ORIGIN AND COMPOSITION OF SOILS 23 



Boron occurs chiefly in volcanic regions, but is 

 much more widely distributed in the soil than was 

 formerly believed. It is a constituent of the ash 

 of many plants, and is thought by some to be a 

 true plant food. A more reasonable theory is 

 that its presence in vegetable products is a mere 

 incident to its occurrence in the soil. It is one of 

 the least abundant of the elements, not occurring 

 in sufficient quantity to find a place in our inves- 

 tigation. Boric acid and borax are used to some 

 extent as a preservative. 



Fluorine does not occur free in nature, but it 

 exists chiefly in combination with calcium, form- 

 ing fluorspar, traces of which are found widely 

 distributed. In combination chiefly with lime 

 it occurs in bone and many other substances. 

 It is one of the elements which does not combine 

 with oxygen, and can be isolated only with the 

 greatest difficulty. At most, very small traces of 

 it are found in soils. Fluoride of lime is found, 

 however, in considerable quantities in certain 

 phosphate deposits. 



Aluminum is, probably, next to oxygen and 

 silicon, the most abundant element of the earth's 

 crust, of which it is estimated to form about one- 

 twelfth. It has never been found, in nature, in 

 the free state, but commonly occurs in combina- 

 tion with silicon and oxygen, in which it is an 

 abundant constituent of feldspar, mica, kaolin, 

 clay, slate, and many other rocks and minerals. 



By the weathering of feldspar, mica, and other 



