BIOLOGY OP SOILS 153 



Calcium carbonate, being insoluble, was precipitated, 

 and the potassium nitrate which remained in solution was 

 then crystallised out. The process is necessarily a slow 

 one, and, since the discovery of natural deposits of nitrates, 

 is no longer required. 



Composts. For agricultural purposes it is generally 

 better to bury the organic matter in the soil and allow nitri- 

 fication to take place there. The process above described 

 is, however, still resorted to as a means of dealing with 

 substances which cannot be directly incorporated with the 

 soil. The nitrifying heaps are known as composts, and, when 

 the process is complete, the whole of the material is spread 

 on the land without converting the nitrates into other forms. 



The practice of composting the farmyard manure, i.e., 

 of mixing it with two or three times its weight of ordinary 

 soil, usually without lime, before applying it to the land, 

 is ^greatly favoured by some farmers. It hastens the action 

 of the manure, and, as the farmers say, makes it go further, 

 i.e., makes it more easy to spread uniformly over a 

 large area. Turning farmyard manure is a laborious and, 

 as will be shown later, a wasteful process. It promotes 

 rapid fermentation and loss of nitrogen results. The 

 admixture of soil would tend to minimise the loss, but the 

 operation cannot be recommended for general use. 



NITRIFICATION. 



Nature of the Change. The conversion of the nitrogen 

 of organic compounds, more particularly of proteids, into 

 nitric acid, by the action of micro-organisms, is a very 

 complex process. It is essentially a process of oxidation, 

 but probably hydrolytic and other changes are also 

 involved. Carbon dioxide and water are produced. The 

 change takes place in several successive stages marked by 

 the formation of definite intermediate products, of which 

 ammonia and nitrous acid are the most important. 



