MINERAL CONSTITUENTS OF SOIL. 205 



I. Mineral constituents of soils. The mineral substances 

 which enter into the composition of all soils, are few in num- 

 ber, and most of them easily detected. They may be divided 

 into 3 classes : 1. earths ; 2. alkalies and metallic oxides ; 

 3. salts and urets. 



As the rocks are mostly made up of silica, alumina, lime 

 and magnesia, the great mass of the soil is composed of these 

 substances, which are commonly called earths. The alkalies, 

 potassa, soda and ammonia, the metallic oxides of iron and 

 manganese, exist in all fertile soils in small quantities. The 

 phosphates of lime and magnesia, nitrate of potash, sulphates 

 of lime and ammonia, chloride of sodium, carbonates and 

 other salts, are almost always present in soils ; and, in some 

 cases, sulphurets, phosphurets and carburets of iron exist in 

 very small quantities. 



In one view of the subject, the soils are a mass • of 

 salts, mostly silicates, silicic being the most abundant and 

 most powerful acid in nature. We should expect, from the 

 composition of the simple minerals which form the rocks, that 

 the soils, considered chemically, would be a mass of silicates. 

 But it will be a more practical view, and better accord with 

 the general representation of agricultural writers, to describe 

 silicic acid among the earths, alumina, lime and magnesia. 

 We have already described these earths, in their principal 

 chemical characters and in their relations to the rocks. We 

 come now to consider them agriculturally, and shall notice 

 their amount in soils, their relations to the vegetable king- 

 dom, and their fitness to perform the duties assigned them in 

 the vegetable economy. 



When soils are examined by chemical analysis, they are 

 found to be composed of the following mineral substances. 



1. Silica or silicic acid, also called silex and siliceous earth, 

 constitutes about 40 or 45 per cent, of the crust of the globe, 

 and 66 per cent, of all the rocks and soils of New England. 

 This proportion varies but slightly, with regard to all soils, 



