ACTION OF THE ELEMENTS OF SOIL. 219 



distributed into three classes ; silicates, that is silicic acid 

 united to the several bases, as in the simple minerals ; salts, 

 such as phosphates, carbonates, etc. ; and humus or geine, 

 which may include all the organic portions. 



1. The silicates appear to act but slightly if at all upon each 

 other, and hence, were there no agent external to them, would 

 remain without change for ages. But the carbonic acid of 

 the air combines with the bases of the silicates, the potash and 

 soda, and forms soluble salts. These are removed by water, 

 and the silica and alumina remain. By this action, the soil 

 is rendered gradually and constantly finer, more clayey and 

 tenacious. 



2. The earthy carbonates, such as limestone, act in the 

 same manner upon the silicates as carbonic acid, hence the 

 utility of lime to set the alkalies and oxides free. 



3. The alkaline bases potash, soda, lime, magnesia and 

 alumina, which are thus set free, combine with the humic, 

 crenic and apocrenic acids, or, according to Dana, with the 

 geic acid and form geates, which are converted into soluble 

 super-geates, by the action of carbonate of lime. 



4. These bases not only combine with the geic acid, but 

 they act by their presence or catalytic power ; and convert 

 insoluble into soluble geine. The power of hastening decay, 

 is greatest in potash and lime, next in alumina, and finally in 

 oxide of iron while passing from the protoxide to the peroxide, 

 hence the utility of these substances in rendering the vegeta- 

 ble matter soluble and available to the roots of plants. 



5. The oxygen of the air and of the water hastens the pro- 

 cess of decay, and by liberating the carbonic acid, tend to 

 keep up the process of decomposition. 



II. Mutual action of growing plants, silicates, salts and 

 geine. The action of salts and silicates upon each other, 

 even when aided by the humus of the soil, is not very rapid. 

 But when a living plant is introduced into the soil, it exerts a 



