668 PRINCIPLES OF SOIL MICROBIOLOGY 



(fig. 51). Although microorganisms require small amounts of min- 

 erals, they are capable of developing in media having considerable 

 concentrations of the various salts. 67 Investigators frequently differ- 

 entiate between fixed and free salts, in regard to their influence upon 

 bacteria. 68 The presence of various minerals in the soil is essential 

 not only to the nutrition of microorganisms, but also for the purpose of 

 neutralizing unfavorable reactions, which result from the activities of 

 various organisms. The soil bases, including iron and aluminum 

 hydrates, 69 are especially important in this respect. The maintenance 

 of a proper reaction in the soil is very essential for the activities of such 

 important organisms as nitrate-forming, 70 nitrogen fixing and various 

 cellulose decomposing bacteria. 71 



Microorganisms thus play a manifold part in the transformation of 

 minerals in the soil. The activities of these organisms which result 

 in a change of the minerals from one chemical state into another may 

 be classified as follows: (1) Heterotrophic energy utilization of micro- 

 organisms leads to a mineralization of the soil organic matter or to the 

 liberation of minerals from their combination with organic compounds. 

 (2) A part of the minerals thus liberated or added to the soil in the form 

 of inorganic fertilizers may be reassimilated by various soil organisms 

 and thus changed from a soluble into an insoluble condition. (3) The 

 autotrophic bacteria, utilizing minerals as sources of energy, bring about 

 a change in the chemical nature of the minerals in question. (4) The 

 interaction between insoluble minerals in the soil with the products 

 formed by the activities of microorganisms, especially organic and in- 

 organic acids, results in an increase in solubility of these minerals. 



The stimulating effect of small amounts of various minerals, such as 

 zinc, iron and copper, upon the activities of various specific organisms, and 

 of arsenic upon certain soil processes in general has already been noted. 

 The action of arsenic is probably of the nature of partial sterilization 

 of soil. 72 



67 Sperlich, A. tJber Salztoleranz bezw. Halophilie von Bakterien der Luft, 

 der Erde und des Wassers. Centrbl. Bakt. II, 34: 406-430. 1912. 



68 Guillemin, M., and Larson, W. P. The relation between the fixed and free 

 salts of bacteria. Jour. Inf. Dis., 31: 349-355. 1922. 



69 Whiting, A. L. Inorganic substances, especially aluminum, in relation to 

 the activities of soil organisms. Jour. Amer. Soc. Agron., 15: 277-289. 1923. 



70 Ashby, 1907 (p. 394). 



71 Further information on the influence of minerals and salts upon bacteria 

 and their activities in the soil is given by Greaves, J. E. and Carter, E. G. 

 The action of some common soil amendments. Soil Sci., 7: 121-160. 1919. 



72 Greaves, J. E. Stimulating influence of arsenic upon the nitrogen-fixing 

 organisms of the soil. Jour. Agr. Res., 6: 389-416. 1916. 



