Nitrogen management this fall
To lengthen the period of bacterial inhibition, it is 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 N against loss and increase the amount of N present in the ammonium form the following spring. Just like 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 to enhancng ammonium recovery. However, the practice will overall ensure the greatest chance to protect your N investment and at the same time enhance environmental protection.
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 be dissolved and be 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 keep in mind is that ammonium sulfate is more acidifying than other N sources, so be sure to monitor soil pH. As a general rule, 5 pounds of lime is needed to neutralize 1 pound of N from ammonium sulfate, compared with 2 pounds of lime per pound of N from anhydrous ammonia.
Lastly, organic fertilizers derived from animals (manure, poultry litter, etc.) are good fertilizer sources that can be used in the fall. These products supply N as well as phosphorus, potassium, and other crop nutrients. Often these organic fertilizers represent a less expensive source of nutrients than inorganic fertilizers.
Timing N applications. In years like this, when harvest is done so early, it is critical to keep in mind that soil temperature can impact to a large extent the efficiency of fall N applications and the effectiveness of nitrification inhibitors. Nitrifying bacteria are active till soils freeze (32°F), but their activity is greatly reduced once soil temperature goes below 50°F. For this reason, it is recommended that the start of fall N applications be directed by soil temperature and not by calendar date, harvest date, or any other consideration. The temperature guideline applies equally for anhydrous ammonia, ammonium sulfate, and manure/organic fertilizers that can be used in the fall. Because the efficiency of nitrification inhibitors also decreases with warm temperatures, higher temperatures result in faster breakdown of the molecule responsible for inhibition of nitrifying bacteria. The cooler the temperature, the greater the efficiency of the inhibitor and the greater the chance that ammonium does not convert to nitrate.
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