The Science Behind 50-degree Soil and Nitrogen Application
Every year we hear a chorus of reminders to wait until soil temperatures at the 4-inch depth are 50°F and trending cooler before applying anhydrous ammonia, and those of us in the manure world tend to echo these comments. That is if you are applying an ammonia-rich manure, liquid/slurry hog manure wait until soils start to cool before applying. So, what is the science behind this recommendation, especially for manures?
This recommendation is based on the potential for nitrogen loss. Remember, there are a few forms of nitrogen that can be applied or are found in soils. These include ammonia/ammonium, nitrate, and organic nitrogen. Of these forms, all forms can be lost, but ammonia and nitrate tend to be the most mobile.
When it comes to manures, it’s pretty much nitrate-free when we apply it, but microbes in the soil will process ammonium nitrogen and turn it to nitrate. The activity level of these microbes is controlled by how much ammonia is present, the amount of water and oxygen in the soil, and the soil temperature. Although all these variables are important, for now let’s focus just on temperature. A good rule of thumb is that microbial activity will double for every 18°F increase in temperature (so if our soil is at 68°F those microbes will be turning the ammonia in the manure into nitrate at about 2x the rate they would if our soil was at 50°F). Often this means that not only will the microbes have more time to cause the conversion to nitrate, but they might be doing it much faster than if fertilizer application had waited.