This is an unstable compound, and readily lets go of
its contained ammonia, which produces the characteristic sharp smell of manure.
What science is working to develop (and what gardeners would introduce into the soil artificially if they were available) are large colonies of Nitrosomonas and Nitrococcus bacteria, which convert the ammonium products into nitrites. To carry on the process, these bacteria would then be supplemented by even larger numbers of Nitrobacter-the forms that convert nitrites into nitrates.
Temperature affects the nitrification process. Most of the bacteria involved do their most effective work in soil temperatures from about 70 to 85 degrees F. At temperatures below 50, they are quite inactive. Excessively dry or wet soil conditions also interfere with their effectiveness.
As always, these bacteria require energy foods to keep them alive -the starches and sugars that they can get only from decaying organic matter.
Unfortunately, the nitrification process is reversable, so if conditions are not right we have denitrification-nitrates produced as the end product of this long chain of organic breakdown revert to nitrites and ammonia. Several microorganisms exist which will do this. They are favored by anaerobic conditions (lack of oxygen) and by presence of liberal amounts of fresh, decomposing organic matter. This is one reason for maintaining two or more compost piles instead of continually adding fresh material to one old pile.
The role of fungi in the soil has not been as thoroughly studied as has the role of bacteria. Fungi, however, must also find their energy foods in organic matter. In breaking down organic material, fungi often do a better job of dissolving cellulose than do bacteria. They can often work where bacteria cannot. For example, fungi often invade a duff or mull on the surface of soil and begin working on the material before it can sift down far enough for bacteria to attack. Fungi need less soil moisture to survive and so continue to work after drought has checked bacterial decay action.
Fungi survive and remain active at pH readings much lower than those tolerated by bacteria. No fungus, however, with the possible exception of mycorrhizae, can fix nitrogen.
The exact role of antibiotics in soil is uncertain. Whether these substances (which are produced by a number of different fungi) are helpful or harmful to higher plants has not been fully explored, although at least one such product is used to control fire blight in pears and apples. We do know that these antibiotic substances are plant antagonists, not affecting the fungus that produces them but inhibiting or poisoning other fungi and bacteria.