THE SOIL POPULATION 37 



bers of the soil population may also be directly responsible for the rapid 

 destruction of the added organisms. It has been shown (635), for ex- 

 ample, that subsurface soils contain microbiological populations that 

 are smaller, less versatile, and less adaptable than surface soil popula- 

 tions. Some factor in the subsurface soils was believed to cause the pre- 

 vention of the rapid development of the introduced organisms. Anti- 

 biotic or inhibitory substances were said to be responsible for this effect, 

 these substances being of microbial origin. Aqueous extracts of soil did 

 not adversely affect the growth of soil bacteria in vitro, but alcohol ex- 

 tracts reduced their activity in the soil and in artificial culture media. 



