86 THE PRINCIPLES OF AGRICULTURE. 



The most of them need to undergo chemical treatment to 

 render their elements available. The work of preparing 

 and mixing is mostly done at large factories, from which 

 the ordinary commercial fertilizers are shipped and sold 

 to farmers in all parts of the country. 



The aim in preparing these is to produce a mixture 

 containing the three elements, nitrogen, phosphoric acid, 

 and potash, in the proportions in which they are needed 

 for average soils and crops. 



To meet the wants of particular crops, so far as they 

 are understood, special fertilizers are sometimes prepared 

 and sold for each crop, containing the three elements in 

 varying proportions. 



In addition to the substances mentioned, which furnish 

 the three most essential elements of fertilizers, there are 

 certain others which serve some purpose, either directly 

 or indirectly, in improving the fertility of the soil. 



Lime (CaO). Lime, or oxide of calcium, is obtained 

 in large quantities, in various localities, from " limestone 

 quarries." In its natural state it is found combined with 

 carbonic acid, for which it has a strong attraction, form- 

 ing calcium carbonate (CaC0 3 ). 



Calcium oxide, or quicklime, is obtained by heating the 

 calcium carbonate until the carboni'c acid is driven off. 



Quicklime has a strong attraction for both carbonic 

 acid and water. When exposed to the atmosphere, it 

 slowly absorbs both these substances, forming air-slaked 

 lime. . If brought in contact with water, it unites with it 

 so rapidly as to cause great heat, producing calcium 

 hydrate (Ca0 2 H 2 ). 



Lime is beneficial to soil in various ways : 



1. It serves directly as plant food. All plants require 

 a small quantity. 



