ABUNDANCE OF BACTERIA IN SOIL 



57 



50,000,000 frequently found in well-manured and cultivated 

 soils. The nature of the medium employed for preparing the 

 plates is of considerable importance in this connection, peptone- 

 meat-extract media giving a smaller number of organisms than the 

 so-called synthetic media, since the former media favor the rapid 



Fig. 41. — Plate preparation showing development of spreading colonies of 

 bacteria (from Lipman). 



development of certain spreading colonies which rapidly over- 

 grow the plate (Fig. 41). 



When the direct microscopic examination of stained soil prep- 

 arations is used, the bacteria per gram of soil are numbered in the 

 hundreds of millions or even billions (Table 7). By the use of 

 elective culture methods, lower numbers are found and the organ- 

 isms are limited to those microbes favored by the specific media. 

 Table 8 gives a comparison between the abundance of bacteria 

 determined by the use of various elective culture methods and the 



