NITRIFICATION. 67 



The change from organic nitrogen to ammoniacal 

 nitrogen is always accompanied by oxidation of carbo- 

 naceous matter and the consequent production of carbon 

 dioxide, the formation of which probably supplies the 

 energy necessary for the reaction. 



The other two stages of the reaction, ammoniacal 

 nitrogen to nitrites and nitrites to nitrates, are them- 

 selves processes of oxidation and are consequently 

 sources of energy. It is found that both the nitrous 

 and the nitric organisms can effect their work in solu- 

 tions free from organic matter and assimilate the carbon 

 which they require for their growth from carbonates 

 (Winogradski) or carbon dioxide" and without the aid of 

 sunlight. Winogradski found that on the average 

 35 parts of nitrogen were oxidised for each part of car- 

 bon assimilated from carbonates. The necessary energy 

 for this assimilation of carbon must be derived from 

 the oxidation of the nitrogen. The oxidation of am- 

 monia to a nitrite evolves about four times as much 

 heat as the oxidation of the nitrite to a nitrate. Even 

 the latter process evolves more heat than is necessary to 

 account for the energy required in the experiments of 

 Winogradski, 9 parts of nitrogen as nitrite oxidised to 

 nitrate yielding sufficient energy to allow of 1 part of 

 carbon being converted from carbon dioxide into cellu- 

 lose.! 



2. Nitrification can only occur under favourable conditions. 

 The main essentials are : 



(a) Suitable food. Certain minerals, particularly potash, 



lime, sulphates, and phosphates, must be present, 

 and carbon dioxide is also essential. Organic matter 

 is not necessary for either the nitrous or nitric orga- 

 nism. Ammonium compounds appear to be most 

 easily nitrified, but the pure nitrous organism can 

 apparently attack certain organic nitrogenous bodies, 

 e.<j., asparagine, milk, casein, urea. 



(b) The presence of a basic material in order to combine 



with the nitrous and nitric acid. The medium in 



* Godlewski, J.C.S. 1896, abst. ii., C68. t Warington, J.C.S. Trans. 1891, p. 521. 



F 2 



