64 Occurrence of Microorganisms in Soil 



the upper layers. Warington showed that the process of nitrification 

 consists of two stages: (a) the oxidation of the ammonium salt to 

 nitrite; (b) the oxidation of the nitrite to nitrate. 



In crude cultures prepared from soil, the nitrite-forming bacteria 

 are present together with the nitrate-formers, and, even when the 

 development of the former reaches its maximum, the latter may still 

 be dormant. As soon as all the ammonia has been transformed into 

 nitrite, the nitrate-formers become active. When transfers are made 

 from the crude cultures thus obtained into fresh media, the stage of 

 oxidation of the ammonium salt will influence the type of organism 

 that will be prevalent in the subsequent culture. If transfers are 

 made at an early stage of oxidation, when the ammonium ion is still 

 present, the nitrate organism may be entirely eliminated from the 

 culture even after only a few such consecutive transfers. If, however, 

 nitrite is substituted for the ammonium salt in the medium, which is 

 then inoculated with soil or with a previous culture at an active stage 

 of nitrate formation, the nitrite-forming organism may be entirely 

 eliminated. The two bacteria can thus be separated from one an- 

 other when their characteristic metabolism is recognized. 



The culture media in which these organisms develop show at 

 first no turbidity or pellicle formation, because of the scarcity of 

 growth of the corresponding organisms. After repeated additions 

 to the media of ammonium salt or of nitrite, a bluish slime is pro- 

 duced on the bottom and on the wall of the flask. When this slime 

 is examined microscopically, it is found to consist of a layer of minute 

 rods staining with difficulty. After several transfers into fresh media, 

 the culture becomes sufficiently enriched so that plates can be pre- 

 pared for the isolation of pure cultures. 



All soils that are not very acid in reaction contain bacteria capable 

 of oxidizing ammonium salts to nitrites and the latter to nitrates. 

 The limiting acidity for the dexelopment of these bacteria is pU. 4.0- 

 3.7, whereas their optimum reaction is at pH 6.8-7.3. When a sofl 

 more acid in reaction than the minimum and lacking the nitrifying 

 organisms is treated with lime, the organisms will gradually appear, 

 although inoculation with a good fertile soil is often practicable, so 

 as to introduce the organisms immediately. This is true of acid peat 

 soils and certain acid forest sofls. 



The nitrifying bacteria are not very sensitive to drying, but steam 

 or volatile antiseptics are highly injurious, resulting in their rapid 

 destruction in the soil. 



