102 PRINCIPLES OF SOIL MICROBIOLOGY 



iron, the latter being added to the sterilized medium, and with hydrogen 

 in the atmosphere. 



The optimum reaction is at pH 6.8 to 8.7, the limits being pH 5.2 to 

 9.2. Growth takes place on the surface, at the boundary between the 

 gas and the liquid; in some cases, the liquid becomes turbid. Partial 

 pressure of the gases (H 2 and C0 2 ) has an inappreciable influence upon 

 growth. Respiration of the organism is discussed in detail elsewhere 

 (p. 403). 



