580 THE PHILOSOPHY OF FARMING. 
otherwise buried beneath the constant accession 
of this all-important substance. Permeability is 
not thus necessary to make a soil fertile; but a 
sufficient and constant filtration, or drainage, in 
order that the access of one element, or the 
supply of one portion of the raw material may not 
interfere with, or prevent the accession of, due 
supplies of the rest. Whatever moisture exists 
within the soil, beyond that which the wants of 
healthy plants require, must be injurious to 
growth; because it keeps off the accession of 
other essentials, in the exact proportion of that 
excess. From these facts we infer that a thorough 
percolation of water through the soil, and a 
constant accession of air are conditions absolutely 
necessary to full growth, and therefore are the 
two fundamentals of all good farming, inasmuch 
as no methods of treatment, however good and 
appropriate in themselves, can fully answer their 
purposes without them. 
Soils may be classed as generally consisting of 
three distinct kinds—wet, dry, and intermediate. 
Wet soils arise either from the total, or almost 
total impermeability of the sub-strata—whether 
they be stiff clays or impenetrable rocks—or from 
water which has filtered down from higher grounds 
