It is very well known that a young orchard will not, usually, flourish
on the site of an old one; for the older trees are supposed to have
withdrawn from the soil certain elements necessary to their growth;
and as necessary to the growth of the young tree, should it be planted
there. There is no “like” or “dislike” of the soil to the tree; it is
a plain case of starvation. The tree needs, and the soil cannot supply
certain elements of its wood.
But if, after a plant has abstracted from the soil certain
ingredients, the whole plant is decomposed and returned to the earth,
the soil repossesses itself of the lost elements, and is ready to
yield them up again to a plant of the same kind. If the straw of wheat
be burned upon the field, annually, the soil would yield fine crops
for a thousand successive years, that is so far as the _straw is
concerned_. But if the grain is removed, and nothing resupplies the
drain of phosphates which it makes from the soil, the soil will in due
time, according to the original quantities in the soil, cease to yield
_grain_, although the straw may be admirable. But if both straw and
kernel were every year burned upon the field, as grass and its seed is
upon the prairies, wheat would grow for a thousand years in
succession. The same is true of corn, of potatoes, and of any annual
crop. When the annual growth is restored to the soil, it is
repossessed of all its treasure which had been loaned for a season. If
a part of the crop is removed, the soil is poorer by just so much as
the portion removed contained within it of the elements necessary to
that crop, and it must be restored artificially, _i. e. by manuring_;
or by allowing the earth to prepare (by disintegration or
decomposition of its minerals) a new supply; _i. e. by fallowing_. A
forest will grow for ages on the same spot, for it returns annually
its leaves, and, gradually, by force of accidents and the elements,
its twigs, branches, trunks, etc., to the soil again. But let the
whole product be gradually removed, and the soil would soon be unable
to supply the trees their nourishment, except in cases where the soil
was very rich in the materials of growth. The forests of Germany, like
our mines, are under the management of the government. It was
customary, for a time, to allow the peasants the use of the _twigs_
and _smaller branches_; but analysis has shown that in these,
especially, resides the large proportion of potash entering into the
composition of trees; the annual removal of it debilitated the trees
to an extent that obliged the Conservators to change their mode of
proceeding.
On the other hand, in one of Mr. Horsford’s letters from Germany, we
have the question of growing plants upon their own ashes, brought, by
the ablest chemist of the age, directly to the test of experiment.
“In the spring preceding my arrival in Giessen, Professor
Liebig planted some grape scions under the windows of the
laboratory. He fed them, if I may use such an expression,
upon the ashes of the grape vine—or upon the proper
inorganic food of the grape, as shown by analyses of its
ashes. The growth has been enormous, and several of the
vines bore large clusters of grapes in the course of the
season. Indeed, I know not but all, as my attention was
drawn to them particularly only since the fruit has been
gathered. The soil otherwise is little better than a
pavement—a kind of fine gravel, in which scarcely anything
takes root.
“I was shown pots of wheat, in different stages of their
growth, that had been fed variously—some upon the inorganic
matters they needed, according to the analyses of their
ashes—others had merely shared the tribute of the general
soil. The results in numbers I don’t yet know. In
appearance, no one could be at a loss to judge of what might
be expected.”
The fact that depopulated forest-grounds change the character of their
growth, is quite familiar to all; and the reasons of it have been
variously debated.