much more than the amount of nitrogen in the annual yield of hay per
acre, if raised without any nitrogenous manure. Obviously, so far as the
nitrogen in the castings is derived from surface-growth or from
surface-soil, it is not a gain to the latter; but so far as it is derived
from below, it is a gain.
The several humus-acids, which appear, as we have just seen, to be
generated within the bodies of worms during the digestive process, and
their acid salts, play a highly important part, according to the recent
observations of Mr. Julien, in the disintegration of various kinds of
rocks. It has long been known that the carbonic acid, and no doubt
nitric and nitrous acids, which are present in rain-water, act in like
manner. There is, also, a great excess of carbonic acid in all soils,
especially in rich soils, and this is dissolved by the water in the
ground. The living roots of plants, moreover, as Sachs and others have
shown, quickly corrode and leave their impressions on polished slabs of
marble, dolomite and phosphate of lime. They will attack even basalt and
sandstone. {222} But we are not here concerned with agencies which are
wholly independent of the action of worms.
The combination of any acid with a base is much facilitated by agitation,
as fresh surfaces are thus continually brought into contact. This will
be thoroughly effected with the particles of stone and earth in the
intestines of worms, during the digestive process; and it should be
remembered that the entire mass of the mould over every field, passes, in
the course of a few years, through their alimentary canals. Moreover as
the old burrows slowly collapse, and as fresh castings are continually
brought to the surface, the whole superficial layer of mould slowly
revolves or circulates; and the friction of the particles one with
another will rub off the finest films of disintegrated matter as soon as
they are formed. Through these several means, minute fragments of rocks
of many kinds and mere particles in the soil will be continually exposed
to chemical decomposition; and thus the amount of soil will tend to
increase.
As worms line their burrows with their castings, and as the burrows
penetrate to a depth of 5 or 6, or even more feet, some small amount of
the humus-acids will be carried far down, and will there act on the
underlying rocks and fragments of rock. Thus the thickness of the soil,
if none be removed from the surface, will steadily though slowly tend to
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