THE NITROGEN CYCLE 217 



Nitrification. — Not only can plants absorb tbeir nitro- 

 gen in the form of ammonia, but they can also make use 

 of products of oxidation of ammonia, viz., nitrites and 

 nitrates. The chemical equations showing the relation 

 between ammonia and nitrous and nitric acids are as 

 follows : — 



NH3 + 2O2 = HNO2 + H2O + = HNO3 



It is possible in the laboratory directly to oxidise ammonia to 

 nitrous acid by passing electric sparks through a mixture of 

 ammonia and oxygen, or by passing the mixture over heated 

 spongy platinum. 



It was Pasteur who first suggested that the oxidation of 

 ammonia to nitric acid, which evidently takes place in nature, 

 was really due to micro-organisms, and two French chemists, 

 Schlosing and Muntz, actually proved that this was the case. 

 They found that if solutions containing ammonia were allowed 

 to percolate through soil, which was well aerated at regular 

 intervals, the ammonia was mainly converted into nitrate ; 

 but that if any hving energy in the soil was paralysed, e.g. 

 by the introduction of chloroform vapour, or by other anti- 

 septics, no nitrification took place. The study of the conditions 

 of nitrification has engaged the attention of a great number 

 of workers both in England and on the Continent, and is of 

 the very greatest importance from the point of view of agricul- 

 ture, and the kindred subject of sewage purification. In order 

 to have a Hving idea of the sequence of changes which take 

 place when the nitrogenous solution undergoes nitrification, 

 the following experiment may be undertaken : 10 c.c. of 

 urine may be added to a Htre of water in a Winchester bottle, 

 together with about a gram of good garden mould, and the 

 solution, which will occupy rather less than half of the bottle, 

 may be continually aerated, either by drawing air through by 

 means of a Bunsen water pump, or by attaching the bottle to a 



