64 CONDITIONS FOR NITRIFICATION 



monia to a nitrite and from a nitrite to a nitrate are each apparently 

 the work of only one particular organism called by Winogradski, 

 Nitroso-monas and Nitroso-coccus, capable only of affecting the oxidation 

 of ammonia to a nitrite, and Nitro-bacter, capable only of converting 

 nitrites into nitrates. The simpler terms nitrous organism (of which 

 possibly two or more species exist) and nitric organism, used by 

 Warington, are equally distinctive. 



The change from organic nitrogen to ammoniacal nitrogen is always 

 accompanied by oxidation of carbonaceous matter and the consequent 

 production of carbon dioxide, the formation of which probably supples 

 the energy necessary for the reaction. 



The other two stages of the reaction, ammoniacal nitrogen to 

 nitrites and nitrites to nitrates, are themselves 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 solutions free from 

 organic matter and assimilate the carbon which they require for their 

 growth from carbonates (Winogradski) or carbon dioxide l and without 

 the aid of sunlight. Winogradski found that on the average 35 parts 

 of nitrogen were oxidised for each part of carbon assimilated from car- 

 bonates. 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 oxida- 

 tion of the nitrite to a nitrate. Even the latter process evolves more 

 heat than is necessary to account for the energy required in the experi- 

 ments of Winogradski, 9 parts of nitrogen as nitrite oxidised to ni- 

 trate yielding sufficient energy to allow of 1 part of carbon being 

 converted from carbon dioxide into cellulose. 2 



2. Nitrification can only occur under favourable conditions. 



The main essentials are 



(a) Suitable food. Certain mineral substances, 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 organism. Ammonium compounds appear to be most 

 easily nitrified, but the pure nitrous organism can apparently attack 

 certain organic nitrogenous bodies, e.g., asparagine, casein, urea. 



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

 nitrous and nitric acid. The medium in which the process occurs 

 may be slightly alkaline or neutral, but acidity or much alkalinity 

 prevents nitrification. Calcium carbonate acts very efficiently as a 

 basic material, the carbon dioxide being easily expelled or perhaps used 

 for the assimilation of carbon by the organisms. Sodium bicarbonate, 

 NaHCO 3 , is also suitable ; but sodium carbonate, Na 2 CO 3 , hinders or 

 entirely prevents nitrification. 



(c) Suitable temperature. Nitrification probably ceases about the 

 freezing point of water and it is stopped at a temperature of about 

 50 C. or 55 C. It is most active about 36 C. 



(d) Sufficient moisture. The action is suspended if a soil be air- 

 dried. 



^odlewski, Jour. Chem. Soc., 1896, Abstracts, ii. 668. 

 2 Warington, Jour. Chem. Soc., 1891, Trans., 521. 



