248 THE SOIL. 



tion ; the commonest compounds of hydrogen, and 

 indeed almost the only ones from which plants could 

 obtain that element, are water and ammonia. It 

 may be laid down as a rule, that plants can only 

 absorb oxygen, hydrogen, carbon, and nitrogen, in a 

 state of combination, and moreover that those com- 

 pounds, to be absorbed, must be either fluid or gas- 

 eous. 



642. The soil consists of silica, alumina, lime, mag- 

 nesia, oxide of iron, small quantities of various 

 alkaline and earthy salts, and a portion of decaying 

 organic matter. It likewise contains water, and the 

 small quantity of ammonia and carbonic acid which 

 the rain has brought down from the air. Plants 

 cannot derive the elements of organic matter from 

 the earthy constituents of the soil, or from the or- 

 ganic matter which it may contain, unless there is 

 air present ; by the action of air these substances 

 decay, and are gradually changed into gases, which 

 plants can absorb. 



643. It is commonly supposed that plants derive 

 the whole of their food from the soil ; but this is an 

 error: it is a fact well ascertained by chemical ex- 

 periments, that plants derive the greater part of their 

 nourishment from the air, although the soil is equally 

 essential to their growth. 



644. The earthy substances contained in plants are 

 principally obtained from the soil: it is true that the 

 air contains exceedingly minute traces of various 

 earthy and saline substances, which are suspended in 



