Assimilation of atmospheric nitrogen by a variety of microorganisms which live freely in soil. Once the nitrogen has been captured by one of the microorganisms, there are many different routes by which it is handled. Some is retained in the soil as decomposing plant matter, waiting to be released and taken up by new crops as a nitrate. That nitrate is produced by nitrifying bacteria living in the soil that thrive on ammonia, which is produced by decaying plant and animal material. In processing nitrogen the nitrifying bacteria produce nitrate that can be absorbed by the roots of plants.