Should You be Taking PSNTs Right Now?

Sydney Riggi, Extension Agent – Nutrient Management; Amy Shober, Extension Specialist – Nutrient Management and Environmental Quality; Phillip Sylvester, Extension Agent – Agriculture, Kent County, Jarrod Miller, Extension Specialist – Agronomy,

The compressed corn planting window this spring will result in a narrow window for in-season N applications. We recommend that growers take pre-sidedress nitrate testing (PSNT) samples prior to sidedressing corn. The goal of the PSNT is to provide a real-time recommendation while there is still the opportunity to adjust nitrogen (N) applications to corn. Regional research has shown that producers who use the PSNT can save time and money, while reducing the risk of nitrogen loss.

The PSNT is recommended for fields that have a history of manure, biosolids, or compost applications. The PSNT is also appropriate for fields with forage legume residues. As such, we suggest that fields that had a cover crop that produced significant biomass (i.e., late killed or planted green) may also benefit from a PSNT; however, no research-based recommendations are available specifically for fields planted in cover crops.

In our region, the PSNT is not recommended for corn fields where commercial fertilizer is the only N source for three or more years because there was no regional PSNT calibration under these conditions. In addition, the PSNT is not recommended for manured fields that received commercial fertilizer (pre-plant or pre-emergence) at rates exceeding 40 to 50 lbs N/ac. Under both of these conditions, the PSNT may overestimate the amount of soil N that will be available to the corn crop over the season. For fields with no recent manure history, we feel that growers can better calculate N-needs based on realistic yield goal (1 lb N/bu corn expected), adjusting for the amount of N applied prior to sidedress. We caution against using PSNT recommendations from other regions for fields with no recent manure history, because their soil conditions and fertilizer management practices are often very different from ours.

It is important to note that the PSNT is less accurate during cool, wet growing seasons like we have had this season. Cooler temperatures early this spring, may have reduced early season N mineralization, which can lead to lower than normal PSNT results. Plus, the 5-10 inches of rain across our region may have moved soil nitrate below the 12 inch depth (the recommended depth of the PSNT sample), but this N will not be out of reach of corn roots later in the season. While regional PSNT recommendations were developed based only on soil nitrate concentrations, we believe that it may also prove valuable to evaluate soil ammonium concentrations this year, especially if soil nitrate concentrations come back lower than normal (based on past history) and manure was applied recently. Elevated soil ammonium concentrations may indicate that mineralization of manure (organic) N was delayed. In our opinion, evaluating soil nitrate + ammonium concentrations can help eliminate the chance of a false positive during cool, wet seasons. However, there are admittedly no hard and fast research-based recommendations for this particular situation.

If you decide to collect a PSNT sample, remember that the samples should be collected:

  • When corn plants are 10-12 inches tall (growth stage V5 to V6)
  • To a depth of 12 inches from 15 to 20 locations in a field (not to exceed 20 acres per sample)
  • No sooner than 2 to 3 days after significant rainfall

Prior to taking samples, there should be a plan as to where and when the sample will go to be dried and analyzed. If the sample will be stored for more than an hour, then it should be held in a cooler until it can be dried for analysis. Properly drying the sample is critical to getting an accurate analysis. A subsample of one to two cups of soil should be spread thinly (½ or less) on newspaper in a warm, dry place and allowed to dry. Once you receive the results of a PSNT, we refer you to the following documents for state-specific guidance on determining the sidedress N rate:

Delaware – Nitrogen Management for Corn in Delaware: The Pre–Sidedress Nitrate Test

Maryland – Soil Fertility Management