Nitrogen management this fall
An early harvest combined with fields that were chopped, plowed under, or harvested for biomass earlier in the summer because they did not produce grain is resulting in many fields cleared and ready for fall operations. Decisions about applying nitrogen this fall rank high in producers' priorities because the application can affect profitability and the environment. Fabián Fernández, University of Illinois, reviews important guidelines developed through years of research and experience. He acknowledges that any given recommended management practice may not work very well every year, usually because of environmental conditions beyond our control, but he also emphasizes that following these guidelines will ensure the greatest chance to both protect your N investment and enhance environmental protection.
Nitrogen sources. For fall application, the only recommended N sources are anhydrous ammonia (NH3) and ammonium sulfate ([NH4]2SO4). Ammonia transforms quickly to ammonium (NH4+), and N in ammonium sulfate is already in the ammonium form. Ammonium is adsorbed onto the exchange sites in soil particles and organic matter and thus is protected from leaching. On the other hand, N 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 easily be 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 N source is urea (CO[NH2]2). Urea converts to NH3 and then to NH4+ within a few days of application. However, research has demonstrated 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 polymer 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. This is especially true when the application is done too early in the fall or in cases where inappropriate handling of the fertilizer damaged the coating, allowing for quicker dissolution of the urea granule.
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 within the ammonia retention zone. This high pH also inhibits activity of nitrifying bacteria for a while. However, these effects are temporary.