BACTERIA OF THE SOIL 269 



organisms are known which oxidize the reduced iron to the 

 ferric condition, and other organisms which, under ana- 

 erobic conditions in the presence of organic matter will 

 reduce ferric iron to ferrous iron. The latter are of some 

 practical significance as the ferrous iron compounds may 

 be somewhat poisonous to plant roots. 



Bacteria oxidizing ferrous to ferric iron are of consider- 

 able importance in the deposition of iron ore. It is prob- 

 able that most of the great iron ore beds, consisting of iron 

 oxide and iron carbonate, have been formed through the 

 action of microorganisms. Bacteria belonging to this group 

 for the most part are forms which oxidize the soluble salts 

 of ferrous iron to an insoluble ferric condition and deposit 

 them in the sheaths of the filaments. In the city water sup- 

 plies they may accumulate in quantities such as to form 

 reddish or yellowish deposits in the pipes, the so-called 

 deposit of iron rust. 



