340 Recent Developments in Soil Microbiology 



utilization of these activities for soil improvement, soil conservation, 

 plant productivity, and combating of plant and animal pathogens. 



The Soil as a Living System 



Because of the extensive microbiological population inhabiting it, 

 the soil must be considered not merely a dynamic or even a biological 

 system, but a living system. This assumption can be substantiated 

 as follows: (a) living organisms, belonging both to plant and to 

 animal systems, have taken an active part in the processes of rock 

 weathering and soil formation; ( b ) these organisms have contributed 

 to the formation and accumulation of one of the most important 

 and characteristic soil constituents, humus, which is largely re- 

 sponsible for differentiating a soil from a mere mass of inorganic 

 debris; (c) the soil processes are continuous both in summer and 

 in winter, and are affected by temperature, aeration, moisture, and 

 supply of fresh plant and animal residues; (d) the extensive flora and 

 fauna representing numerous forms of life that inhabit the soil 

 range from the smallest bacteria to the large burrowing animals and 

 the roots of higher plants. 



The Soil Microbiological Population 



One could discover in the soil most forms of life, within proper 

 dimensions of size and space, if one would only search for them long 

 enough and develop the proper methods for their demonstration. 

 Exclusive of higher plants, which find in the soil a support and a 

 medium for their growth and from which they derive most of their 

 nutrients, and exclusive of the numerous animals that spend the 

 whole or a part of their life cycle in the soil, there exists in the soil 

 an extensive population of microorganisms. This comprises forms 

 which are characteristic of the soil and which seldom live in a natural 

 state under other conditions, as well as forms which find in the soil 

 only a temporary habitat. 



The soil population also varies considerably, both in kind and in 

 abundance, depending upon the nature of the soil, its treatment, 

 and various environmental conditions. This can easily be demon- 

 strated by comparing the population of an undistiu-bed virgin soil 

 with that of the same soil after it has been cultivated and has re- 

 ceived various added organic and inorganic substances. In the 

 virgin soil, the microorganisms are in a state of equilibrium, where 



