THE PHILOSOPHY OF FARMING. 579 
be augmented, must, therefore, be essential, before 
any just knowledge of its proper treatment for 
the cultivation of plants can be acquired. 
To be a proper medium for the growth of 
plants, the soil must contain water, or moisture, 
carbonic acid gas, or carbonaceous substances, 
from which that gas may be liberated ; or azotised 
matter, from which azote also may be eleminated 
within the plant. These the soil cannot contain 
in due proportions, except it be to a certain extent 
permeable. If the soil be not sufficiently perme- 
able, it will contain an excess of moisture or water, 
and no plants but aquatics can thrive there. 
Stagnant water, therefore, destroys all such plants 
as farmers wish to cultivate, and therefore must 
be dispensed with ; and this destruction of useful 
plants is occasioned by the displacement of the 
other elements. Hence the sterility of imper- 
meable clays,—no air, or gaseous elements can 
penetrate them ; and the roots of plants are thence 
cut off from extending within them ; or, if extend- 
ing, from obtaining therein their complement of 
elements. This want of a due circulation of air 
throughout the insterstices of the soil is the cause 
of the destruction of trees, when they are sur- 
rounded with artificial mounds, or their roots 
