the formation of vegetable mould-第19章
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worms were scanty； and the mould would then have accumulated
slowly。 The average annual increase of thickness for the whole
period is 0。19 of an inch。
Two other cases are worth recording。 In the spring of 1835； a
field； which had long existed as poor pasture and was so swampy
that it trembled slightly when stamped on； was thickly covered with
red sand so that the whole surface appeared at first bright red。
When holes were dug in this field after an interval of about 2。5
years； the sand formed a layer at a depth of 0。75 in。 beneath the
surface。 In 1842 （i。e。； 7 years after the sand had been laid on）
fresh holes were dug； and now the red sand formed a distinct layer；
2 inches beneath the surface； or 1。5 inch beneath the turf； so that
on an average； 0。21 inch of mould had been annually brought to the
surface。 Immediately beneath the layer of red sand； the original
substratum of black sandy peat extended。
A grass field； likewise not far from Maer Hall； had formerly been
thickly covered with marl； and was then left for several years as
pasture； it was afterwards ploughed。 A friend had three trenches
dug in this field 28 years after the application of the marl； ｛42｝
and a layer of the marl fragments could be traced at a depth；
carefully measured； of 12 inches in some parts； and of 14 inches in
other parts。 This difference in depth depended on the layer being
horizontal； whilst the surface consisted of ridges and furrows from
the field having been ploughed。 The tenant assured me that it had
never been turned up to a greater depth than from 6 to 8 inches；
and as the fragments formed an unbroken horizontal layer from 12 to
14 inches beneath the surface； these must have been buried by the
worms whilst the land was in pasture before it was ploughed； for
otherwise they would have been indiscriminately scattered by the
plough throughout the whole thickness of the soil。 Four…and…a…half
years afterwards I had three holes dug in this field； in which
potatoes had been lately planted； and the layer of marl…fragments
was now found 13 inches beneath the bottoms of the furrows； and
therefore probably 15 inches beneath the general level of the
field。 It should； however； be observed that the thickness of the
blackish sandy soil； which had been thrown up by the worms above
the marl…fragments in the course of 32。5 years； would have measured
less than 15 inches； if the field had always remained as pasture；
for the soil would in this case have been much more compact。 The
fragments of marl almost rested on an undisturbed substratum of
white sand with quartz pebbles； and as this would be little
attractive to worms； the mould would hereafter be very slowly
increased by their action。
We will now give some cases of the action of worms； on land
differing widely from the dry sandy or the swampy pastures just
described。 The chalk formation extends all round my house in Kent；
and its surface； from having been exposed during an immense period
to the dissolving action of rain…water； is extremely irregular；
being abruptly festooned and penetrated by many deep well…like
cavities。 ｛43｝ During the dissolution of the chalk； the insoluble
matter； including a vast number of unrolled flints of all sizes；
has been left on the surface and forms a bed of stiff red clay；
full of flints； and generally from 6 to 14 feet in thickness。 Over
the red clay； wherever the land has long remained as pasture； there
is a layer a few inches in thickness； of dark…coloured vegetable
mould。
A quantity of broken chalk was spread； on December 20； 1842； over a
part of a field near my house； which had existed as pasture
certainly for 30； probably for twice or thrice as many years。 The
chalk was laid on the land for the sake of observing at some future
period to what depth it would become buried。 At the end of
November； 1871； that is after an interval of 29 years； a trench was
dug across this part of the field； and a line of white nodules
could be traced on both sides of the trench； at a depth of 7 inches
from the surface。 The mould； therefore； （excluding the turf） had
here been thrown up at an average rate of 0。22 inch per year。
Beneath the line of chalk nodules there was in parts hardly any
fine earth free of flints； while in other parts there was a layer；
2。25 inches in thickness。 In this latter case the mould was
altogether 9。25 inches thick； and in one such spot a nodule of
chalk and a smooth flint pebble； both of which must have been left
at some former time on the surface； were found at this depth。 At
from 11 to 12 inches beneath the surface； the undisturbed reddish
clay； full of flints； extended。 The appearance of the above
nodules of chalk surprised me； much at first； as they closely
resembled water…worn pebbles； whereas the freshly…broken fragments
had been angular。 But on examining the nodules with a lens； they
no longer appeared water…worn； for their surfaces were pitted
through unequal corrosion； and minute； sharp points； formed of
broken fossil shells； projected from them。 It was evident that the
corners of the original fragments of chalk had been wholly
dissolved； from presenting a large surface to the carbonic acid
dissolved in the rain…water and to that generated in soil
containing vegetable matter； as well as to the humus…acids。 ｛44｝
The projecting corners would also； relatively to the other parts；
have been embraced by a larger number of living rootlets； and these
have the power of even attacking marble； as Sachs has shown。 Thus；
in the course of 29 years； buried angular fragments of chalk had
been converted into well…rounded nodules。
Another part of this same field was mossy； and as it was thought
that sifted coal…cinders would improve the pasture； a thick layer
was spread over this part either in 1842 or 1843； and another layer
some years afterwards。 In 1871 a trench was here dug； and many
cinders lay in a line at a depth of 7 inches beneath the surface；
with another line at a depth of 5。5 inches parallel to the one
beneath。 In another part of this field； which had formerly existed
as a separate one； and which it was believed had been pasture…land
for more than a century； trenches were dug to see how thick the
vegetable mould was。 By chance the first trench was made at a spot
where at some former period； certainly more than forty years
before； a large hole had been filled up with coarse red clay；
flints； fragments of chalk； and gravel； and here the fine vegetable
mould was only from 4。125 to 4。375 inches in thickness。 In another
and undisturbed place； the mould varied much in thickness； namely；
from 6。5 to 8。5 inches； beneath which a few small fragments of
brick were found in one place。 From these several cases； it would
appear that during the last 29 years mould has been heaped on the
surface at an average annual rate of from 0。2 to 0。22 of an inch。
But in this district when a ploughed field is first laid down in
grass； the mould accumulates at a much slower rate。 The rate；
also； must become very much slower after a bed of mould； several
inches in thickness； has been formed； for the worms then live
chiefly near the surface； and burrow down to a greater depth so as
to bring up fresh earth from below； only during the winter when the
weather is very cold （at which time worms were found in this field
at a depth of 26 inches） and during summer； when the weather is
very dry。
A field； which adjoins the one just described； slopes in one part
rather steeply （viz。； at from 10 degrees to 15 degrees）； this part
was last ploughed in 1841； was then harrowed and left to become
pasture…land。 For several years it was clothed with an extremely
scant vegetation； and was so thickly covered with small and large
flints （some of them half as large as a child's head） that the
field was always called by my sons 〃the stony field。〃 When they
ran down the slope the stones clattered together； I remember
doubting whether I should live to see these larger flints covered
with vegetable mould and turf。 But the smaller stones dis
