Soil temperature can significantly impact the efficiency of fall nitrogen applications. Nitrifying bacteria are active until soils freeze (32° F), but their activity is greatly reduced once soil temperatures drop below 50° F. For this reason, Fernandez says the start of fall N applications should be directed by soil temperature and not by date of year. This guideline applies equally for anhydrous ammonia, ammonium sulfate and manure/organic fertilizers that can be used in the fall.

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, he says.

"In most years, the 50° F temperature allows for N applications before soils become too wet or frozen," Fernandez says. "There is no need to increase the risk of N loss by starting applications too early. Also, applying once temperatures are 50° F does not automatically ensure no N 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 approaching 50° F, historically the chance that they will continue to decline without a significant bounceback to warmer levels is 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 decrease the first week of November in central and northern Illinois.

Up-to-date soil temperatures can be accessed online.