312 MICROBIOLOGY OF SOIL 



nitrate agar of R. C. Cook. It is evident, therefore, that the results 

 secured in the counting of soil bacteria have only a relative value. 

 With the same media and methods some information may be secured 

 concerning the influence of fertiUzation, tillage, liming, etc., on certain 

 of the soil bacteria. But even this information must be properly 

 discounted, since equal numbers 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 accompUsh 

 exactly as much decomposition as the same number of similar organ- 

 isms 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 OmeUanski 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 could be adapted 

 for the quantitative-qualitative determination of urea, denitrifying, 

 methane, and still other physiological groups of microorganisms. 



There is no doubt that the quantitative-quaHtative 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. 



TEANsroRMATioN 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 



