146 



SOILS. 



FIG. 15. Nitrosomonas. (Winogradsky). 



,,. -:-.- 



ditions of their activity have been largely developed by R. 



Warington. 



Nitrifying Bacteria. The conversion of ammonia into ni- 

 trates is accomplished under 

 proper conditions by two or- 

 ganisms, or groups of organ- 

 isms; the first stage being the 

 formation of nitrites by the 

 round, often flagellate cells of 

 nitrosomonas (or nitrosoco- 

 cus). The second, the oxid- 

 ation of the nitrites into ni- 

 trates by very minute rod- 

 shaped bacilli, named nitrobac- 

 teria. The conditions under 

 which these bacteria can act 

 are quite definite in that, aside 



FIG. i6.-Nitrobacterium. (Winogradsky). from 3. SUpply of the nitHfiable 



substance, a fairly high temperature (24 C. or 75 F.) and 

 a moderate degree of moisture, there must be a free access of 

 oxygen (air) ; and there must be present a base (or its car- 

 bonate) with which the acids formed by oxidation can imme- 

 diately unite. In an acid medium ("sour" soils) nitrifica- 

 tion promptly ceases ; as it also does whenever the amount of 

 base present has been fully neutralized. The bases most favor- 

 able to nitrification are lime and magnesia in the form of car- 

 bonates, an excess of which does no harm ; while in the case 

 of the carbonates of potash and soda, the amount must be 

 strictly limited. 



Conditions of Activity. Dumont and Crochetelle found that up to 

 .25 per cent, potassic carbonate acted favorably on the process; which 

 was, however, completely stopped by as much as .8 per ct. War- 

 ington has shown that ammonic carbonate similarly prevents nitrifi- 

 cation when exceeding about .37 per ct. Ammonia salts in general 

 appear to be antagonistic to the transformation of nitrites into nitrates. 



Aside from the carbonates, some neutral salts favor nitrifi- 

 cation very markedly; while others tend to depress it. 

 Deherain found that .5 per cent of common salt suffices to pre- 



