302 Bacteria in Relation to Country Life 



injurious to vegetation when present in any but small 

 proportions in the soil. In the presence of lime, the iron- 

 bacteria are capable of converting the copperas into the 

 harmless oxide of iron and gypsum. 



Iron in the soil and decay bacteria. Apart from the 

 iron bacteria proper, the fortunes of iron in the soil are 

 affected by the great host of decay bacteria. The soil- 

 humus and the more recent plant residues contain more 

 or less iron. When these substances are broken down 

 by the soil bacteria, the iron may be changed to a fer- 

 rous carbonate which, in the presence of carbon dioxid, 

 becomes soluble and is diffused in the soil-water. Com- 

 ing in contact with sulfuretted hydrogen, formed in 

 the decay of protein substances, the iron may be changed 

 ultimately to copperas, and, finally, to iron oxide and 

 gypsum. 



It has been demonstrated in the study of pure cul- 

 tures of common decay bacteria, that such reactions 

 play an important part in the formation of black sand 

 in seas, bays, and lakes. The iron bacteria and the 

 decay bacteria, widely distributed on land and sea, 

 may, therefore, be regarded as geological agents of some 

 moment, as well as direct factors in providing to crops 

 the iron compounds essential to their growth. 



Influence of iron on bacteria. In so far as the in- 

 fluence of iron on the bacteria themselves is concerned, 

 our knowledge is meager. It is now known that salts of 

 iron exert a stimulating effect on the development of 

 the nitrogen-fixing bacteria of the azotobacter group, 

 and probably also on other soil-organisms, as shown by 

 applications of ferrous sulfate. 



