corn planting. As a result, soil nitrate levels in the early growth stages of corn will have a lower starting point after a cover crop than after a fallow field.

Variations in soil management can be accounted for by calibrating the PSNT results to make a fertilizer recommendation under different management systems. The new calibration is based on experiments done between 2012 and 2022, and includes both on-farm and research station experiments.

Previous crop types included corn, soybeans, wheat, pumpkins and an alfalfa or orchardgrass hay. Previous cover crops included various grasses and legumes in monoculture; grass or legume mixtures; fallow; and two sites that were in a previous alfalfa or orchardgrass hay.

The sites also had a variety of different long-term and recent manure histories. All sites were managed no-till when corn was grown. Sites had less than 50 pounds N per acre applied at planting, followed by a gradient of N fertilizer application rates applied at sidedress to determine economically optimum N fertilizer rate, which is what the PSNT recommendation is calibrated to predict.

Key differences to previous PSNT

Because the calibration process selected a single history factor for long-term manure (5.1), PSNT samples should only be collected for fields with a long-term manure history (manure applied in two or more years of the past five, not including the current year). Fields with no long-term manure history should have no adjustments to sidedress N fertilizer rates based on PSNT results. 

The single history factor also indicates that the PSNT is a robust tool to assess and quantify the variations in N availability that can occur following a variety of cover crop and manure management practices.

Compared to the previous PSNT calibration, the history factor is generally higher, meaning that a unit of soil nitrate measured under these management scenarios (for example, no-till soil management) has a greater contribution to reducing the N fertilizer requirement than in the older dataset.

This could be explained by no-till generally having a slower rate of increase in N mineralization during the early growing season because of cool soils. So, the nitrate concentration measured at the time of PSNT sampling in a no-till system may need to be given a greater credit.

Another difference from the original calibration is the coefficient multiplied by corn yield is reduced from 1.0 to 0.72. We believe this reduction reflects improvements in nitrogen-use efficiency in modern corn production systems from better corn genetics and improved fertilizer management practices.

Finally, we did not detect a fixed critical soil nitrate threshold at a point where no sidedress N fertilizer is recommended.

More about manure

The dataset used to calibrate the equation included several sites with side-by-side broadcast manure vs injected manure comparisons. The calibration process showed that the same history factor can be used for both injected and broadcast manure.

The effect of increased N conservation by manure injection was realized by a higher PSNT nitrate concentration in the sites that were injected, because more ammonium was retained in the soil and ultimately nitrified, resulting in lower sidedress N fertilizer recommendations. While no special attention needs to be paid to injected vs. broadcast manure when calculating the N fertilizer recommendation using the calibrated equation, a different soil sampling methodology is recommended when collecting PSNT soil samples in a field that has had manure injected recently.

In fields where manure has been injected recently, the increased rates of N mineralization will be concentrated in the injection bands. If the injection bands are not sampled proportionally with the rest of the bulk soil, the measured soil nitrate concentrations could be biased.

To overcome this problem, Penn State researchers have designed a special soil sampling method for collecting PSNT soil cores in fields after manure injection. The sampling protocol requires knowing the direction where the manure injector was traveling, but you do not need to know exactly where each injection band lies. 

To sample in- and out-of-band soil proportionally, take five soil cores spaced 6 inches apart in a row perpendicular to the direction of the manure injection bands. The 6-inch spacing between soil cores in each row ensures areas of the soil that are both close to and far away from the injection band are sampled proportionally.

Each soil core should be taken to the typical PSNT sampling depth of 12 inches. Repeat this process at three to four points in the field, ultimately compositing 15 to 20 soil cores in the whole sample. 

All other aspects of regular PSNT soil sampling remains the same; do it when corn is 12 inches tall, avoid sampling immediately after a heavy rainstorm, and dry the soil sample immediately or keep it refrigerated until lab analysis to preserve nitrate concentration.

In the coming months, we will revise the formal PSNT factsheet to reflect the outcome of this recalibration process.

White is an assistant professor of soil fertility and nutrient management at Penn State. Contributing authors include Heather Karsten, associate professor of crop production/ecology; Robert Meinen, assistant research professor; and John Spargo, director of the agricultural analytical lab at Penn State.

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Nitrates
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