32 Microbiological Population of the Soil 



of soil, they allow the development of only a small part of the micro- 

 organisms which are present in a particular soil, depending on the 

 composition of the medium and conditions of incubation. The re- 

 sults thus obtained tended to obscure the importance of the micro- 

 biological population in the various soil processes. The information 

 obtained by these simple cultural methods gave, therefore, only a 

 limited insight into the composition of the complex population, into 

 the numerous processes for which this population was responsible, 

 and into the complex associative and antagonistic interrelationships 

 among the various soil-inhabiting microorganisms. 



At first, bacteria were considered to comprise the all-important 

 group of microorganisms; the various processes which influence soil 

 fertility and for which microbes were known or were believed to be 

 responsible were associated with the occurrence and abundance of 

 bacteria. When it was recognized that various other groups of micro- 

 organisms must receive attention, and their importance in numerous 

 soil processes could no longer be ignored, there came a change in 

 the general concept of the population and its importance. 



These changes in our understanding of the nature of the soil popu- 

 lation were also accompanied by a growing realization of the rela- 

 tion of this population to soil processes. In order to measure the 

 activities of the population in the soil, a small amount of soil was at 

 first added to aqueous solutions containing varying concentrations 

 of different substances, such as peptone, cellulose, ammonium salts, 

 or nitrates. These solutions were incubated for a few days in the 

 laboratory, and the chemical changes that took place in the composi- 

 tion of the specific substance were determined by various analytical 

 procedures. The results thus obtained were believed to serve as a 

 measure of the activities of the microbiological population in the soil. 

 It was later found that the chemical changes brought about in the 

 solution cultures might not have been due at all to the important 

 groups of organisms, but only to those forms that could adapt them- 

 selves more readily to the artificial conditions created by the par- 

 ticular selective media. This tended to give to such organisms an 

 exaggerated importance, out of all proportion to the part that they 

 actually play in soil processes. The more predominant and perhaps 

 the more important groups of microorganisms frequently had great 

 difficulty in developing in the artificial media and under the arti- 

 ficial conditions of culture. 



The "solution" methods were gradually replaced by the "soil" 

 methods, whereby a small amount of a known substance was added 



