52 



THE SOIL POPULATION AND ITS DISTRIBUTION 



at any one time. Figs. 38 and 39 are preparations of soil showing 

 the occurrence of some of the microbial cells in stained specimens. 

 The results obtained by this method indicate that bacteria 

 exist in the soil not only in millions but in tens and hundreds of 

 millions per gram. Most of the bacteria found by the direct 

 microscopic method are non-spore-forming, rod-shaped or spherical 

 cells. Many of the non-spore-forming organisms are found within 

 the colloidal films which surround the soil particles, while the 

 spore-forming organisms occur in the spaces between the individual 

 particles. The cells may occur in large aggregates or colonies 

 enclosed in an abundance of capsular material. The protozoa, 

 fungus mycelium, and portions of actinomyces growth are fre- 

 quently disintegrated by the process of staining. Table 7 shows 

 the large numbers of bacteria and fungi which have been observed 

 in soil by the direct microscopic method. 



TABLE 7 



Numbers of Microorganisms in One Gram of Soil as Determined 

 BY THE Direct Microscopic Method (from Richter) 



Plate Method for Counting Microbes. — The plate method 

 for counting bacteria as well as certain other soil microbes makes 

 use of the fact that media containing agar and gelatin can be kept 

 liquid at temperatures which are not destructive to microbes and 

 form gels at temperatures favorable to growth of the organisms. 

 A highly diluted suspension of soil is prepared in sterile water in 

 such a manner that the liquid contains only a limited number of 

 microbes; these are as completely dispersed in the water as pos- 



