Fall N application: What, when, where and how
It is once again harvest season! Although combines recently started to roll, it will not be long before most Illinois fields will be cleared of crops, and farmers will start operations to prepare for the next growing season.
Nitrogen application is one of the many decisions to be made at this time of year, and it is an important one. Proper nitrogen management is critical to sustainable corn production, and what is done in this regard can impact farmers' profitability and the environment in which we all live. For these reasons it would be wise to review important guidelines developed through years of research and experience.
I acknowledge that a given recommended practice may not work very well every year--mostly because of environmental conditions beyond our control--but these guidelines, if followed, will ensure the best chance for protecting your nitrogen investment and at the same time enhancing environmental protection.
What to Apply
While many inorganic nitrogen sources are available in the marketplace, for fall application the only recommended sources are anhydrous ammonia (NH3) and ammonium sulfate ([NH4]2SO4). Ammonia transforms quickly to ammonium (NH4+), and nitrogen in ammonium sulfate is already in the ammonium form. Ammonium is adsorbed onto the exchange sites in soil particles and organic matter, and thus it is protected from leaching. By contrast, nitrogen sources containing nitrate (NO3-) should not be used in the fall because nitrate does not become adsorbed onto exchange sites in the soil and can be easily leached or denitrified long before corn plants are ready to use it. Common fertilizers that contain nitrate include ammonium nitrate (NH4NO3) and urea ammonium nitrate (UAN).
Another common nitrogen source is urea (CO[NH2]2). Urea converts to NH3 and then to NH4+ within a few days of application. However, research results indicate that this fertilizer should not be used in the fall because it has a greater risk of loss compared with anhydrous ammonia before rapid nutrient uptake by the crop the following spring. The same can be said of polymer-coated ureas. While the coating protects urea for a while, often urea starts to diffuse out of the granule too early, and the loss potential is higher than for anhydrous ammonia.
One of the benefits of anhydrous ammonia is that it kills nitrifying bacteria (which are responsible for the transformation of ammonium to nitrate) at the point of application. In addition, as ammonia reacts with water to form ammonium, the reaction creates an alkaline (high pH) environment in the ammonia retention zone. This high pH also inhibits activity of nitrifying bacteria for a while, but the effects are temporary.