Applying anhydrous ammonia: Why we wait for soils to cool to 50° F at the 4-inch depth.
With the good weather we've experienced throughout most of the Midwest, growers have been completing a lot of tillage and many have applied anhydrous ammonia. In retrospect, with the warm weather we have recently experienced throughout most of the Midwest, we could question whether the nitrogen application, even if done with a nitrification inhibitor, was the best decision. We are all aware of the problems with the loss of anhydrous in soils that are too wet or too dry to seal properly at the time of application, and we also know that N losses can be significant now that many soils are above the critical 50° F point. Exactly how much nitrogen we are losing after the application is difficult to estimate. However, there are four factors each grower can consider to determine if more N was lost than expected.
1.) Number of days soils remained above 50° F. (Although the nitrification process probably continues, in a slowed down way, to temps of about 40° F.)
2.) Soil moisture at the time of application. (Too wet or too dry accelerates initial losses.)
3.) Soil bulk density and tilth. (Sandy soils and soils with fractured structures tend to have accelerated nitrification and nitrogen losses.)
4.) Whether a nitrification inhibitor was used. (Inhibitors slow down nitrification but do not stop it. Inhibitors are broken down too over time and lose their effectiveness as soil temperatures warm up.)
In addition, the rate of N loss from an anhydrous application is somewhat affected by organic matter levels, soil pH, and precipitation that may leach nitrates out of the soil.
What we might expect from warm weather this fall is that crop residue that has been incorporated via tillage might have accelerated breakdown due to warm and moist soils this fall....perhaps giving us a bit of an advantage to the emergence of corn next spring.