After an early harvest and exceptionally dry fall, a Purdue University agronomist says it's important for farmers to pay close attention to soil temperature and moisture levels before applying anhydrous ammonia.

The rule of thumb is to apply anhydrous ammonia after soil temperatures at a depth of 4 inches fall below 50 degrees and are getting colder, said Jim Camberato.

"Low soil temperature hinders the bacterial conversion of ammonium nitrogen (NH₄) to nitrate nitrogen (NO₃)," he said. "Slowing this reaction is critical to the efficient use of anhydrous ammonia because ammonium nitrogen is retained in the soil, whereas nitrate nitrogen is easily lost through leaching to tile drains or denitrification to the air.

"The longer nitrogen remains in the ammonium nitrogen form in the fall, the lower the potential for nitrogen loss in the early spring when warm soil temperatures and excess soil moisture invariably occur."

Soil temperatures finally have fallen below 50 degrees in northern Indiana. But, while many farmers are eager to begin anhydrous ammonia applications following the early harvest, Camberato cautions that the extremely low soil moisture in most Indiana soils can also be a problem because moisture helps keep the fertilizer in the soil.

"Soil moisture helps the soil seal and also captures ammonia (NH₃), enabling its reaction with water to form ammonium nitrogen," Camberato said. "Currently, some soils may be too dry to seal properly and capture all the ammonia that will be applied - especially if the soils are loosened by tillage prior to anhydrous ammonia application.

"Some farmers have reported smelling ammonia a day or two after application, which is a good indicator that the soil did not seal well after application and that ammonia is being lost to the air."

The one positive of the low soil moisture is that it will slow the rate of conversion from ammonium nitrogen to nitrate nitrogen, even if soil temperatures become warm again.

With weather so unpredictable in the Eastern Cornbelt, Camberato said farmers should consider applying a nitrification inhibitor, nitrapyrin, with anhydrous ammonia to further delay the conversion of ammonium nitrogen to nitrate nitrogen. Nitrapyrin offers some insurance against fluctuating soil temperatures in fall and early winter.

The bottom line, Camberato said, is to make sure the soil is moist enough and in good enough physical condition to capture the anhydrous ammonia regardless of temperature and hope soils remain cold before they become wet.