Any of various soil bacteria that change ammonia or ammonium into nitrite or change nitrite into nitrate as part of the nitrogen cycle. Bacteria of the genus Nitrosomonas are the primary converters of ammonium into nitrite (which is actually toxic to plants), and bacteria of the genus Nitrobacter oxidize the nitrite to form nitrate ions (which are readily absorbed and usable by plants). Most of the nitrogen contained in fertilizer is made available to plants by these bacteria.
