the formation of vegetable mould-第32章
按键盘上方向键 ← 或 → 可快速上下翻页，按键盘上的 Enter 键可回到本书目录页，按键盘上方向键 ↑ 可回到本页顶部！
————未阅读完？加入书签已便下次继续阅读！
dust； and when fields are harrowed during dry and windy weather；
clouds of dust may be seen to be blown away。 But in all these
cases the surface…soil is merely transported from one place to
another。 The dust which falls so thickly within our houses
consists largely of organic matter； and if spread over the land
would in time decay and disappear almost entirely。 It appears；
however； from recent observations on the snow…fields of the Arctic
regions； that some little meteoric dust of extra mundane origin is
continually falling。
The dark colour of ordinary mould is obviously due to the presence
of decaying organic matter； which； however； is present in but small
quantities。 The loss of weight which mould suffers when heated to
redness seems to be in large part due to water in combination being
dispelled。 In one sample of fertile mould the amount of organic
matter was ascertained to be only 1。76 per cent。； in some
artificially prepared soil it was as much as 5。5 per cent。； and in
the famous black soil of Russia from 5 to even 12 per cent。 ｛62｝
In leaf…mould formed exclusively by the decay of leaves the amount
is much greater； and in peat the carbon alone sometimes amounts to
64 per cent。； but with these latter cases we are not here
concerned。 The carbon in the soil tends gradually to oxidise and
to disappear； except where water accumulates and the climate is
cool； ｛63｝ so that in the oldest pasture…land there is no great
excess of organic matter； notwithstanding the continued decay of
the roots and the underground stems of plants； and the occasional
addition of manure。 The disappearance of the organic matter from
mould is probably much aided by its being brought again and again
to the surface in the castings of worms。
Worms； on the other hand； add largely to the organic matter in the
soil by the astonishing number of half…decayed leaves which they
draw into their burrows to a depth of 2 or 3 inches。 They do this
chiefly for obtaining food； but partly for closing the mouths of
their burrows and for lining the upper part。 The leaves which they
consume are moistened； torn into small shreds； partially digested；
and intimately commingled with earth； and it is this process which
gives to vegetable mould its uniform dark tint。 It is known that
various kinds of acids are generated by the decay of vegetable
matter； and from the contents of the intestines of worms and from
their castings being acid； it seems probable that the process of
digestion induces an analogous chemical change in the swallowed；
triturated； and half…decayed leaves。 The large quantity of
carbonate of lime secreted by the calciferous glands apparently
serves to neutralise the acids thus generated； for the digestive
fluid of worms will not act unless it be alkaline。 As the contents
of the upper part of their intestines are acid； the acidity can
hardly be due to the presence of uric acid。 We may therefore
conclude that the acids in the alimentary canal of worms are formed
during the digestive process； and that probably they are nearly of
the same nature as those in ordinary mould or humus。 The latter
are well known to have the power of de…oxidising or dissolving per…
oxide of iron； as may be seen wherever peat overlies red sand； or
where a rotten root penetrates such sand。 Now I kept some worms in
a pot filled with very fine reddish sand； consisting of minute
particles of silex coated with the red oxide of iron； and the
burrows； which the worms made through this sand； were lined or
coated in the usual manner with their castings； formed of the sand
mingled with their intestinal secretions and the refuse of the
digested leaves； and this sand had almost wholly lost its red
colour。 When small portions of it were placed under the
microscope； most of the grains were seen to be transparent and
colourless； owing to the dissolution of the oxide； whilst almost
all the grains taken from other parts of the pot were coated with
the oxide。 Acetic acid produced hardly any effect on his sand； and
even hydrochloric； nitric and sulphuric acids； diluted as in the
Pharmacopoeia； produced less effect than did the acids in the
intestines of the worms。
Mr。 A。 A。 Julien has lately collected all the extant information
about the acids generated in humus； which； according to some
chemists； amount to more than a dozen different kinds。 These
acids； as well as their acid salts （i。e。； in combination with
potash； soda； and ammonia）； act energetically on carbonate of lime
and on the oxides of iron。 It is also known that some of these
acids； which were called long ago by Thenard azohumic； are enabled
to dissolve colloid silica in proportion to the nitrogen which they
contain。 ｛64｝ In the formation of these latter acids worms
probably afford some aid； for Dr。 H。 Johnson informs me that by
Nessler's test he found 0。018 per cent。 of ammonia in their
castings。
It may be here added that I have recently been informed by Dr。
Gilbert 〃that several square yards on his lawn were swept clean；
and after two or three weeks all the worm…castings on the space
were collected and dried。 These were found to contain 0。35 of
nitrogen。 This is from two to three times as much as we find in
our ordinary arable surface…soil； more than in our ordinary pasture
surface…soil； but less than in rich kitchen…garden mould。
Supposing a quantity of castings equal to 10 tons in the dry state
were annually deposited on an acre； this would represent a manuring
of 78 lbs。 of nitrogen per acre per annum； and this is very 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。
｛65｝ 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 increase； but the a
