Now that we are officially in spring, it is old news that this winter was unusually warm. Those who applied anhydrous ammonia last fall are now concerned that they might have lost some of their nitrogen (N). The question is how much?
Anyone with some experience working with N knows that there is no way to provide an answer applicable to every acre of agricultural land in the state. This is because N transformations and eventually N losses are dependent on many variables and complex interactions.
Some of these variables are soil temperature, timing of fall N application, use of a nitrification inhibitor, rate of biological activity, drainage, amount and frequency of rain, and soil type. Despite these factors, all of which create uncertainty in predicting how much nitrification has occurred or how much N has been lost in any given situation, I would like to offer some guiding principles for you to consider.
The first question to try to answer is how much of the applied N has been transformed to nitrate (NO3-). When anhydrous ammonia (NH3) is applied, it quickly reacts with soil water to convert to ammonium (NH4+). In the ammonium form, N is held by the soil and cannot be leached out of the root zone or denitrified. However, once ammonium transforms to nitrate, this form of N can be leached out with rainwater moving through the soil profile or denitrified when soils are warm and saturated with water.
Nitrification is a bacteria-mediated transformation. Nitrifying bacteria are most active in aerobic conditions (when soils are not saturated with water) and warm temperatures. Nitrification stops at 32°F and increases slowly as soil temperatures increase to about 50°F. Above 50°F, the activity of nitrifying bacteria increases quickly.
Monthly mean soil temperatures have been about 5°F warmer than average this fall and winter in Champaign (Table 1; similar weather and soil temperature information for other parts of Illinois can be accessed at www.isws.illinois.edu).
In Champaign, during January and February daily minimum soil temperatures at 4 inches below the soil surface ranged from 35.7 to 45°F. The fact that temperatures never reached 32°F would indicate that nitrifying bacteria have been slightly active most of the fall and winter.