THE SOIL. 63 



mentation. This next step, the oxidation of ammonia to 

 nitric acid, is brought about by the " Nitrifying" bacteria. 

 These organisms require for their growth a suitable tem- 

 perature, moisture, darkness and a good supply of air. 

 The nitrification of ammonia takes place in two stages. 

 The first stage, the oxidation to nitrous acid, is brought 

 about by the " Nitrous " bacteria ; the oxidation of the 

 nitrous to nitric acid is the work of the "Nitric" bacteria. 

 To prevent the accumulation of too much acid, where 

 fermentation is taking place, the soil must provide a suffi- 

 ciency of lime to neutralise the acid as it is produced. The 

 changes taking place in organic matter in its preparation 

 for plant feeding may be thus represented : 



ORGANIC MATTER 



HUMUS 



I 



It is evident that farmyard manure, and organic matter 

 generally, is only valuable as plant food provided it comes 

 under the action of the bacteria just considered. Besides 

 producing nitrogenous and other plant food from organic 

 matter, these bacteria are indirectly of the greatest help 

 in rendering the mineral part of the soil of use to the 

 crop. The acids produced during their activity, and in 



