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Applying Fall Nitrogen: Should I or Shouldn’t I?

by Nov 19, 2018Agronomically Speaking

This fall has been a challenge for many producers across the U.S. pertaining to the amount of rain that you may have had. Now that harvest is finished or is near completion, fall tillage and fertility is next on many of your lists. Corn production requires nitrogen (N) fertilization to supplement what is available from the soil. After N, phosphorous (P) is the nutrient that is likely to be deficient for effective corn production.

Nitrogen Use Efficiency (NUE) is very important when trying to decide whether to apply N in the fall or spring. Placement and timing of N affect the NUE and management decisions within your production system. As a reminder weather, soil type, and timing of crop N uptake are important to N loss potential. Applying N in the fall is thought of as the riskiest N management practice. The reason being is there is a high loss potential from leaching, volatilization, and denitrification compared with later N application. The time between fall N application and when N is needed by the corn crop in the spring is a large enough time gap where the factors mentioned previously can take place.
The goal should be to apply enough N to corn when the crop needs it without supplying too much that can be potentially lost. If N is lost it not only reduces your profits it can also harm the environment.

N is needed in large quantities by the corn plant because N is an essential component of all protein within the corn plant. When N is deficient, normal growth and development are hindered. In fact, when corn is stressed by a lack of N at any time during the plant’s growth and development the yield is reduced similar to any other plant stress. The following chart shows the approximate amount N removed in corn grain and stover which represents the minimum amount of N required to grow your crop.

When applying N in the fall, the rule of thumb is, not to apply fall N until the ground temperatures remain below 50°F from the time of application to spring (the colder the better). Cool soil temperatures help to reduce the activity of nitrifying soil bacteria that convert ammonium to nitrate forms of N. If applying N in the fall this practice should be managed carefully.

Fall N management tips include:

  • Only Ammonium sources of N should be used such as Anhydrous ammonia
    • Other N sources such as Urea and UAN nitrify too quickly which increases the chance for loss
  • Nitrification inhibitor should be considered to further slow conversion to nitrate
    • Examples of Nitrification inhibitors include:  N-Serve®, and Instinct
  • Avoid soils that are more prone leaching
    • Sandy Soils
    • Poorly drained soils