Fall N application: What, when, where and how
This fall, just as in the fall of 2010 to a certain extent, harvest is happening earlier than is typical. In years like this, it is critical to keep in mind that soil temperature can significantly impact the efficiency of fall nitrogen applications and the effectiveness of nitrification inhibitors. Nitrifying bacteria are active until soils freeze (32°F), but their activity is greatly reduced once soil temperatures go below 50°F. For this reason, it is recommended that the start of fall nitrogen applications be directed by soil temperature and not by date, when harvest is complete, 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. As I mentioned earlier, 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.
While I realize that every year anxiety levels rise when soil temperatures are not getting down to 50°F and falling steadily, I would remind readers that in most years, the 50°F temperature allows for nitrogen applications before soils become too wet or frozen. There is no need to increase the risk of nitrogen loss by starting applications too early. Also, applying once temperatures are 50°F does not automatically ensure no nitrogen loss, though it does provide a better chance to protect your investment. Air temperatures in Illinois can vary substantially during the early fall. Even if temperatures are getting to 50°F, historically the chance that they will continue to decline without a significant bounceback to warmer levels are very rare before the second week of October in northern Illinois and the third week in central Illinois. On average, soil temperatures reach 50°F and continue to go down the first week of November in central and northern Illinois. Daily maximum 4-inch bare-soil temperatures for Illinois this week have been bouncing between the 60s and the 70s.
Up-to-date soil temperatures can be accessed on the web. However, these values should be used as a reference. Since soil temperatures can be influenced by a number of factors (such as residue cover, soil color, and drainage), it is always best to monitor temperatures in individual fields prior to nitrogen application.