Nitrification describes the process by which soil nitrogen (e.g. ammonia NH3) is converted into nitrate (NO3-) by microorganisms. During this conversion, nitrogen from dead material is recycled and made available to plants again.
Explanation for children:
Nitrification involves the conversion of nitrogen from one form to another by microorganisms. For example, plants need nitrogen so they can grow a lot, but they only like it in a very specific form. Imagine the nitrogen in the fertilizer is circular. Although there is also a lot of nitrogen in the air, the nitrogen in the air is rectangular. The plants need the circular nitrogen, so they cannot eat air. Fortunately, there are organisms that convert nitrogen from rectangular to circular form. The process is called nitrification because the atmospheric nitrogen (rectangular) is turned into nitrate (circular).