SAPROPHYTIC ORGANISMS IN THE SOIL 15 



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. 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 

 host plant that protects the specific bacteria from attack by antagonistic 

 organisms. In the absence of the host plant, the bacteria seem to disap- 

 pear gradually, and reinoculation becomes advisable when the host is 

 again planted in the given soil. It has been observed also that specific 

 strains of bacteria tend to deteriorate in the soil, and that it is necessary 

 to reinoculate the soil with more vigorous strains of the organisms in 

 question. 



Some bacteria, notably members of the Azotobacter group, are able 

 to fix nitrogen independently of host plants but these organisms are 

 absent from many soils. The suggestion was made that such soils might 

 benefit from inoculation. However, it has been found that when soils 

 and peats are inoculated with A. chroococcum large-scale destruction 

 of the latter often occurs (814), due, it is believed, to the presence in 

 th9 soil of antagonistic organisms as well as toxic substances (492, 687, 

 980). 



Certain fungi are unable to grow in fresh nonsterilized soil but are 



