Experiments and analyses were conducted to define conditions under which microbial activity in the soil, leading to corrosion of iron, will take place. The basic elements whose presence is necessary for microbial activity of any kind are listed. Research in this field has not yet yielded complete and precise information on the corrosion processes induced by microorganisms. A schematic explanation of the mechanism of metal salts formation is given. A number of microorganisms that cause corrosion are named and their metabolism explained. -- AATA