16 Historical 



(various fatty acids such as formic, butyric, acetic, propionic, valeri- 

 anic), various nitrogen compounds (leucin, tyrosin, indol, skatol, 

 amines, amides, as well as ammonia and sometimes nitrite); the 

 minerals were said to be found largely in unassimilable forms. 



c. Other Decomposition Processes. These were frequently 

 placed between the two previously mentioned categories, as in the 

 case of "Vermoderung," which was said to occur in nitrogen-poor 

 substances with a moderate water content and in the presence of air. 

 "Decay" and "putrefaction" were believed to take place at times 

 side by side, according to the oxygen content of the various layers, 

 as in manures. The various alcoholic and acid "fermentations" were 

 also included in these intermediary reactions. 



These confusing ideas took no consideration of the microbes as 

 living organisms which possess a distinct metabolism and which use 

 organic matter as a source of energy and of nutrients. This con- 

 fusion carried over to more recent times. It was particularly evident 

 in the ideas current even as late as the end of the second decade of 

 this century, concerning the "humification" of organic matter in 

 nature, especially in soil, in peat bogs, and in coal formation. 



The Golden Age of Soil Microbiology (1891-1910) 



Once the foundation for soil microbiology was laid, the field was 

 open for contributions dealing with the population of the soil, espe- 

 cially the bacteria, and its role in soil processes. Numerous organ- 

 isms were described and many important soil processes were eluci- 

 dated. Among the outstanding investigators during this period, 

 especially during its early phases, were M. W. Beijerinck and S. N. 

 Winogradsky, who laid the foundation for much of the subsequent 

 work. A number of chemical processes involved in the various trans- 

 formations of nitrogen in soils and in composts were studied in detail. 

 This element was recognized as frequently limiting plant growth. It 

 was established that the transformation of nitrogen in nature is largely 

 dependent upon the activities of various groups of soil microorgan- 

 isms, especially the processes of ammonification, nitrification, and 

 nitrogen fixation. These comprise the liberation of nitrogen, in an 

 available form as ammonia, from complex organic nitrogenous sub- 

 stances; the oxidation of ammonia to nitrate; and the fixation of 

 atmospheric nitrogen by microorganisms when an available source 

 of energy is supplied. The final elucidations of these processes were 

 made, respectively, by Marshal and by Miintz and Coudon in 1893, 



