/5-. 



Microorgaiiisins and Soil Fertility 



DOMESTlCATIOx\ OF MICROORGANISMS 



The control of a given reaction that occurs in nature and the ap- 

 plication of such reaction or system for the benefit of man and his 

 economy, especially when this involves complex biological processes, 

 gradually lead to the domestication of this system and these proc- 

 esses. Men learned to domesticate animals and plants in prehistoric 

 times. Only very few animals and very few plants have been intro- 

 duced into human economy since history began. In the case of 

 microorganisms, the picture is quite different. The ability of man 

 to domesticate microorganisms, including those living below ground 

 and those living above it, those that are able to control diseases and 

 those that bring about useful processes, may be looked upon as one 

 of the greatest triumphs of modern civilization. This has been ac- 

 complished in the brief span of less than about three-quarters of a 

 century. Some of these domestications have been brought about 

 only within the last decade, as in the manufacture of antibiotics. 

 Soil microorganisms have contributed their share to the growing 

 family of domesticated forms of life which man has placed under 

 his control. 



Within this category, by far the most important group of organ- 

 isms, from the point of view of soil processes and crop production, 

 are the root-nodule or legume bacteria, or those organisms that form 

 nodules on the roots of leguminous plants. The solution of the prob- 

 lem of soil inoculation and development of an understanding of the 

 strain specificity of the organisms concerned in the inoculation of 

 specific plants have in many instances revolutionized agricultural 

 practice. Such organisms have come to occupy a highly important 

 place in rural economy and also in soil conservation and soil im- 

 provement. 



In addition to root-nodule bacteria, various other microorganisms 

 have been utilized for soil inoculation. Although but seldom is a 



