836 PRINCIPLES OF SOIL MICROBIOLOGY 



ment and specific culture media, the anaerobic methods, etc., all of which 

 were necessary steps in the development of the science. The develop- 

 ment of media proceeded from the complex organic media, introduced by 

 R. Koch, to the special inorganic media such as silica jelly, introduced by 

 Winogradsky, and synthetic media introduced by Beijerinck. These 

 artificial media finally led to the use of the soil itself as a culture medium 

 for the growth and activities of microorganisms. Any modification of 

 the physical or chemical condition of this medium, either as a result 

 of addition of nutrients or stimulants, or as a result of change of en- 

 vironmental conditions, leads to a change in the numbers and activi- 

 ties of the microorganisms. 



Three distinct biological processes had been clearly outlined and 

 partly understood by the middle of last century: (1) Decomposition 

 of organic matter. This was known to give rise to humus which was 

 believed to be one of the fundamental principles in soil fertility. Some 

 investigators (Liebig, 1840) considered humus only as an intermediary 

 product and not as a plant food; organic matter was believed to de- 

 compose slowly by chemical oxidation ("eremacausis"). The work 

 of Schloesing, Wollny and others finally led to a better understanding 

 of the process. (2) Nitrification. The accumulation of nitrates in 

 the soil as a result of decomposition of organic matter was known in 

 17th and 18th centuries, but only Boussingault connected this process 

 with soil fertility. (3) Nitrogen fixation. The use of legumes for 

 enrichment of the soil was known to the ancient Romans. Berthelot 

 was the first to suggest that nitrogen fixation may be accomplished 

 also by non-symbiotic bacteria. The isolation of the organisms con- 

 cerned both in nitrification and nitrogen fixation took place only at 

 the close of last century. 



Soil microbiology as an independent science. Three definite and 

 often distinct conceptions are included in the science of soil micro- 

 biology, namely: (1) a knowledge of the organisms occurring in the 

 soil, their numbers, types and relationships; (2) the biochemical activi- 

 ties of these organisms, under laboratory conditions and in pure culture ; 

 (3) the role of these activities in the soil processes and their application 

 to agriculture. Any advance in the science of botany, zoology, or bacte- 

 riology, which throws light upon the nature of organisms which occur 

 in the soil, such as the development of new methods, a better system 

 of classification in bacteriology, the role of bacteria in the nutrition of 

 protozoa, the question of the physiology and classification of filamentous 

 fungi, the role of mycorrhiza in plant nutrition, etc. can be considered 



