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Young Folks' Library: Wonders of Earth, Sea and Sky

How The Soil Is Made
(From the Formation of Vegetable Mould.)
By
CHARLES DARWIN.
Worms have played a more important part in the history of the world than most persons
would at first suppose. In almost all humid countries they are extraordinarily numerous,
and for their size possess great muscular power. In many parts of England a weight of
more than ten tons (10,516 kilogrammes) of dry earth annually passes through their
bodies and is brought to the surface on each acre of land; so that the whole superficial
bed of vegetable mould passes through their bodies in the course of every few years.
From the collapsing of the old burrows the mould is in constant though slow movement,
and the particles composing it are thus rubbed together. By these means fresh surfaces are
continually exposed to the action of the carbonic acid in the soil, and of the humus-acids
which appear to be still more efficient in the decomposition of rocks. The generation of
the humus-acids is probably hastened [pg
The finely levigated castings, when brought to the surface in a moist condition, flow
during rainy weather down any moderate slope; and the smaller particles are washed far
down even a gently inclined surface. Castings when dry often crumble into small pellets
and these are apt to roll down any sloping surface. Where the land is quite level and is
covered with herbage, and where the climate is humid so that much dust cannot be blown
away, it appears at first sight impossible that there should be any appreciable amount of
sub-aerial denudation; but worm castings are blown, especially while moist and viscid, in
one uniform direction by the prevalent winds which are accompanied by rain. By these
several means the superficial mould is prevented from accumulating to a great thickness;
and a thick bed of mould checks in many ways the disintegration of the underlying rocks
and fragments of rock.
The removal of worm-castings by the above means leads to results which are far from
insignificant. It has been shown that a layer of earth,.2 of an inch in thickness, is in many
places annually brought to the surface per acre; and if a small part of this amount flows,
or rolls, or is washed, even for a short distance, down every inclined surface, or is
repeatedly blown in one direction, a great effect will be produced in the course of ages. It
was found by measurements and calculations that on a surface with a mean inclination of
9° 26', 2.4 cubic inches of earth which had been ejected by worms crossed, in the course
of a year, a horizontal line one yard in length; so that two hundred and forty cubic inches
would cross a line one hundred yards in length. This latter amount in a damp state would
weigh eleven and one-half pounds. Thus, a considerable weight of earth is continually
moving down each side of every valley, and will in time reach its bed. Finally, this earth
will be transported by the streams flowing in the valleys into the ocean, the great
receptacle for all matter denuded from the land. It is known from the amount of sediment
annually delivered into the sea by the Mississippi, [pg 138] that its enormous drainage-area must
on an average be lowered.00263 of an inch each year; and this would suffice in four and a
during the digestion of the many half-decayed
leaves which worms consume. Thus the particles of earth, forming the superficial mould,
are subjected to conditions eminently favorable for their decomposition and
disintegration. Moreover, the particles of the softer rocks suffer some amount of
mechanical trituration in the muscular gizzards of worms, in which small stones serve as
mill-stones.
136]
 
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