Ii8 BACTERIOLOGY OF THE SOIL 



cultivated land are problems which have yet to be dealt 

 with. 



2. Numbers of Bacteria in the Soil. It is a difficult 

 matter to determine the number of bacteria in a sample 

 of soil, and no really reliable method exists at present 

 for the accurate numerical estimation of all the various 

 kinds which may be present in it. 



The counting of the colonies on agar or gelatine plates, 

 inoculated either with a definite amount of soil or with a 

 measured volume of sterilized water mixed with the soil 

 to be examined, is generally adopted. However, many 

 kinds pf soil organisms, and especially those concerned 

 with the fixation of nitrogen and the process of nitrifica- 

 tion, do not grow at all on these media, and unless 

 special precautions are taken to exclude air no colonies 

 of the anaerobic forms appear upon the plates. 



The method adopted to free the bacteria from the soil 

 particles, the nature and chemical reaction of the media 

 employed, and many other factors influence the result. 



The figures given below are comparative only, and 

 have mostly been determined by plate cultures on neutral 

 or slightly alkaline gelatine or agar media. 



Very little is known in regard to the relationship 

 between the number of organisms in a soil as determined 

 by the plate methods in ordinary agar and gelatine 

 media and its fertility. That an increase or decrease in 

 the number of bacteria present has an important bearing 

 on the material available for the nutrition of crops is 

 certain, but much research must be prosecuted before the 

 connection is clearly understood. 



The soil, containing as it does considerable amounts 

 of water and stable organic matter, is an excellent 

 medium for the development and multiplication of many 



