Amy Shober, Extension Specialist – Nutrient Management and Environmental Quality, ashober@udel.edu; Jarrod O. Miller, Extension Agronomist, jarrod@udel.edu; Phillip Sylvester, Extension Agent – Agriculture, Kent County, philllip@udel.edu; Cory Whaley, Extension Agent – Agriculture, Sussex County, whaley@udel.edu
Are you planning to plant small grains following corn this fall? If so, you might want to consider conducting a fall soil nitrate test (FSNT) to help guide fall nitrogen (N) management for wheat and barley. This quick test was developed by researchers at the University of Maryland to determine if there is enough N remaining in the soil to support adequate fall tillering of wheat and barley. We believe this test could be very useful this year, especially if you decided to apply additional N to corn due to the challenging weather conditions.
Collecting the Sample: Collect 15-20 soil cores per field or management units to a depth of 8 inches, avoiding areas that are not representative of field conditions (including, but not limited to, sidedress or starter bands, wet spots, areas with significantly different soil types, etc.). Mix soil cores thoroughly. Remove approximately 1 cup of soil from the bucket and spread it in a thin layer on paper to dry overnight. Submit the dried soil samples to an approved soil testing laboratory for nitrate (NO3-N) analysis. If necessary, lightly crush large soil pieces to make sure the lab gets a representative sample. If you are unable to dry the sample, it should be kept cool and delivered to the lab as soon as possible.
Interpreting the Results: The University of Maryland provides the following guidance for fall N fertilization based on soil test results. For wheat, no fall N application is recommended if the FSNT ≥11 parts per million NO3-N. Up to 30 lb/ac of fall N is recommended for wheat if soil NO3-N concentrations are <11 parts per million. For barley, no fall N application is recommended if the FSNT ≥8 parts per million NO3-N. Up to 30 lb/ac of fall N is recommended for barley if soil NO3-N concentrations are <8 parts per million.
University of Maryland researchers based these interpretations on profitable yield responses to N application of 30 lb/ac in field trials conducted between 2011 and 2014 at various soil NO3-N concentrations. Fall N applications of 30 lb/ac to wheat resulted in average increase in yield of 2.3 bu/ac when compared to plots receiving no N. Over 21 site-years, this fall application of N resulted in a yield increase 67% of the time when soil NO3-N concentrations were ≤10 ppm and 52% of the time when soil NO3-N concentrations were >10 ppm. However, probability of profitable response to 30 lb/ac decreased rapidly when fertilizer prices increased, small grain prices decreased, and soil NO3-N were >10 ppm. For example, University of Maryland researchers reported that a 5 bu/ac increase in yield was needed to provide a profitable outcome for application of 30 lb/ac fall N when wheat price was $5/bu, N cost was $0.60/lb, and application cost was $7/acre. During the Maryland field trials, the researchers reported that a 5 bu/ac yield increase due to fall N application (30 lb/ac) to soils occurred only 15% of the time when FSNT >10 ppm.
As such, we believe that the FSNT can be a useful tool to guide N applications to small grains this year. If you applied more N than needed, it is possible that residual N will be available to meet the early growth needs of your small grain. In this case, you may be able to skip the N applications and save some money without affecting yield. In contrast, if all of the N you applied to corn was taken up by the crop or lost via leaching or denitrification, then low soil nitrate concentrations will reveal the need for fall N applications to your small grains. If you do decide to apply fall N, you can often skip the ammonium sulfate. Sulfur needs are low for small grains in the fall. Instead use cheaper sources of N (e.g., urea, 30% UAN), which will improve your chances for a profitable response. If your soil test reveals the need for phosphorus, both MAP and DAP are options that contain N and P.