Many cleared fields in Illinois are ready for fall operations due to the very early harvest and the chopping, plowing-under, or harvesting for biomass that took place earlier in the summer in fields that did not produce grain.
University of Illinois assistant professor of crop sciences Fabián Fernández says decisions on nitrogen applications this fall rank high in the list of producers' priorities because the application can affect profitability and the environment.
Fernández offers some guidelines for nitrogen application. "Although recommended management practice may not work very well every year because of environmental conditions beyond our control, I emphasize that these guidelines, if followed, will ensure the greatest chance to protect your nitrogen investment and at the same time enhance environmental protection," he adds.
Anhydrous ammonia (NH3) and ammonium sulfate ([NH4]2SO4) are the only sources recommended for fall application. Ammonia transforms quickly to ammonium (NH4+), and nitrogen in ammonium sulfate is already in the ammonium form. Ammonium is adsorbed into the exchange sites in soil particles and organic matter, protecting it from leaching.
Nitrogen sources containing nitrate (NO3-) should not be used in the fall. Nitrate does not become adsorbed into exchange sites in the soil and can be easily leached or denitrified long before the 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), which 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, compared to anhydrous ammonia, it is more likely to be lost before rapid nutrient uptake by the crop the following spring.
The same can be said of polymer-coated ureas. While the coating initially protects urea, it can diffuse out of the granule too early and the loss potential is higher than for anhydrous ammonia. This is especially true when it is applied too early in the fall or when the coating is damaged during handling, allowing for quicker dissolution of the urea granule.
One of the benefits of anhydrous ammonia is that it kills the nitrifying bacteria that transform ammonium to nitrate at the application point. Moreover, the reaction of ammonia with water to form ammonium creates an alkaline (high pH) environment within the ammonia retention zone, which inhibits activity of nitrifying bacteria.
However, these effects are temporary. Including a nitrification inhibitor with anhydrous ammonia applications can lengthen the period of bacterial inhibition. Research indicates that nitrification inhibitors, such as dicyandiamide (DCD) and N-serve, can protect fall nitrogen against loss and increase the amount of nitrogen present in ammonium form the following spring.
"As with most practices, using a nitrification inhibitor might not pay every year," Fernández says. "If the following spring is dry and cool, the inhibitor might not be as effective at enhancing ammonium recovery. However, this practice will, overall, offer the best chance to protect your nitrogen investment and the environment."
Ammonium sulfate is an excellent source for no-till fields where broadcast applications are preferred. It is always best to apply it before soils freeze so the fertilizer can dissolve and rain can incorporate it into the soil. In fields with minimal slope (less than 5 percent) with low run-off potential, ammonium sulfate can be applied on frozen ground because volatilization losses are unlikely.
"Ammonium sulfate is more acidifying than other nitrogen sources, so make sure to keep an eye on soil pH," Fernández advises. "As a general rule, 5 pounds of lime is needed to neutralize 1 pound of nitrogen from ammonium sulfate compared to 2 pounds of lime per pound of nitrogen from anhydrous ammonia."
Organic fertilizers derived from animals (manure, poultry litter) are good fertilizer sources that can be used in the fall. These products supply nitrogen as well as phosphorus, potassium, and other crop nutrients, and are often less expensive than inorganic fertilizers.