1. _Planting._—It has been observed that lands which are unfit for
arable culture, and which yield only a trifling rent as natural
pasture, are yet in many cases capable of growing profitable
plantations, and of being greatly increased in permanent value by the
prolonged growth of wood. Not only, however, do all trees not thrive
alike on the same soil, but all do not improve the soil on which they
grow in an equal degree.
Under the Scotch fir, for example, the pasture is not worth 6d. more
per acre than before it was planted—under the beech and spruce, it
is worth even less than before, though the spruce affords excellent
shelter;—under ash, it gradually acquires an increased value of 2s.
or 3s. per acre. In oak copses, it becomes worth 5s. or 6s., but only
during the last eight years (of the twenty-four), before it is cut
down. But under the larch, after the first thirty years, when the
thinnings are all cut, land not worth originally more than 1s. per
acre, becomes worth 8s. to 10s. per acre for permanent pasture.[20]
[20] The result of trials made on the _mica slate_ and _gneiss soils_
(see page 100) of the Duke of Atholl.
The cause of this improvement is to be found in the nature of the soil,
which gradually accumulates beneath the trees by the shedding of their
leaves. The shelter from the sun and rain which the foliage affords,
prevents the vegetable matter which falls from being so speedily
decomposed, or from being so much washed away, and thus permits it to
collect in larger quantities in a given time, than where no such cover
exists. The more complete the shelter, therefore, the more rapid will
the accumulation of soil be in so far as it depends upon this cause.
But the quantity of leaves which annually falls has also much influence
upon the extent to which the soil is capable of being improved by any
given species of tree, as well as the degree of rapidity with which
those leaves, under ordinary circumstances, undergo decay. The broad
membranous leaf of the beech and oak decay more quickly than the
needle-shaped leaves of the pine tribes, and this circumstance may
assist in rendering the larch more valuable as a permanent improver.
We should expect likewise that the quantity and quality of the
inorganic matter contained in the leaves,—brought up year by year from
the roots, and strewed afterwards uniformly over the surface where the
leaves are shed,—would materially affect the value of the soil they
form. The leaves of the oak contain about 5 per cent. of saline and
earthy matter, and those of the Scotch fir less than 2 per cent.; so
that, supposing the actual weight of leaves which falls from each kind
of tree to be equal, we should expect a greater depth of soil to be
formed in the same time by the oak than by the Scotch fir. I am not
aware of any experiments on the quantity of ash left by the leaves of
the larch.
The improvement of the land, therefore, by the planting of trees,
depends in part upon the quantity of _organic_ food which the trees
can extract from the air, and afterwards drop in the form of leaves
upon the soil, and in part upon the kind and quantity of _inorganic_
matter which the roots can bring up from beneath, and in like manner
strew upon the surface. The quantity and quality of the latter will, in
a great measure, determine the kind of grasses which will spring up,
and the consequent value of the pasture in the feeding of stock. In the
larch districts of the Duke of Atholl, the most abundant grasses that
spring up are said to be the holcus mollis and the holcus lanatus, (the
_creeping_ and the _meadow_ soft-grasses.)
2. _Laying down to grass._—On this point two facts seem to be pretty
generally acknowledged:
_First_, that land laid down to artificial grasses for one, two, three,
or more years, is in some degree rested or recruited, and is fitted for
the better production of after-corn crops. Letting it lie a year or two
longer in grass, therefore, is one of the received modes of bringing
back to a sound condition a soil that has been exhausted by injudicious
cropping.
_Second_, that land thus laid down with artificial grasses deteriorates
more or less after two or three years, and only by slow degrees
acquires a thick sward of rich and nourishing herbage. Hence the
opinion, that grass-land improves in quality the longer it is
permitted to lie,—the unwillingness to plough up old pasture,—and the
comparatively high rents which, in some parts of the country, old grass
lands are known to yield.
Granting that grass lands do thus _generally_ increase in value,
_three_ important facts must be borne in mind before we attempt to
assign the cause of this improvement, or the circumstances under which
it is likely to take place for the longest time and to the greatest
extent.
1. The value of the grass in any given spot may increase for an
indefinite period—but it will never improve beyond a certain extent—it
will necessarily be limited, as all other crops are, by the quality
of the land. Hence the mere laying down to grass will not make _all_
land _good_, however long it may lie. The extensive commons, heaths,
and wastes, which have been in grass from the most remote times, are
evidence of this. They have in most cases yielded so poor a herbage
as to have been considered unworthy of being enclosed as a permanent
pasture.
2. Some grass lands will retain the good condition they thus slowly
acquire for a very long period, and _without manuring_, in the same
way, and upon the same principle, that some rich corn lands have
yielded successive crops for 100 years without manure. The rich grass
lands of England, and especially of Ireland, many of which have been in
pasture from time immemorial, without, it is said, receiving any return
for all they have yielded, are illustrations of this fact.
3. But that others, if grazed, cropped with sheep or meadowed, will
gradually deteriorate, unless some proper supply of manure be given to
them,—which required supply must vary with the nature of the soil, and
with the kind of treatment to which it has been subjected.
In regard to the acknowledged benefit of laying down to grass, then,
two points require consideration,—what form does it assume?—and how is
it effected?
1. The improvement takes place by the gradual accumulation of a
dark-brown soil on the surface, rich in vegetable matter: and which
soil thickens or deepens in proportion to the time which elapses from
its being first laid down to grass.
If the soil be very light and sandy, the thickening is sooner arrested;
if it be moderately heavy land, the improvement continues for a longer
period; and some of the heaviest clays in England are known to bear
the richest permanent pastures. On analyzing the soils of the richest
of these pastures, whatever be the degree of tenacity of the clays
or loams (the subsoils) on which they rest, or their deficiency in
vegetable matter,—they are found to be generally characterized by
containing from 8 to 12 per cent. of organic, chiefly vegetable matter,
from 5 to 10 only of alumina, and from 1 to 6 per cent. of lime.
Thus the soil formed on the surface of all rich old pasture lands
is possessed of a remarkable degree of uniformity,—both in physical
character and in chemical composition. This uniformity they gradually
_acquire_, even upon the stiff clays of the Lias and of the Oxford
clay, which originally, no doubt, have been,—as many clay lands still
are,—left to natural pasture from the difficulty and expense of
submitting them to arable culture.
2. But how do they acquire this new character, and why is it the work
of so much time? When the young grass throws up its leaves into the
air, from which it derives so much of its nourishment, it throws down
its roots into the soil in quest of food of another kind. The leaves
may be mown or cropped by animals, and carried off the field, but the
roots remain in the soil, and, as they die, gradually fill its upper
part with vegetable matter. It is not known what average proportion the
roots of the natural grasses bear to the leaves; no doubt it varies
much, both with the kind of grass and with the kind of soil. When wheat
is cut down, the quantity of straw left in the field, in the form of
stubble and roots, is sometimes greater than the quantity carried off
in the sheaf. Upon a grass field two or three tons of hay may be reaped
from an acre; and if we suppose only a tenth part of this quantity to
die every year in the form of roots or parts of roots, or of excretions
from roots, we can easily understand how the vegetable matter in
the soil thus gradually accumulating, should at length become very
considerable in quantity. In arable land this accumulation is prevented
by the constant turning up of the soil, by which the vegetable fibres
being exposed to the free access of air and moisture, are made to
undergo a more rapid decomposition.
But the roots and leaves of the grasses contain inorganic earthy and
saline matter also. Dry hay leaves from an eighth to a tenth part of
its weight of ash when burned. Along with the dead vegetable matter of
the soil, this inorganic matter accumulates also on the surface, in the
form of an exceedingly fine earthy powder; hence _one_ cause of the
universal fineness of the surface mould of old grass fields. And the
earthy portion of this inorganic matter consists chiefly of silica and
lime, with scarcely a trace of alumina, so that, even on the stiffest
clays, a surface soil may be ultimately formed, in which the quantity
of alumina will be comparatively small.
But there are still other agencies at work by which the surface of
stiff soils is made to undergo a change. As the roots penetrate into
the clay, they more or less open up a way into it for the rains. Now
the rains in nearly all lands, when they have a passage downwards,
have a tendency to carry down the clay along with them. They do so,
it has been observed, on sandy and peaty soils, and more quickly when
these soils are laid down to grass. Hence the mechanical action of the
rains,—slowly in many localities, yet surely,—has a tendency to lighten
the soil, by removing a portion of its clay. They constitute one of
those natural agencies by which, as elsewhere explained, important
differences are ultimately established, almost everywhere, between the
surface crop-bearing soil and the subsoil on which it rests.
But further, the heats of summer and the frosts of winter aid this slow
alteration. In the extremes of heat and of cold, the soil contracts
more than the roots of the grasses do; and similar though less striking
differences take place during the changes of temperature experienced
in our climate in a single day. When the rain falls on the parched
field, or when a thaw comes on, the earth expands, while the roots of
the grasses remain nearly fixed; hence the soil rises up among the
leaves, mixes with the vegetable matter, and thus assists in the slow
accumulation of a rich vegetable mould.
The reader has witnessed in winter how, on a field or a by-way side,
the earth rises above the stones, and appears inclined to cover them;
he may even have seen in a deserted and undisturbed highway, the stones
gradually sinking and disappearing altogether, when the repetition of
this alternate contraction and expansion of the soil for a succession
of winters has increased in a great degree the effects which follow
from a single accession of frosty weather.
So it is in the fields. And if a person skilled in the soils of a given
district can make a guess at the time when a given field was laid
down to grass, by the depth at which the stones are found beneath the
surface, it is because this loosening and expansion of the soil, while
the stones remain fixed, tends to throw the latter down by an almost
imperceptible quantity every year that passes.
Such movements as these act in opening up the surface-soil, in mixing
it with the decaying vegetable matter, and in allowing the slow action
of the rains gradually to give its earthy portion a lighter character.
But with these, among other causes, conspire also the action of living
animals. Few persons have followed the plough without occasionally
observing the vast quantities of earthworms with which some fields seem
to be filled. On a close shaven lawn many have noticed the frequent
little heaps of earth which these worms during the night have thrown
out upon the grass. These and other minute animals are continually
at work, especially beneath an undisturbed and grassy sward—and they
nightly bring up from a considerable depth, and discharge on the
surface, their burden of fine fertilizing loamy earth. Each of these
burdens is an actual gain to the rich surface soil, and who can doubt
that in the lapse of years, the unseen and unappreciated labours of
these insect tribes must both materially improve its quality and
increase its depth?
There are natural causes, then, which we _know_ to be at work, that are
sufficient to account for nearly all the facts that have been observed,
in regard to the effect of laying lands down to grass. Stiff clays will
gradually become lighter on the surface, and if the subsoil be rich in
all the kinds of inorganic food which the grasses require, will go
on improving for an indefinite period without the aid of manure. Let
them, however, be deficient, or let them gradually become exhausted of
any one kind of this food, and the grass lands will either gradually
deteriorate after they have reached a certain degree of excellence—or
they must be supplied with that ingredient—that manure of which they
stand in need. It is doubtful if any pasture lands are so naturally
rich as to bear to be cropped for centuries without the addition of
manure, and at the same time without deterioration.[21]
[21] See pages 240 and 241.
On soils that are light, again, which naturally contain little clay,
the grasses will thrive more rapidly, a thick sward will be sooner
formed, but the tendency of the rains to wash out the clay may prevent
them from ever attaining that luxuriance which is observed upon the old
pastures of the clay lands.
On undrained heaths and commons, and generally on any soil which is
deficient in some fertilizing element, neither abundant herbage, nor
good crops of any other kind, can be expected to flourish. Laying such
lands down, or permitting them to remain in grass, may prepare them
for by-and-by yielding one or two average crops of corn, but cannot be
expected _alone_ to convert them into valuable pasture.
Finally, plough up the old pastures, on the surface of which this light
and most favourable soil has been long accumulating—and the heavy soil
from beneath will be again mixed up with it—the vegetable matter will
disappear rapidly by exposure to the air,—and if again laid down to
grass, the slow changes of many years must again be begun through the
agency of the same natural causes, before it become capable of again
bearing the same rich herbage it was known to nourish while it lay
undisturbed.
Many have supposed that by sowing down with the _natural_ grasses, a
thick sward may at once be obtained—and on light loamy lands, rich in
vegetable matter, this method may, to a certain extent, succeed—but on
heavy lands, in which vegetable matter is defective, disappointment
will often follow the sowing of the most carefully selected seeds.
By the agency of the causes above adverted to—_the soil gradually
changes_, so that it is unfit, when first laid down, to bear those
grasses which, ten or twenty years afterwards, will naturally and
luxuriantly grow upon it.