MICROBES IN PLANT GROWTH 



13 



It may make up 5 per cent (in the case of sands) to 93 per cent 

 (in the case of peats) of the total volume of soil. (3) The gaseous 

 phase, or the soil air, which may differ greatly in composition 

 from ordinary atmosphere, because of the higher content of 

 carbon dioxide, lower content of oxygen, and frequently the 

 presence of some methane and hydrogen. 



The existence of these three phases alone would hardly be 

 sufficient to make the soil a medium fit for the growth of higher 

 plants. The presence in the soil of another factor, namely, the 

 microbes, makes the soil a living medium and renders it dynamic. 



Fig. 5. — Schematic representation of the relation of microorganisms to the 

 phj-sical structure of soil (from France). 



Role of Microbes in Plant Growth. — The soil may be 

 looked upon not only as a medium supporting growth of higher 

 plants but as a complex natural environment inhabited by a popu- 

 lation more diverse and active in a greater variety of affairs than 

 the visible inhabitants of the earth. Economically the major 

 interest in soils is concerned with the growth of higher plants, 

 but, since there is such a close correlation between the develop- 

 ment of plants and microbial activities, a comprehensive appre- 

 ciation of the microbial life is of much more than academic interest. 



These microbes are largely responsible for the numerous 



