Foundation of Soil Microbiology 



13 



was allowed to pass \ xuy slow!)' through a column of sand and lime- 

 stone, the ammonia in the sewage was at first unaffected, but, after 

 twenty days, it became conxertcd into nitrate, so that later the am- 

 monia disappeared and nitrate was found in its place. Addition of 

 a little chloroform vapor stopped the process completely; when the 



Fig. 8. Robert Koch introduced the gelatin plate method for the enumeration of 

 bacterial population of the soil and made other important contributions to micro- 

 biology. 



chloroform was removed and a little soil suspension added, the 

 process was started again. The role of "organized ferments" in the 

 process of nitrification was thus established. 



The enrichment culture method was introduced by Beijerinck. It 

 consisted in using a selecti\e medium containing special nutrients, 

 for the purpose of stimulating the development of specific bacteria, 

 which were then isolated in pure culture. This method was utilized 

 by many bacteriologists, especially Winogradsky, and resulted in 

 the isolation of a number of organisms concerned in important soil 



