NITRIFICATION 789 



9. Nitrification. — It has long been known that when dung, 

 urine, flesh of animals, tissues of plants, or any substance 

 containing organic nitrogenous compounds are spread over and 

 dug into the soil, the compounds become oxidised and the 

 whole or a great portion of the nitrogen ultimately takes the form 

 of a nitrate, chiefly nitrate of calcium or potassium. Compounds 

 of ammonia are likewise oxidised to nitrates under similar 

 circumstances. 



This production of nitrates is termed nitrification, and was 

 formerly considered a simple chemical process. It does not, 

 how€ver, take place in soil which has been sterilised, and is 

 now ascertained to be the result of the vital activity of certain 

 species of bacteria. 



Apparently in every case of the nitrification of complex 

 organic substances the formation of ammonium compounds by 

 the uro-bacteria and putrefactive organisms always precedes the 

 production of nitrates and it is upon the ammonium salts thus 

 produced that the special nitrifying bacteria exercise their 

 powers. 



Moreover, the formation of nitrates takes place in two distinct 

 stages, namely, first the partial oxidation of the ammonium com- 

 pounds into nitrites, after which the nitrites are further oxidised 

 into nitrates. No single bacterium appears to be alone capable 

 of effecting both these changes, the two steps of the work being 

 carried out by two different types of bacteria. 



The special organism which completes the first part of the 

 nitrification process in all European soils is Nitrosomonas europcea 

 Win., a very minute motile bacterium. Slightly different species 

 of the same genus carry out the work in Asiatic and African soils, 

 while in South American and Australian soils the nitrite-forming 

 organisms are cocci. 



The second part of the process, namely, the formation of 

 nitrates from nitrites, is brought about by pear-shaped non-motile 

 bacteria included in the genus Nitrobacter : they are all ex- 



