362 MICROBIOLOGY OF SOIL 



Compounds of iron are of considerable significance in the life 

 processes of many bacterial species. For instance, it was shown by 

 Lipman and after him by Koch, that Azotobactermll not develop in cul- 

 ture media devoid of iron compounds. In field practice smaU applica- 

 tions of ferrous sulphate often seem to exert a favorable effect on crop 

 growth, and there is reason to suspect that soil-microbial activities are 

 of some moment in bringing about the results noted. 



Aluminum, Manganese, Copper.— Weathering processes and the 

 relation of carbon dioxide to these processes have already been dis- 

 cussed in connection with calcium and potassium compounds. To a 

 great extent aluminum is affected by these reactions, for in the decompo- 

 sition of feldspar, kaolinite is one of the important products formed. 

 Hence, bacteria become a factor of considerable importance in the forma- 

 tion of hydrated silicates of aluminum, at least, in the presence of 

 organic matter. Moreover, it is recognized in the ceramic industries 

 that after it is dug clay must undergo ripening in order to be suitable 

 for certain purposes. The ripening process involves the activities of 

 bacteria. Unfortunately very little is known about the reactions that 

 occur in the ripening of clay. 



As to manganese and copper there is scarcely any experimental evi- 

 dence available as to the part played by their compoimds in the soil, 

 particularly in so far as they affect microorganic life. To some extent, 

 it is known that where Bordeaux mixture has been employed for spray- 

 ing potatoes, cranberries, fruit trees, etc., plant growth is Subsequently 

 stimulated to a striking extent. In view of the very slight quantities of 

 copper that are actually added to the soil by these sprays, it is possible 

 that the effects noted are caused by stimulated or changed microbial 

 activities. This view finds some support in the influence exerted by 

 copper sulphate on the growth of algae in lakes, ponds, and shallow 

 streams. 



It has also been reported that the decomposition of complex sihcates 

 has been effected from powdered minerals by nitrite bacteria. 



Antagonism 



A subject which bids fair to become a fertile source of investigation 

 is the application of certain biochemical laws, as established by Loeb 

 and Osterhout in the animal and plant worlds respectively, to the 



