Part III.] OR POISONED BY VEGETABLE GROWTH? 
If it were owing to the poisonous excretions 
of the roots that the same crops cannot be taken 
year after year from the same land, this cause 
would apply equally to all lands; but there is 
an infinite variety in soils in this respect. And 
this infinite variety, and the infinite gradations 
in richness and productiveness of different soils, 
prove that the nutriment of plants comes from 
the soil, not from the air. Yet the very same 
physiologist who makes the plant imbibe its 
food from the air by its leaves, will force it to 
swallow poison from the soil with its roots ! If 
plants feed through the medium of their leaves 
from the air, why manure the soil ? 
It is perhaps possible that the reason why 
each plant appears to have its favourite soil is, 
that it finds there in the greatest abundance the 
particular inorganic or incombustible matters 
adapted to its peculiar constitution; that the 
reason why particular plants will not grow in 
particular soils is, the absence of the particular 
inorganic matters adapted to their peculiar con¬ 
stitution ; and that the reason why particular 
plants cease to grow on particular lands is, their 
having taken up those peculiar inorganic con¬ 
stituents necessary to them, and these being 
together with the crop abstracted from the soil 
