Management and Environmental Quality, University of Delaware, ashober@udel.edu, Sydney Riggi, Extension Agent – Nutrient Management, University of Delaware, sydney@udel.edu; Jarrod Miller, Extension Specialist – Agronomy, University of Delaware, jarrod@udel.edu; Phillip Sylvester, Extension Agent, University of Delaware, phillip@udel.edu; Karen Gartley, Soil Testing Program Director, University of Delaware, kgartley@udel.edu
As we have highlighted in previous weeks, managing nitrogen (N) when it won’t stop raining is a difficult task. The PSNT is meant to help measure the N that is available from organic amendments, like animal manures. Organic forms of nitrogen and ammonium are rapidly converted by soil microbes to nitrate (NO3–) once soil temperatures exceed 60°F, conditions we have observed since late April/early May. As such, we expect that organic forms of N, like those in animal manures, were converted to ammonium (NH4+) earlier in the season. Once present as ammonium, conversion to nitrate by soil microbes is rapid (unless a nitrification inhibitor was applied to manure or soil). This nitrate was then susceptible to leaching below the root zone, runoff from fields where rainfall exceeded infiltration, or loss to the atmosphere from soils that were flooded for more than 48 hours.
We still advocate using a PSNT for fields with a recent history of manures or cover crops. However, the copious amounts of rain this year, coupled with rapid conversion of N in the soil, makes it difficult to rely solely on results of a PNST to determine your sidedress needs. Part of the difficulty we are having in making sidedress N recommendations this year is related to the huge variability in PSNT values we are seeing in fields with similar management. For example, we have received reports of pre-sidedress nitrate test (PSNT) values that are well below the 25 ppm NO3-N threshold (e.g., 8-10 ppm NO3-N) in soils that received moderate amounts of manure (e.g., 2-3 tons/ac) and commercial starter N fertilizers (30-80 lb/ac). However, we have also seen samples with PSNT values as high as 45 ppm NO3-N from nearby fields under the same management conditions. In a “normal year”, PSNT-based sidedress recommendations are determined by the concentration of nitrate in the PSNT sample, which is then adjusted for manure application (based on the timing and amount of manure applied) and any starter N applied prior to or at planting. But this year, Mother Nature is making it harder for us to make the call on if there is really manure N left to mineralize. We have also been measuring soil ammonium concentrations to look for evidence of delayed mineralization. Unfortunately, low soil ammonium concentrations (< 5 ppm) suggest that there is not much delay in the mineralization and that much of the organic N has been lost. As such, the University of Delaware Soil Testing Lab is recommending a range of sidedress N rates based on the PSNT.
Recognizing that N is important for adequate crop growth but that too much can result in environmental issues, we ask growers and consultants to consider the following before deciding exactly how much sidedress N to apply to corn (when you finally get out into the field):
- Are there visible signs of significant crop stress that will likely reduce the chance of achieving your expected yield goal (e.g., poor stand uniformity, dead or dying plants, poor root development) or does your corn look pretty healthy (e.g., even stand, healthy green color, healthy roots)?
If you have poor stand uniformity or signs that plants were drowned out, this is a good indication that you may lose yield. In this case, we recommend that you reduce your yield goal and fertilize at a lower N rate. Remember, fertilizing dead or dying corn will not bring it back. In contrast, if your crop looks to be in pretty good shape, then you can consider adding N at a higher sidedress rate to account for early season N losses.
- Will putting equipment into the field to sidedress cause more damage than it’s worth?
If you put equipment in the field while it is still too wet, you risk getting stuck and/or causing significant compaction. Both will cost you money and time to fix. If you can’t get equipment in the field before the corn is too tall, and you feel that you must get some N out, you have the option to fly on granular urea. While some sources recommend that you keep rates low (<60 lb/ac) to prevent leaf burn, research out the the University of Missouri suggests that applications of up to 150 lb/ac of urea are possible without significant leaf burn to the plant. If you do consider aerial urea application, you should consider that urea must be watered in (by rainfall or irrigation) to prevent significant ammonia volatilization; a urease inhibitor can help reduce ammonia loss, to some extent. The economics of aerial N applications should be considered before going this route.
- Can you fertigate?
If you are set up to fertigate and you can get your pivot around without causing compaction or getting stuck, we recommend putting out a modest amount of N now, then take tissue test later in the season and, if needed, add more N through the irrigation system later in the season.
- Will an additional N application cause you to add more than was in your nutrient management plan?
In Delaware, you should contact your consultant to have them write a justification for additional fertilizer applications. A copy of the addendum should be included with the original nutrient management plan before the end of the calendar year and will be essential if the farm is audited. The Delaware Department of Agriculture can be contacted for guidance and clarification. Consultants and growers from other states should contact State regulatory agencies for guidance. Click here for Delaware Department of Agriculture contact information.