16 MICROORGANISMS IN SOILS AND WATER BASINS 



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. 



The ability of other bacteria, notably members of the Azotobacter 

 group, to fix nitrogen independently of host plants and the fact that 

 these organisms are absent in many soils led to the suggestion that such 

 soils might benefit from inoculation. It has been found, however, that 

 when soils and peats are inoculated with Azotobacter chroococcum 

 large-scale destruction of the latter often occurs isil^)- This is believed 

 to be due to the presence in the soil of antagonistic organisms as well as 

 toxic substances (477, 648, 951). 



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

 capable of growing in heated soil. This was found to be due to the fact 

 that normal soils contain certain substances that render the growth of 

 the fungus impossible j these substances are destroyed by heating. An 

 extract of fresh soil was found to act injuriously upon the growth of the 

 fungus Pyronema; the injurious effect was partly removed on boiling 

 (500). 



The survival of microorganisms added to soil or water is thus influ- 

 enced by the nature of the native soil or water population, the organ- 

 isms added, the composition of the substrate, and various environ- 

 mental conditions. 



SAPROPHYTIC AND PATHOGENIC NATURE OF 

 CERTAIN SOIL MICROORGANISMS 



Various fungi and actinomycetes causing animal diseases, notably 

 skin infections, appear to resemble very closely the corresponding soil 

 saprophytes. It was therefore suggested that many of the dermato- 

 phytic fungi normally lead a saprophytic existence in the soil but are 

 also capable of developing on epidermal tissue, and bringing about in- 

 fection of the tissues. This was found to be true especially of species of 



