838 PRINCIPLES OF SOIL MICROBIOLOGY 



Hellriegel and Wilfarth on the nodule bacteria and fixation of nitrogen 

 by leguminous plants; the work of Frank and Beijerinck on the iso- 

 lation of the organisms and their cultivation in pure culture, and 

 finally the work of Winogradsky on the autotrophic bacteria were the 

 contributions which transformed the science from its preparatory into 

 the building period. 



With the introduction by Robert Koch, in 1881, of the gelatin plate 

 for the study of bacteria, a stimulus was given to the systematic study 

 of soil microorganisms, although the earliest bacteriologists were 

 medical men and were more interested in public health and hygiene 

 than in soil processes. They limited themselves entirely to a study 

 of the numbers of bacteria and fungi in various soil layers, that 

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

 on the plate was not considered to be of importance. The occur- 

 rence of specific organisms was studied chiefly from the point of view 

 of finding out whether the soil contained pathogenic organisms. Here 

 may be mentioned, in addition to Koch, Frankel in Germany and 

 Houston in England. 



Of biochemical processes in the soil, the first to attract universal 

 attention, as said before, were those of nitrification and nitrogen-fixa- 

 tion. Pasteur suggested in 1862 that nitrification is a bacterial process 

 and Schlosing and Miintz definitely proved that in 1877. This was 

 soon followed by the work of Warington, who in a series of splendid 

 contributions, beginning with 1878, established some of the most 

 fundamental principles of the process of nitrification in the soil, outside 

 of actual isolation of the organisms concerned. This was accomplished 

 in 1890, by Winogradsky. In reference to nitrogen-fixation, universal 

 attention was first centered upon the symbiotic process. Boussingault 

 emphasized in 1838 that the favorable action of legumes upon the 

 soil is due to their power of fixing atmospheric nitrogen. Frank demon- 

 strated in 1879 that the nodules on the roots of the plants are formed 

 as a result of inoculation with microorganisms. This was definitely 

 demonstrated in 1886 by Hellriegel and Wilfarth. The organism 

 Bad. radicicola was isolated and described by Beijerinck in 1888. The 

 non-symbiotic nitrogen-fixing bacteria were isolated in 1893 by Wino- 

 gradsky (Clostridium) and in 1901 by Beijerinck (Azotobacter). 



The names of Winogradsky and Beijerinck stand for the most 

 fundamental work that has been done in building up the science of 

 soil microbiology. While Winogradsky limited himself to the study 

 of autotrophic and the anaerobic nitrogen-fixing organisms, Beijerinck 's 



