12 Historical 



dantly in the soil and to be responsible for many soil transformations. 

 Attention may be directed, for example, to the work of Kette, 

 who emphasized in 1865 that the importance of addition of stable 

 manure to the soil was due to the fact that the manure could not 

 be replaced by inorganic nitrogen compounds and minerals or by 

 purely vegetable matter, because these lack "a true vibrion fermen- 

 tation." 



Fig. 7. Ferdinand Cohn laid the basis for die classification of bacteria, including 

 many that occur in the soil. 



Robert Koch introduced in 1881 the gelatin plate for the study of 

 bacteria, thus laying the basis for a systematic study of soil micro- 

 organisms. The gelatin was soon replaced by agar-agar as a solidify- 

 ing agent for bacteriological media. Those who followed Koch were 

 medical men who were more interested in public health and hygiene 

 than in soil processes. They limited themselves to a study of various 

 soil layers for the presence of bacteria and fungi that would develop 

 on the gelatin plate. Any organism that did not develop on the 

 plate was considered to be of no importance. The occurrence of 

 specific bacteria was studied primarily for the purpose of establishing 

 whether the soil contained pathogenic organisms. The pure-culture 

 techniques that were thus developed made, however, an important 

 contribution to bacteriology and had a significant effect upon the 

 study of soil bacteria. 



In a study of the purification of sewage water, Pasteur made the 

 passing suggestion in 1862 that nitrification was due to bacterial 

 action. Schloesing and Mijntz found that, when a stream of sewage 



