NITRIFICATION 



121 



Fig. 27. — Nltrlfy- 



i n g bacteria 



(nitrosococcus) 



(after Bonaz- 



zi). 



ceased and the ammonia in the sewage passed through unchanged. 

 A water extract of fresh garden soil added to the tuhe soon re- 

 started the process. 



This was confirmed by Warington, who further found that 

 the oxidation of ammonia occurs in two dis- 

 tinct stages. The first 'stage involves the 

 changing of ammonia to nitrates, the second 

 of changing the nitrites to nitrates. Only 

 after many years of study was he even able 

 to obtain very active impure cultures of the 

 organism which changes nitrites to nitrates, 

 and although he observed other organisms 

 which change nitrites to nitrates he was un- 

 able to obtain them in pure culture. 



In 1890 a Russian investigator, Wino- 

 gradski, by a series of very ingenious experi- <; 

 ments isolated the organisms which transform ammonia into ni- 

 trites and the ones which transform nitrites into nitrates. He 



showed that the 

 nitrifying organ- 

 isms are sensitive 

 to organic matter 

 and hence do not 

 develop on the 

 gelatin plates used 

 in the study of the 

 ordinary soil or- 

 ganisms. He grew 

 them for some 

 time on cultural 

 media containing 

 only mineral salts. 

 He then trans- 

 ferred them to 

 gelatin plates on 



Fig. 28. — Nitrifying bacteria (nitrobacter) (after which Very feW 



GIbbs). colonies d e v e I - 



