The shift from bacterial to fungal dominance, for instance, can enhance the conditions favoring weed invasions on rangelands. Bacteria are particularly important in nitrogen cycling. Free-living bacteria fix atmospheric nitrogen, adding it to the soil nitrogen pool. Other nitrogen-fixing bacteria form associations with the roots of leguminous plants such as lupine, clover, alfalfa, and milkvetches. Some soil nitrogen is unusable by plants until bacteria convert it to forms that can be easily assimilated.
Some bacteria exude a sticky substance that helps bind soil particles into small aggregates.
So despite their small size, they help improve water infiltration, water- holding capacity, soil stability, and aeration. The key is in maintaining a healthy system so that the good guys can do their work. Bacteria are capable of filtering and degrading a large variety of human-made pollutants in the soil and groundwater so that they are no longer toxic. The list of materials they can detoxify includes herbicides, heavy metals, and petroleum products. Ingham, Elaine. 1998.