NITRIFICATION 127 



lot now is sterilized by heating, that all bacteria may be 

 killed. The other lot is undisturbed. Both lots are 

 treated alike in all other respects from now on. When 

 compared later, it will be found that the treated lot 

 shows no increase of nitrates of available food ; while 

 the other lot where bacteria were permitted to go on 

 shows an increase in this respect. Hence, nitrification, 

 now, is believed to be a biological process : to be actually 

 caused, governed and controlled by the bacterial life of 

 the soil. Moreover, it is a two-fold process, for the rea- 

 son that two sets of bacteria are 

 at work. One set oxidizes am- 

 monium compounds into ni- A** 

 trous acid nitrite; the other A,^'- 



* % ft * 



oxidizes the nitrites into ni- & t 



trates the final form. A Rus- '* 



V * 



sian scientist has demonstrated 



i 

 that these two sets are com- 



NITRIFYING BACTERIA 



pletely separated, that neither A .-Nitrococcus 



crosses the line into the other's B - and ( A^ i c r onn) aciena 

 territory, that each class does 



its own work. only. In short, that neither class is able to 

 do the other's work, even if it would do so. 



The bacteria that cause nitrification. These workers 

 are known now as nitrobacteria. The two classes are: 

 Nitrous bacteria, called also nitrosomonus, and nitric 

 bacteria, called also nitrobacter. As stated before, ni- 

 trous bacteria begin the work of nitrification : they 

 change ammonium compounds into nitrites. When this 

 is done, their work stops : they go no farther, for they 

 cannot. However, nitrification is not stopped, for at 

 this point the nitric bacteria take up the work, change 

 nitrites into nitrates, thereby completing the work origi- 

 nally begun by putrefaction bacteria. 



