654 
FOOD OF PLANTS. 
manures are generally in a direct ratio with the quantity 
applied, and with the character of the upper soil with respect 
to quality and depth, it may be concluded that the superiority 
of manures in the latter case arises from the greater quantities 
of each substance affording- a greater number of affinities and 
combinations that promote the growth of plants. And ex¬ 
perience has also taught that if manures of any kind be applied 
to any subsoil prepared as the upper soil generally is, no such 
fertilizing effects will ensue; that subsoils differ in quality, 
and that some are very pernicious to vegetation, and require 
long exposure, working, and manuring to dispel the noxious 
qualities. And it may be inferred that the failure in the 
action of the manures arises from the substances brought into 
contact having less chemical affinity, and consequently not 
forming the combinations that are necessary for the growth 
of plants, and that the upper stratum, or cultivable soil, has 
undergone a long series of preparations, which has fitted it 
for entering into immediate action w r ith the manures applied 
to furnish food for vegetables; and the mode of that prepara¬ 
tion may never be known. The action of manures, and the 
benefits derived from them, are much affected and modified 
by local circumstances and external agencies, and by a soil 
too wet or too dry, and by a climate being too dry or too 
humid ; and as similar constitutional qualities in soils are 
rendered very different by these causes, many cases will find 
that manures are similarly affected, and give proportional 
results. 
It seems evident that organic matter furnishes food for 
plants; and. recent chemical analysis has found the im¬ 
poverishing effects inflicted on soils by cropping to consist in 
a diminution of that substance, and not of any of the earthy 
constituents. The insensible progress of fermentation and 
the suction of vegetables impoverish it, and deprive it of 
organic remains, and leave the metallic residue or earths,— 
forming very poor soils,—and ochres where the ferruginous 
principle abounds. The organic matter would seem destined 
to repair the losses of the vegetable kingdom, and the earthy 
residue that of the mineral; for there we find diamonds, spars, 
and gypsum, and similar productions. 
The pure earthy part of the dung is very small, and, after a 
perfect putrefaction, bears an inconsiderable proportion to the 
soil it was intended to manure. The great use of soils seems 
to be in affording a bed to plants, in which to fix the roots, 
and derive nourishment slowly and gradually by the roots from 
the earth and from atmospheric air. In order to account for 
the presence of carbon in plants, it is supposed that vegetables 
