SAPROPHYTIC ORGANISMS IN THE SOIL 15 



it again attains a normal form of life, it is designated as a carrier of the 

 disease-producing agent. 



Pathogenic organisms pass their existence in the living body of the 

 plant or animal. They spread from one host to another by contact or 

 through a neutral medium, such as water, milk, or dust where they may 

 remain alive and active for varying lengths of time, or they reach the 

 soil or water basins in the excreta of the host. If the host is killed by 

 the infecting microbes, they may survive for some time upon the rem- 

 nants of what was once a living animal or plant and thus find their way 

 into the soil and water basins. 



Considering the millions of years that animals and plants have ex- 

 isted on this planet, one can only surmise the great numbers of microbes 

 causing the numerous diseases of all forms of life that must have found 

 their way into the soil or into streams and rivers. What has become of 

 all these pathogenic bacteria? This question was first raised by medical 

 bacteriologists in the eighties of the last century. The soil was searched 

 for bacterial agents of infectious diseases. It was soon found that, with 

 very few exceptions, organisms pathogenic to man and animals do not 

 survive very long. This was at first believed to be due to the filtration 

 effect of the soil upon the bacteria (32). It came to be recognized, how- 

 ever, that certain biological agents are responsible for the destruction 

 of the pathogenic organisms. These investigations led to the conclusion 

 that the soil can hardly be considered as a carrier of most of the infec- 

 tious diseases of man and animals. The fact that many pathogens can 

 grow readily in sterilized soil but do not survive long in normal fresh 

 soil tends to add weight to the theory of the destructive effect upon 

 pathogens of the microbiological population in normal soil. 



INTRODUCTION OF SAPROPHYTIC ORGANISMS 

 INTO THE SOIL 



It often becomes necessary to inoculate the soil with organisms not 

 usually found there. The common practice of inoculating soil with bac- 

 teria capable of forming root nodules on leguminous plants is a case in 

 point. It is essential, therefore, to know how long these organisms will 

 survive. The survival period is influenced greatly by the presence of a 



