50 



Microbiological Population of the Soil 



Elective Culture Methods. These methods consist of diluting 

 the soil with sterile water and adding definite volumes, such as 1-ml 

 portions of the final dilutions, to special nutrient media adapted for 

 the growth of specific groups of microorganisms. The highest dilu- 

 tion which allows positive growth permits the calculation of the 



4A 5A 7A 7B 9B llA IIB 18A 19A 19B 



Fig. 24. Crop yields and bacterial numbers (from Waksman). 



approximate number of specific organisms present in the particular 

 soil. Thus, if a dilution of soil of 1:100,000 placed in a pectin 

 medium allows the decomposition of the pectin, whereas a dilution 

 of 1 : 200,000 does not, one may conclude that the soil contains 100,000 

 pectin-decomposing organisms per gram. 



These methods are rather cumbersome, since they involve the 

 preparation of a large number of media for the development of vari- 

 ous physiological groups of organisms, and the need for a number 

 of containers for making the \arious dilutions. Furthermore, the 



