6 PRINCIPLES OF SOIL MICROBIOLOGY 



Various media and different methods have to be used for the study of the 

 different groups. In some cases, special enrichment culture media favor- 

 ing the development of particular organisms have to be devised, so that 

 the growth of these will take place in preference to that of all the 

 other organisms. We thus often create artificial conditions which are 

 distinctly different from those of the soil and conclusions, based on the 

 results of growth of the organisms under such artificial conditions, often 

 do not hold true for the soil. To be able to grow the organisms in pure 

 culture in the soil, the latter must be first sterilized. No method of 

 sterilization has yet been devised which would not modify, in a funda- 

 mental manner, the chemical conditions of the soil. What will hold 

 true for sterilized soil, then, may not hold true for unmodified soil. 

 Again, the various organisms exist in the soil in large numbers, with a 

 number of associative and antagonistic influences at work (both by 

 living microorganisms and their products). Each organism has 

 adapted itself to its environmental conditions and to the other organisms 

 and may be, so to speak, in a condition of "unstable equilibrium." 

 When this same organism is cultivated, in pure culture, upon a favor- 

 able medium, its activities are very likely to be different from those in 

 the normal soil. Before we can conclude that a microorganism is 

 active in the soil and that certain chemical transformations are produced 

 by this organism under ordinary soil conditions, certain requirements 

 must be satisfied. The following postulates, applied by Koch to 

 pathogenic bacteria, and modified by Conn 3 in their application to soils 

 should hold true for soil microorganisms: (1) The organism must be 

 shown to be present in the soil in an active form when the chemical 

 transformation under investigation is taking place. (2) The organism 

 must be shown to be present in larger numbers in such soil than in 

 similar soil in which the chemical change is not taking place. (3) 

 The organism must be isolated from the soil and studied in pure cul- 

 ture. (4) The same chemical change must be produced by the or- 

 ganism in experimentally inoculated soil, making the test, if possible, 

 in unsterilized soil. (5) The organism must be found in the inoculated 

 soil. 



Methods of study. The methods generally employed for the study of 

 soil bacteria can be divided into those of direct microscopic observation 

 and cultural methods. The former have been suggested by Conn and 

 further developed by Winogradsky. The latter have been used by the 



3 Conn, H. J. The proof of microbial agency in the chemical transformation 

 of soil. Science. N. S. 46: 252-255. 1917. 



