Sidedress N and PSNT Sampling for Corn

As this year’s corn crop begins to emerge and stretch out, it is time to prepare for sidedress N applications. Sidedress N for corn can be applied any time after planting, but the target window is generally between growth stages V4 and V8, before rapid corn N uptake occurs. Sidedress N application may be considered for several reasons:

  • In areas that have received excess rainfall this spring, soils that are highly susceptible to in-season N loss from leaching or denitrification may have lost considerable N, and corn may require supplemental sidedress N.
  • In areas that were dry last year and continued to be dry this spring, some growers may have applied lower preplant N rates in anticipation of drought and lower corn yield. If timely rainfall arrives in June, corn yield potential may increase and require supplemental sidedress N.
  • Some growers have experienced too many years of one or both situations above, and they have made planned split N applications part of their standard N management strategy.

Whether your N management strategy includes a preplanned sidedress N application or not, the Pre-Sidedress Soil Nitrate Test (PSNT) is one tool to help make decisions about in-season N application. You may also hear this test called the Late-Spring Soil Nitrate Test (LSNT) in Iowa. PSNT is an in-season soil nitrate test used during the early growing season to determine if additional sidedress N fertilizer is needed. PSNT helps assess available soil nitrate-N prior to rapid corn N uptake and the likelihood of corn yield response to sidedress N application.

PSNT requires a 0-12 inch depth soil sample taken when corn plants are 6-12 inches tall (at the whorl), usually in late May or early June. Late-planted corn may not reach that height before mid-June, but PSNT soil samples should still be collected during the first two weeks of June. The recommend soil sampling procedure requires 16 to 24 soil cores taken randomly, staggering your soil cores across the row as you go (see ISU publication below). All soil cores should be placed in the soil sample bag and submitted to the laboratory within 24 hours or stored in the refrigerator.

You can submit PSNT soil samples using the online AGVISOR program by choosing the “Corn Sidedress N” crop choice and submitting a 0-12 inch soil sample for nitrate analysis (see online submission instructions below). AGVISOR will generate sidedress N fertilizer guidelines, using the PSNT critical level of 25 ppm nitrate-N (0-12 inch depth). If PSNT is greater than 25 ppm nitrate-N, then the probability of any corn yield response to additional sidedress N is low. If spring rainfall was above normal, then the PSNT critical level of 20 to 22 ppm nitrate-N (0-12 inch depth) should be used. The ISU publication discusses PSNT interpretation for excessive rainfall and manured soils.

If PSNT is utilized after excessive rainfall, the soil cores will be wet and difficult to mix in the field. Therefore, it is best to send all soil cores to the laboratory to be dried and ground, ensuring a well-blended soil sample for analysis. Although in-field soil nitrate analyzers have improved over the years, the difficult task of blending wet, sticky soil cores in the field still remains. The only way to get accurate, repeatable soil analysis results is to dry, grind, and blend the entire soil sample in the laboratory before analysis.

AGVISE provides 24-hour turnaround on PSNT soil samples. The soil samples are analyzed and reported the next business day after arrival. Soil test results are posted on the online AGVISOR program for quick and easy access. With AGVISE, you get not only great service but also the highest quality data with four decades of soil testing experience.

Pre-Sidedress Soil Nitrate Test (PSNT) resources
Instructions for submitting PSNT online
Late-Spring Soil Nitrate Test, Iowa State University
PSNT in a delayed spring, Iowa State University

Please call our technical support staff if you have any questions on PSNT and interpreting the soil test results for sidedress N application.

Updated: 4 June 2020