Ill SOILS 25 



soluble in acids has been removed, be spread out in the air 

 and sun for some weeks and then be again soaked in pure 

 water in an acid solution, it will be found that a small portion 

 of it has become soluble and thus available for plant food. 

 This part was, therefore, in the first instance, in a dormant 

 condition. The alteration brought about by the exposure of Theparti- 



, . • -1 •, , -r , f , . , cles of the 



the soil to the air is easily understood if the way in which soil merely 



rocks are weathered be remembered. For the small particles rock!^ ° 



of the earth are merely minute fragments of rock upon which 



the same wasting influences act in the same way as in the 



case of the great masses of hard rock from which the soil 



was originally formed. These facts then, explain why tillage why the soil 



operations, which expose the under layers of the soil to the by tillage. 



air and the sun, render the earth more fertile, and why the 



resting or fallowing of land restores fertility by allowing 



time for the air to permeate the soil and so to increase the 



soluble constituents. 



The Double Silicates. — All soils contain silica and alu- 

 mina, and a considerable portion of these elements are com- 

 bined together with other elements to form what are called 

 double silicates. These are four in number, namely : i — Importance 

 Silicate of alumina and ammonia ; 2 — Silicate of alumina silicates. 

 and potash ; 3 — Silicate of alumina and lime ; 4 — Silicate 

 of alumina and soda. Now all these substances are used by 

 the plant with the exception of alumina — which is the basis 

 of clay. But, although not a plant food, alumina, in its 

 form of a silicate, is an important agent in the preparation 

 of the food, for it combines with ammonia, potash, lime and 

 soda in such a way as to give up these elements readily to 

 the roots of plants in a soluble condition. It has been 

 found that silicate of alumina combines most readily with 

 ammonia, and then, in point of order, with potash, lime and 

 soda. Thus, if silicate of alumina and potash be placed 

 v;here it can get ammonia, it will part with the potash for 

 ammonia and become a silicate of alumina and ammonia. 



