242 



Transformation of Mineral Substances 



Iron interacts with the humus compounds of the soil to give rise 

 to iron humates. It is more readily available in this form to plants 

 growing in alkali soils when it is not precipitated out as inorganic 

 phosphate. 



logC, 

 4.0 



5.0 



6.0 



4.0 



3.0 

 Solution of minerals increase- 



2.0iOgCK,Ca 



Fig. 97. Influence of acidity created by growth of Azotobacter on solution of 

 calcium, magnesium, and potassium from the mineral biotite (from Wright). 



The Role of Microorganisms in the Transformation of 

 Rare or Trace Elements 



Molybdenum, copper, zinc, cobalt, boron, and certain other ele- 

 ments act primarily as catalyzers for the activities of different soil 

 organisms. Molybdenum is essential for the fixation of nitrogen by 

 Azotobacter, a fact utilized for determining the concentration of 

 available molybdenum in the soil. Boron is highly essential to the 

 growth of legume bacteria, though high concentrations are injuri- 

 ous. Copper is essential for a variety of microbial processes, but 

 high concentrations of this element, too, are injurious. Its concen- 

 tration in the soil can be determined by the use of certain fungi and 

 bacteria. Zinc and cobalt form essential constituents of certain en- 

 zyme and vitamin systems. Zinc also favors growths of fungi and 

 represses spore formation. 



