92 AGRICULTURAL BACTERIOLOGY 



may be stored in their cells to be further oxidized to sul- 

 phuric acid, which will combine with bases to form sul- 

 phates, producing compounds in which the sulphur is avail- 

 able to the green plant, Such bacteria are found in the 

 soil, and in great abundance in sulphur springs. Their 

 action results in the formation of a strong mineral acid, 

 which exerts a solvent action on many of the soil minerals. 

 Sulphur is lost in the drainage water in the form of sul- 

 phates. 



The reduction of sulphates by bacteria is a common pro- 

 cess when sea water is mixed with water carrying organic 

 matter. Such conditions occur when sewage is discharged 

 into tidal rivers. The dissolved oxygen of the water is 

 soon exhausted by the bacteria feeding on the organic 

 matter. The facultative bacteria must utilize some other 

 source of oxygen, and find a supply in the sulphates, which 

 are thus reduced to sulphides. This process may be so 

 pronounced as to create a nuisance in cities located on the 

 sea. 



Iron. Iron is often present in ground water as ferrous 

 carbonate. Certain bacteria known as the iron bacteria 

 facilitate the precipitation of the iron from the water. 

 The insoluble iron accumulates in the sheaths of the bac- 

 teria, among which many of the higher bacteria are to be 

 noted. Some of the iron bacteria have peculiar forms, such 

 as flat, twisted ribbons. 



It is believed that they have been the causal agents in 

 the deposition of many of the great iron-ore deposits such 

 as are found in northern Minnesota. They often cause 

 the accumulation of a deposit on the inside of water- 

 mains through which an iron-bearing water passes. They 

 also may cause the plugging of drain-tile in marshy land. 

 The acids produced from the decomposition of organic 

 matter dissolve the iron, which is carried by the percolat- 



