This process is called nitrification. Common sources of ammonium in the soil result from decaying plants and organic matter, or ammonium can come from the application of manure or nitrogen fertilizers. The nitrification process.
Nitrification depends on microorganisms.
Warm, moist soils with good oxygen supply provide favorable conditions for nitrification. Practically speaking, nitrification is very active during the spring and summer months, slows in the fall, and is essentially nonexistent during the winter. The nitrification process can have a direct impact on water quality. When rainfall is heavy, nitrates can move downward in the soil. If the depth to ground water is shallow and the underlying soil is sandy, the potential for nitrates to enter ground water is relatively high.