THE SOIL 55 



Constituents of the soil, until another set of organisms act upon 

 them. The chemical reaction involved is very simple : 



This change, however, can only occur if some basic material 

 be present to neutralise the nitrous acid, probably calcium 

 carbonate — 



OaCO, + 2HN0, - Ca(NO,), + CO, + H,0 

 Calcium Nitrous CaJcium Carbon Water 



carbonate acid nitrite dioxide 



— and is accomplished by the action of a micro-organism, which 

 may conveniently be called the nitrous organism^ or Nitroso' 

 coccus. Lastly, the nitrite is converted by oxidation into a 

 nitrate — 



0a(N0,), + 0, =- Ca(NO,), 



Calcium Oxygen Calcium 



nitrite nitrate 



— by the action of the nitric o^'ganism, or Nitrohacter, 



The conditions favourable for nitrification, assuming the 

 necessary organisms to be present, are : 



1. SuitabU Food. — Mineral matters, especially potassium, 

 calcium, sulphates and phosphates must be present. Carbon 

 dioxide, either as gas, in solution, or as bicarbonates, appears 

 also to be essential. Organic matter is not required for either 

 the nitrous or nitric organism. 



2. The Presence of some basic material, — As already stated, 

 calcium or magnesium carbonate generally acts as the base^ 

 The medium must not be more than slightly alkaline, or it maybe 

 neutral, but if strongly alkaline or acid the process is stopped. 



3. Suitable temperature. — Nitrification ceases about the 

 freezing temperature, is most active about SO^O., and stops at 

 about 50° or 55° 0. 



4. Pref<ence of moifihtr^.. 



