372 MICROBIOLOGY OF SOIL 



do not necessarily mean equal amounts of chemical work accomplished ; 

 for example, there is no certainty that 1,000,000 of decay bacteria 

 derived from one soil will accomplish exactly as much decomposition 

 as the same number of similar organisms from another soil. Otherwise 

 stated, individual cells differ in their physiological efficiency from other 

 cells of the same species. 



QUALITATIVE REACTIONS. By modifying the composition of the 

 culture media different physiological groups may be favored in their 

 development. In this manner the silica jelly medium proposed by 

 Winogradski, or the gypsum plates proposed by Omelianski may be em- 

 ployed for making numerical comparisons of nitro- bacteria in different 

 soils. In like manner Beijerinck's mannit agar may be used for the 

 numerical comparison of Azotobacter, and other media can be adapted 

 for the quantitative-qualitative determination of urea, denitrifying, 

 methane, and still other physiological groups of microorganisms, 

 modified Czapek's agar and Krainsky's agar can be used for actino- 

 mycetes and raisin agar for molds. 



There is no doubt that the quantitative-qualitative method just out- 

 lined may be made to yield valuable information. Yet it, too, possesses 

 defects already noted in connection with the more strictly quantitative 

 method. Apart from the vast amount of work involved in the prepa- 

 ration of a large number of media and in the counting of colonies on 

 many plates, this method fails to indicate differences in physiological 

 efficiency. Furthermore, the colonies of the specific organisms sought 

 are almost invariably accompanied by those of foreign species not 

 always easily distinguished. With these limitations properly recognized 

 and with further improvement in the constitution of special media the 

 method may be made useful in supplementing data secured by other 

 methods. 



TRANSFORMATION REACTIONS. Instead of counting soil bacteria in 

 accordance with colonies produced in general or special media, soil 

 investigators have attempted to measure the bacteriological functions of 

 soils by comparing more or less definite quantities of the latter under 

 known conditions. This method was employed by Wollny and others 

 in studying the factors that affect the formation of carbon dioxide in 

 soils. It was also used by Schloesing and Muntz and their followers in 

 similar studies on nitrate formation. A method somewhat similar in 

 principle but different in its application was proposed by Remy in 



