CHAPTER XIV. 



NITRIFICATION AND DENITRIFICATION. 



1. NITRIFICATION. 



ONE of the results of the decomposition of the dead organic matter 

 which is found in the soil is the production of ammonia gas (NH 3 ). 

 A portion of this escapes into the atmosphere, and it is the liberation 

 of this gas which is responsible for the sharp acrid smell which often 

 accompanies decomposition. A large part of the ammonia, however, 

 combines with the sulphuric acid that is almost certain to be present 

 in the soil, forming with it the compound ammonium sulphate, which in 

 turn is partially or wholly changed into ammonium carbonate. This 

 last-named substance, not being volatile, does not escape into the 

 atmosphere. Now, although a limited number of plants are able to 

 absorb ammonium carbonate, and utilise the nitrogen contained in it as 

 food-material, the majority of plants cannot make use of the nitrogen 

 when combined in this form. Hence it would be a serious matter if 

 the nitrogen were allowed to remain in this inaccessible condition. 



In nature, however, the bulk of the ammonium carbonate is changed 

 into nitrate, in which form the contained nitrogen is accessible to the 

 majority of plants. The change from the ammonium compound into 

 the nitrate is called nitrification. Until comparatively recent times it 

 was supposed that this was accomplished by purely chemical means, 

 for it was known that when nitrogenous organic material was brought 

 into intimate contact with oxygen, as, for instance, through the agency 

 of platinum-black, oxidation took place, and by this means it is 

 possible to transform ammonium into nitrate compounds. This may 

 be represented as follows : 



2NH 3 + 30 2 m 2HN0 2 + 2H 2 0. 



ammonia oxygen nitrous acid water 



