840 PRINCIPLES OF SOIL MICROBIOLOGY 



The soil is a medium, more or less colloidal in nature, containing 

 a great mass of microscopic, forms of life. These produce various 

 physical and chemical changes in the soil which are of greatest im- 

 portance to the growth of higher plants. The pathologist can study 

 the action of his organisms in vivo; the microbiologist working on 

 fermentation processes can sterilize his medium, without altering its 

 composition greatly, and inoculate it with a pure culture of the organism 

 concerned; the soil microbiologist, however, has great difficulties in 

 attempting to learn just what the particular organism does in the soil. 

 When the soil is sterilized, it is no longer, biologically and chemi- 

 cally, a normal soil. In a pure culture, free from stimulating and 

 competing influences of other microorganisms, an organism may 

 manifest certain activities which would not take place in the soil, or 

 vice versa. It is even possible that, in pure culture, different races 

 develop from those present originally in the soil and it is quite probable 

 that the biochemical action is often quite different. As a matter 

 of fact, a very large number of soil organisms develop upon artificial 

 media only with great difficulty and are often repressed there by 

 other organisms which may be only occasional visitors in the soil. 



Recent advances of the science. During the first decade of the present 

 century, the methods used in the study of soil biological processes 

 have undergone various modifications. Some investigators centered 

 their attention upon the study of the metabolism of specific soil micro- 

 organisms, especially the mechanism of transformation of organic or 

 inorganic substances as bearing upon soil processes. This was deter- 

 mined either by adding a small amount of soil to a sterile solution 

 containing the specific substance, then measuring the change that took 

 place after a definite period of incubation; or by adding the specific 

 substance to the soil, keeping it at optimum moisture and temperature 

 for a definite length of time, and then measuring the change. In most 

 of these studies, the organisms responsible for the change were not 

 considered at all. In the study of protein decomposition, ammonia 

 was usually taken as an index, without considering the fact that the 

 process can be carried on by numerous types of organisms and various 

 associations and combinations, each resulting in a different amount of 

 ammonia accumulating. In the study of nitrogen fixation, the fact 

 was usually left out of consideration that different organisms are 

 active at different reactions and, therefore, different amounts of nitrogen 

 are fixed under laboratory conditions, which may or may not hold 

 true in the field. In the study of nitrification, the fact that the addition 



