DIVISION HI.* 

 MICROBIOLOGY OF SOIL. 



CHAPTER I. 



* 



MICROORGANISMS AS A FACTOR IN SOIL FERTILITY. 



INTRODUCTION. 



Rational views on soil fertility were first presented, in a systematic way, 

 by Justus von Liebig in 1840. In his " Organic Chemistry in its Applica- 

 tions to Agriculture and Physiology" he developed important theories on 

 the circulation of carbon and nitrogen in nature, and on the function of 

 the so-called mineral constituents of plants. 



When Liebig 's book appeared many of the leaders and students of 

 agriculture still believed that humus, the partly decomposed residues of 

 plants and animals in the soil, was the direct food of crops. They 

 believed that soils could yield poor or rich harvests in proportion to the 

 amount of humus present in them; they believed, in other words, that 

 plants, like animals, used organic substances as food. 



Liebig rendered a great service to agriculture in emphasizing the sig- 

 nificance of decay processes. He made it evident that humus as such is 

 of no use to plants, and that it becomes valuable only in so far as it is 

 resolved into the simple compounds carbon dioxide, ammonia, nitric acid 

 and various mineral salts. To be sure, he regarded the decomposition 

 of organic matter as a phenomenon purely chemical, nevertheless he 

 succeeded in showing that decay, putrefaction and fermentation are funda- 

 mental facts, connecting links between the world of the living and the 

 world of the dead. 



The research of the following decades brought to light the intimate 

 relation existing between microorganisms and the decomposition of 



* Prepared by Jacob G. Lipman with exception of sub-chapter on " Soil Inoculation" which 

 has been prepared by S. F. Edwards. 



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