DIVISION III* 



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 

 Applications 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 

 significance 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 fundamental 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. The author is indebted to Dr. S. A. Waksman for help 

 in the revision of a portion of the manuscript. 



345 



