SOIL MICROBIAL POPULATION 7 



est variety of organisms or are particularly favorable for the growth of 

 certain special types of organisms. None of the media so far employed 

 allows the growth of the total soil population. The plate method is often 

 supplemented by the selective culture method, in which a great variety 

 of media are used in order to obtain a representative picture of the soil 

 population. Since the number of media required to enable all soil micro- 

 organisms to develop is virtually limitless, the enrichment methods can 

 only give a proximate idea of the nature and abundance of the micro- 

 biological population. Because of the development on the plate of cer- 

 tain organisms that exert a toxic effect upon others, the plate method 

 often shows excessive variation in the numbers of bacteria and fungi 

 (256). 



The microscopic methods have been introduced to fill this gap, since 

 by them the relative abundance of the various groups of organisms 

 found in soils, composts, or other natural substrates can be established. 

 Unfortunately, these methods do not allow any differentiation between 

 living and dead cells, nor do they permit a differentiation between the 

 various physiological types of microorganisms such as pathogens and 

 nonpathogens. A further limitation, especially of the contact slide, is 

 that the fast-growing forms cannot be prevented from overgrowing 

 the slide and repressing the slow-growing types. 



The mechanical separation methods are based upon the use of special 

 sieves or water emulsions and are utilized for the study of the larger 

 forms such as insect larvae and nematodes. 



The relative abundance of the different groups of microorganisms in 

 a given soil, as determined by any one of the foregoing methods, varies 

 with the nature of the soil, amount of organic matter, oxygen sup- 

 ply, moisture content, temperature, acidity, and buffering capacity 

 (Table i), as well as with the nature of the higher plants growing in 

 the given soil (Table 2). Despite all these factors, the microbiological 

 population of the soil throughout the world has certain definite and 

 common characteristics and comprises certain well-defined, specific 

 types. The bacteria usually range in number from a few hundred thou- 

 sand to several hundred million per gram of soil, though many species 

 do not develop on the common plate. Fungi are found in the form of 

 mycelial filaments and as spores and may therefore constitute as large 



