Mid-season soil nitrate testing in corn can provide valuable information on expected nitrogen availability to the crop through the remainder of the growing season. Researchers in Pennsylvania developed a soil nitrate sampling protocol for fields where manure injection causes patterned nitrogen distribution in the soil. Additional work provided updates to the state’s presidedress soil nitrate test (PSNT) that reflect contemporary crop and soil management practices.

A tool for tracking

The PSNT is an affordable tool that allows producers to sample soil and have it analyzed at a soil laboratory for nitrate nitrogen (NO3-N) concentration during corn’s early growth stage. The NO3-N level is indicative of the nitrogen supply available in the soil from manure and other organic sources. The lab results are entered into a simple PSNT formula that provides a recommendation for the amount of sidedress nitrogen expected to provide the economic optimum nitrogen rate (EONR) to the corn in the current growing season.

The test can also indicate if no sidedress nitrogen is needed. Producers can strategically limit manure and fertilizer applications prior to planting and use the PSNT to provide economic and agronomic information on the soil’s resiliency to supply nitrogen for the remainder of the season. Thus, the PSNT is a tool that provides agronomic, economic, and environmental benefits associated with the 4R practices — right source, right rate, right time, and right place.

PSNT soil sampling protocols vary slightly between state land-grant institutions and extension organizations. In Pennsylvania, the PSNT protocol calls for 10 to 20 random soil core samples, taken 12 inches deep, to be collected when corn is 6 to 12 inches tall. The soil is composited and mixed, with a single subsample sent to the laboratory for analysis. Drying or cooling soil samples before shipping is recommended to slow microbial processes and nitrogen transformations. Laboratory analysis results are reported in units of parts per million (ppm) NO3-N.

Measuring for injected manure

The PSNT was developed in the 1990s and did not provide specific soil collection recommendations for fields where manure was injected. Manure injection deposits manure in parallel banded patterns across a field. This results in higher nitrogen concentrations in a zone around the band. Random soil probe placement could collect cores from areas close to the band or in the middle space between bands, each reducing confidence in PSNT results.

Penn State and USDA-ARS researchers designed an experiment that used a unique monolith soil collection system to capture a soil core for every inch across a two-dimensional plane oriented perpendicular to the travel of manure injection (Figure 1). The experiment used 30-inch spacing between manure injection bands, so 30 individual adjacent soil NO3-N levels were known. The work was replicated and repeated over two seasons.

A statistical model was developed to compare NO3-N results from different possible soil sampling protocols to the average NO3-N of all 30 samples combined. A simple sampling spacing protocol emerged as the top choice. Soil sampling where nutrients are banded is done by collecting a set of five equally spaced sets of soil cores in an orientation perpendicular to the nutrient band from four locations in the field (Figure 2).

To determine the specific, equally spaced distance between soil cores for the field, measure the distance between band locations and divide by five. For example, if manure injection bands are 30 inches apart, then collect five soil cores that are 6 inches apart (30 inches divided by five cores equals 6 inches between cores) as a set.

The set of five adjacent cores are collected at four random locations in the field for a total of 20 soil cores. The 20 cores are combined and prepared for shipment to the laboratory as outlined earlier. The protocol when manure is injected requires no more labor or expense compared to the random protocol where there is no injection. Since development, this sampling protocol has been verified in numerous field trials.

The cores must be collected perpendicular to the direction in which the manure band was applied. By PSNT time, manure band locations can be difficult to locate. The precise location of the band does not need to be known because one of the five cores will be close to the band (one sample would be within 3 inches of the center of the manure band in the 30-inch spacing above), as illustrated in Figure 2.

Finding the optimum rate

Historically, laboratory PSNT results provide a level of NO3-N from the soil sample that is entered into a formula to determine sidedress nitrogen needs. Most states provide a threshold NO3-N level that indicates there is enough nitrogen stored in the soil to meet corn growth needs. For example, in Pennsylvania, if the results were greater than 21 ppm NO3-N, no additional nitrogen was recommended. If results were below this threshold, the ppm NO3-N result was entered into a formula along with expected corn grain yield, nitrogen supplied from historic manure application, and nitrogen supplied from previous legume crops to derive a nitrogen sidedress recommendation in pounds per acre.

As time, practices, and yields advanced, the Pennsylvania formula became less reliable in its predictive quality. Changes such as widespread adoption of no-till, cover crops, retention of soil organic matter, and a producer focus on soil health improvements provided resilient pools of immobilized nitrogen in the soil, especially in fields that received historic manure applications. Ever-increasing corn yields mean higher nitrogen crop removal occurs more often today than in previous decades. Penn State researchers utilized data from 47 site-years to develop and verify the updated formula to enter PSNT results to find a recommended nitrogen sidedress rate (see Figure 3).

This formula can account for the wide variety of soil management practices that affect nitrogen fertilizer requirement, such as conservation of manure ammonium with injection and immobilization or mineralization of nitrogen from previous cover crop residues and historical manure application. Research that calibrated nitrogen sidedress recommendations for this formula indicated that the PSNT result is only useful if the field has a long-term manure history of two or more years of manure applied in the last five years prior to the current year.

An additional 50 pounds of nitrogen per acre is added to the sidedress recommendation when a cover crop mixture with different plant families or functional types (such as a mix of grasses, brassicas, and/or legumes) existed in the field in the prior winter.

Unlike the original PSNT approach, the new formula does not contain a fixed critical soil nitrate threshold where no nitrogen sidedressing is recommended. Growers can determine if low sidedress recommendations are worth pursuing. A formula result of zero indicates that no plant response to nitrogen sidedress is expected. When the new formula provides a negative result, no sidedress nitrogen is recommended.

This article appeared in the May 2025 issue of Journal of Nutrient Management on page 16-18.

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