MICROORGANISMS AS A FACTOR IN SOIL FERTILITY. 243 



some information may be secured concerning the influence of fertilization, 

 tillage, liming, etc., on certain of the soil bacteria. But even this infor- 

 mation must be properly discounted, since equal numbers do not neces- 

 sarily 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 Wino- 

 gradski, or the gypsum plates proposed by Omelianski may be employed 

 for making numerical comparisons of nitro bacteria in different soils. 

 In like manner Beyerinck'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-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 make-up 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 Miintz 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 1902. He 



