Fall N application: What, when, where and how
To lengthen the period of bacterial inhibition, it is always a good idea to include a nitrification inhibitor with the application of anhydrous ammonia. Many years of research have indicated that nitrification inhibitors, such as dicyandiamide (DCD) and N-serve, can protect fall nitrogen against loss and increase the amount of nitrogen present in the ammonium form the following spring.
As with most practices, the use of a nitrification inhibitor might not pay every year. For example, if the following spring is dry and cool, the inhibitor might not be as beneficial in enhancing ammonium recovery. However, as I mentioned earlier, this practice will overall ensure the greatest chance to both protect your nitrogen investment and enhance environmental protection.
From the standpoint of production profit, in times when nitrogen prices were low, it was cheaper to buy additional nitrogen rather than the nitrification inhibitor. As nitrogen prices increase and the cost of nitrification inhibitors remains relatively constant, using an inhibitor is becoming more profitable. Still, while an inhibitor represents an added cost, it is important to realize that a reduction in nitrogen efficiency due to losses plus the environmental degradation linked to nitrogen loss also represent added costs. Farmers must carefully consider all these factors when deciding to apply nitrogen in the fall.
Ammonium sulfate is an excellent nitrogen source for no-till fields where broadcast applications are preferred. It is always best to apply it before soils freeze so the fertilizer can be dissolved and incorporated into the soil by rain. In fields with minimal slope (less than 5%) and where the potential for runoff is very low, it is feasible to apply ammonium sulfate on frozen ground because there is no concern of volatilization loss. An important point to remember is that ammonium sulfate is more acidifying than other nitrogen sources, so make sure to keep an eye on soil pH. As a general rule, 5 pounds of lime is needed to neutralize 1 pound of nitrogen from ammonium sulfate compared with 2 pounds of lime needed per pound of nitrogen from anhydrous ammonia.
Last but not least, organic fertilizers derived from animals (manure, poultry litter, etc.) are good for use in the fall. These products supply nitrogen as well as phosphorus, potassium, and other crop nutrients. Often these organic fertilizers represent a less expensive source of nutrients compared with inorganic fertilizers.
When to Apply