Pre-Sidedress Nitrogen Test to Adjust N Rates

By Greg LaBarge

High prices for nitrogen fertilizer has every farmer looking closer at their N program. If your farm works with manure, the Pre-Sidedress Nitrogen Test (PSNT) information can help remove some uncertainty in determining what N credit you should take. Ohio field results with the PSNT show that when test results exceed 25 ppm NO₃-N, additional N is unlikely to increase yield. Purdue University has taken the PSNT a step further, providing guidelines to adjust N rates where the PSNT result are less than 25 ppm NO₃-N.

Collecting a PSNT test sample:

• Corn growth stage V4-V6 (12 inches tall or less).
• The sample core depth is 12 inches.
• A sample should consist of 15 to 25 cores per representative field area.
• Ship to the lab by an overnight carrier or air dry the soil if not immediately shipped.
• Request a PSNT soil nitrate test from the lab. Most labs provide results within 12-24 hours after receiving the sample.

Determine how much N to apply:

Use Table 1 below to determine N rate adjustment if the PSNT - NO₃-N is below 25 ppm. Table 1 is compatible with the Maximum Return To Nitrogen (MRTN)  recommendation calculator (http://cnrc.agron.iastate.edu). You can use the CNRC to determine your base N rate, or last week's CORN newsletter (https://agcrops.osu.edu/newsletter/corn-newsletter/2022-15/update-corn-nitrogen-recommendations-mrtn) has a table with a range corn and nitrogen fertilizers prices.

Table 1. Adjustments to N rates based on Pre-Sidedress Nitrogen Test (PSNT) results.

Example: The suggested MRTN rate from the CNRC with a soybean-corn rotation, $6 per bushel of corn and $0.95 per unit of N is 156 lbs. N/acre. A farmer gets a 19 ppm soil nitrate PSNT test back from the lab. Based on Table 1, we reduce the MRTN rate by 45 lbs. N/A. The suggested sidedress N rate is 111 lbs. N/acre.

Does the PSNT work to adjust N Rates?

For the 2021 corn crop, we used PSNT to adjust the MRTN rate of 186 pounds in an experiment at the Northwest Ag Research Station, Custer, OH. Liquid swine and dairy manure was applied in the fall of 2020 at 5,000, 8,000, or 12,000 gallons per acre, depending on the manure source. The rate reduction used for the project was higher than suggested in Table 1, with the 10-12 ppm PSNT results reduced by 45 pounds per acre and the 14 ppm reduced by 90 pounds. Corn yields from the PSNT adjusted rates were not statistically different from the 28% UAN check with 186 units of N applied. This one-year project's results would indicate that recommended N rate reduction from Table 1 is conservative.

Table 2. Corn yields, PSNT test results, and units of N applied where fall-applied liquid swine or dairy manure was used on Hoytville Clay Loam soil.