CHANGES IN INORGANIC CONSTITUENTS. 29 1 



one in that the microorganisms oxidize ferrous to ferric compounds, and 

 utilize for their growth the energy thus made available. The investi- 

 gations of Molisch, Adler and Ellis show, however, that the iron bacteria 

 can exist very well without iron compounds; and that the precipitation 

 of iron oxide is due to mechanical rather than chemical influences. But 

 whether physiological or mechanical the influence of these microorganisms 

 is felt in the formation of bog iron, and in the filling up of iron pipes; 

 in the latter instance much annoyance is occasionally experienced by 

 those in charge of municipal water supplies. 



Compounds of iron are of considerable significance in the life pro- 

 cesses of many bacterial species. For instance, it was shown by the Lipman 

 and after him by Koch, that Azotobader will not develop in culture 

 media devoid of iron compounds. In field practice small applications 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 discussed 

 in connection with calcium and potassium compounds. To a great 

 extent aluminum is affected by these reactions, for in the decomposition 

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

 bacteria become a factor of considerable importance in the formation 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 compounds 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 spraying 

 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 algse in lakes, ponds, and shallow 

 treams. 



