Nitrogen fixation is important to agriculture because atmospheric nitrogen is unavailable in that form for use by producers. Instead, it must be "fixed" by nitrogen-- fixing bacteria living in the soil and in nodules of leguminous plants, into nitrates. This substance is readily absorbed by the root system of plants and is used to manufacture plant proteins. This nitrogen which is part of the plant proteins are consumed by a heterotrophic organism, and the nitrogen is then incorporated into animal protein. Once living things and their wastes decompose, nitrogen is converted to ammonia and once again into atmosperic nitrogen. The nitrogen cycle repeats and nitrogen is once again fixed into a form useful to plants.