54 ELEMENTARY AGRICULTURAL CHEMISTRY 



from its surface, the total loss of water may be diminished if 

 the upper layers be frequently stirred ; they thus become dry, 

 the particles have fewer points of contact, and the flow of 

 water from below is greatly impeded by abolishing, for a time, 

 the production of the concave surfaces of water films (at the 

 contacts of the particles), which, as described above, are so 

 powerful in assisting the movements of the water, 



NitPification. — Perhaps the most important reactions 

 going on in a soil are those connected with the decay of 

 organic matter and the change in the state of combination of 

 the nitrogen. The organic matter is continually being oxidised, 

 the carbon being mainly converted into carbon dioxide, though 

 certain organic acids are also formed, and may be injurious if 

 the soil be deficient in limestone or other substance capable of 

 acting as a base. The nitrogen originally present as complex 

 organic bodies is eventually converted into nitrates, and this 

 change is known as nitrification. It is really a process of oxida- 

 tion, and may be divided into three stages : 



1. The conversion of the complex nitrogenous organic 

 compound into ammonium compounds. 



2. The oxidation of ammonia into nitrites. 



3. The oxidation of nitrites into nitrates. 



The first step is very easily brought about in some cases ; e.g., 

 urea, the characteristic constituent of urine, only requires to 

 combine with water to give ammonium carbonate : 



CO(NH,), + 2H,0 = (]SrH,),C03. 

 Urea Water Ammonium 



carbonate 



This occurs very readily, and is the cause of the strong 

 ammoniacal smell perceptible in stables. With other nitro- 

 genous compounds the change does not occur so readily. In 

 almost all cases, the reaction is brought about by the life 

 processes of some micro-organisms ; in some cases, moulds, in 

 othei»s, bacteria are the agents which do the work. 



The ammonium compounds thus formed are absorbed by the 



