Assessing Nitrogen Management in Corn After this Challenging Season

Gordon Johnson, Extension Vegetable & Fruit Specialist, gcjohn@udel.edu; Amy Shober, Extension Nutrient Management and Environmental Quality Specialist, ashober@udel.edu; Jarrod Miller, Extension Agronomist; jarrod@udel.edu

Excessive rainfall that occurred early and late in the season made N management for corn (among other things) a challenge for many growers in our region. Early season rainfall resulted in significant N losses via leaching or denitrification, and in some cases delayed mineralization of manure N. We think it is safe to say that no one likely knew how much N was lost or how much additional N should have been applied. Throughout the season, we tried to provide some guidance to help growers and consultants answer those questions. While there is no way to turn back time and change the N management decisions that were made this season, we can take a look back to assess if the decisions to forego or apply additional in-season N were warranted.

The end-of-season corn stalk nitrate test (CSNT) makes use of the fact that corn plants either remove N from, or accumulate N in, the lower stalk based on soil N availability. Studies over a wide range of conditions have found remarkably similar relationships between the amount of N found in the lower stalks late in the growing season and the likelihood that corn has been under or over-fertilized. The test allows growers to identify N deficiencies or surpluses that may not be apparent upon visual inspection of the plant.

Collect stalk samples between ΒΌ milk line and 3 weeks after black layer from areas with uniform soil type and and management history, avoiding areas with severely damaged or diseased stalks. Purdue Extension provides a good review on how to identify corn reproductive stages. Cut an 8-inch segment of stalk starting 6 inches above the soil soil surface. Remove leaf sheaths from the stalk samples. Collect 15 stalk segments for every 10 acres of corn and submit them as a single sample. Store stalk samples in paper bags (not plastic) to allow for some drying and to minimize mold growth. Ship samples (within 24 hrs) for analysis or refrigerate until shipping is possible.

Factors that limit crop yield, like the unusually wet weather we experienced this year, can increase stalk nitrate concentrations. As such, we suggest taking a cautious approach to interpreting the results of the CSNT for this season. However, we believe that growers can still learn a lot about how well they fared with N management based on the results. In general, stalk samples with less than 700 ppm nitrate indicate the crop was under-fertilized with N, while samples with more than 2000 ppm nitrate indicate that the crop was likely over-fertilized with N.

The CSNT does not directly indicate how much N rates should be increased or decreased for a given stalk nitrate concentration. However, use of the CSNT for several years will allow corn producers to identify N management practices, including rates, forms, and times of application, that tend to result in optimum amounts of plant-available N. After appropriate consideration of weather and other factors, growers should consider making adjustments to N fertilizer and/or manure rates based on results of the CSNT over multiple years. In addition, the CSNT can also help growers make in-season N management decisions when faced with excessively wet conditions in future years.

For additional information:
Hansen, D.J., G. Binford, and J.T. Sims. 2014. End-of-season corn stalk nitrate testing to optimize nitrogen management. University of Delaware. Newark, DE.