666 PRINCIPLES OF SOIL MICROBIOLOGY 



Transformation of zinc. Certain microorganisms are capable of 

 oxidizing zinc sulfide (blende) to zinc sulfate. The transformation is 

 favored by the presence of sulfur. 61 Sulfur oxidizing bacteria can 

 bring about the dissolution of natural silicates and carbonates of zinc 

 by the sulfuric acid formed from the oxidation of the sulfur. Traces 

 of zinc will greatly increase the growth of various organisms, especially 

 fungi. 62 



Transformation of iron. A detailed discussion of the transformation 

 of iron by specific bacteria is found elsewhere. Most of the so-called iron 

 bacteria seem to be able to exist heterotrophically or without iron as a 

 source of energy. 63 The precipitation of iron salts seems to be merely 

 a physiological phenomenon accompanying the activities of various 

 bacteria growing in media rich in iron salts. Iron hydroxide and basic 

 ferric salts are precipitated when organic iron compounds and inorganic 

 salts of iron are added to the soil. The precipitation occurs also without 

 the interaction of microorganisms but is much less. In addition to 

 bacteria, some algae, fungi and protozoa are also capable of bringing 

 about precipitation of iron in the form of ferric hydroxide, basic ferric 

 salts, and ferrous sulfide. Indirectly, microorganisms may be of im- 

 portance in depositing ferrous carbonate and ferrous silicate. 64 Micro- 

 organisms also play an important part in the transformation of the 

 insoluble iron compounds into soluble forms by means of the carbon 

 dioxide as well as organic acids formed in the metabolism of the or- 

 ganisms. Small quantities of iron added to nutrient media have a 

 catalytic effect upon the activities of various microorganisms. This 

 is especially true of various autotrophic processes, such as oxidation of 

 hydrogen and sulfur. The precipitation of small amounts of iron as 

 a result of sterilization of an alkaline medium containing phosphate 

 has been shown to stop the development of various organisms, such as 

 the hydrogen bacteria. 



Transformation of aluminum in the soil. Aluminum forms one of 

 the most abundant elements in the soil, especially in clay soils, where it 

 occurs in combination with silicates and with organic substances. It 



61 Rudolfs, W., and Hellbronner, A. Oxidation of zinc sulfide by micro- 

 organisms. Soil Sci., 14: 459^64. 1922; also Compt. Rend. Acad. Sci., 174: 

 1378-1380. 1922. 



62 Steinberg, R. A. A study of some factors in the chemical stimulation of 

 the growth of Aspergillus niger. Amer. Jour. Bot., 6: 330-372. 1919; But- 

 kewitsch, W., and Orlow, W. G. Zur Frage nach den "Okonomischen Koeffi- 

 zienten" bei Aspergillus niger. Biochem. Ztschr., 132: 556-565. 1922. 



63 Some exceptions are noted by Lieske, 1911 (p. 95). 



64 A detailed study of these processes is given by Harder, 1919 (p. 93). 



