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Chapter III
THE AMOUNT OF FINE EARTH BROUGHT UP BY WORMS TO THE
SURFACE.
Rate at which various objects strewed on the surface of grass-
fields are covered up by the castings of worms--The burial of a
paved path--The slow subsidence of great stones left on the
surface--The number of worms which live within a given space--The
weight of earth ejected from a burrow, and from all the burrows
within a given space--The thickness of the layer of mould which the
castings on a given space would form within a given time if
uniformly spread out--The slow rate at which mould can increase to
a great thickness--Conclusion.
We now come to the more immediate subject of this volume, namely,
the amount of earth which is brought up by worms from beneath the
surface, and is afterwards spread out more or less completely by
the rain and wind. The amount can be judged of by two methods,--by
the rate at which objects left on the surface are buried, and more
accurately by weighing the quantity brought up within a given time.
We will begin with the first method, as it was first followed.
Near Mael Hall in Staffordshire, quick-lime had been spread about
the year 1827 thickly over a field of good pasture-land, which had
not since been ploughed. Some square holes were dug in this field
in the beginning of October 1837; and the sections showed a layer
of turf, formed by the matted roots of the grasses, 0.5 inch in
thickness, beneath which, at a depth of 2.5 inches (or 3 inches
from the surface), a layer of the lime in powder or in small lumps
could be distinctly seen running all round the vertical sides of
the holes. The soil beneath the layer of lime was either gravelly
or of a coarse sandy nature, and differed considerably in
appearance from the overlying dark-coloured fine mould. Coal-
cinders had been spread over a part of this same field either in
the year 1833 or 1834; and when the above holes were dug, that is
after an interval of 3 or 4 years, the cinders formed a line of
black spots round the holes, at a depth of 1 inch beneath the
surface, parallel to and above the white layer of lime. Over
another part of this field cinders had been strewed, only about
half-a-year before, and these either still lay on the surface or
were entangled among the roots of the grasses; and I here saw the
commencement of the burying process, for worm-castings had been
heaped on several of the smaller fragments. After an interval of
4.75 years this field was re-examined, and now the two layers of
lime and cinders were found almost everywhere at a greater depth
than before by nearly 1 inch, we will say by 0.75 of an inch.
Therefore mould to an average thickness of 0.22 of an inch had been
annually brought up by the worms, and had been spread over the
surface of this field.
Coal-cinders had been strewed over another field, at a date which
could not be positively ascertained, so thickly that they formed
(October, 1837) a layer, 1 inch in thickness at a depth of about 3
inches from the surface. The layer was so continuous that the
over-lying dark vegetable mould was connected with the sub-soil of
red clay only by the roots of the grasses; and when these were
broken, the mould and the red clay fell apart. In a third field,
on which coal-cinders and burnt marl had been strewed several times
at unknown dates, holes were dug in 1842; and a layer of cinders
could be traced at a depth of 3.5 inches, beneath which at a depth
of 9.5 inches from the surface there was a line of cinders together
with burnt marl. On the sides of one hole there were two layers of
cinders, at 2 and 3.5 inches beneath the surface; and below them at
a depth in parts of 9.5, and in other parts of 10.5 inches there
were fragments of burnt marl. In a fourth field two layers of
lime, one above the other, could be distinctly traced, and beneath
them a layer of cinders and burnt marl at a depth of from 10 to 12
inches below the surface.
A piece of waste, swampy land was enclosed, drained, ploughed,
harrowed and thickly covered in the year 1822 with burnt marl and
cinders. It was sowed with grass seeds, and now supports a
tolerably good but coarse pasture. Holes were dug in this field in
1837, or 15 years after its reclamation, and we see in the
accompanying diagram (Fig. 5), reduced to half of the natural
scale, that the turf was 1 inch thick, beneath which there was a
layer of vegetable mould 2.5 inches thick. This layer did not
contain fragments of any kind; but beneath it there was a layer of
mould, 1.5 inch in thickness, full of fragments of burnt marl,
conspicuous from their red colour, one of which near the bottom was
an inch in length; and other fragments of coal-cinders together
with a few white quartz pebbles. Beneath this layer and at a depth
of 4.5 inches from the surface, the original black, peaty, sandy
soil with a few quartz pebbles was encountered. Here therefore the
fragments of burnt marl and cinders had been covered in the course
of 15 years by a layer of fine vegetable mould, only 2.5 inches in
thickness, excluding the turf. Six and a half years subsequently
this field was re-examined, and the fragments were now found at
from 4 to 5 inches beneath the surface. So that in this interval
of 6.5 years, about 1.5 inch of mould had been added to the
superficial layer. I am surprised that a greater quantity had not
been brought up during the whole 21.5 years, for in the closely
underlying black, peaty soil there were many worms. It is,
however, probable that formerly, whilst the land remained poor,
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 disappeared
before many years had elapsed, as did every one of the larger ones
after a time; so that after thirty years (1871) a horse could
gallop over the compact turf from one end of the field to the
other, and not strike a single stone with his shoes. To anyone who
remembered the appearance of the field in 1842, the transformation
was wonderful. This was certainly the work of the worms, for
though castings were not frequent for several years, yet some were
thrown up month after month, and these gradually increased in
numbers as the pasture improved. In the year 1871 a trench was dug
on the above slope, and the blades of grass were cut off close to
the roots, so that the thickness of the turf and of the vegetable
mould could be measured accurately. The turf was rather less than
half an inch, and the mould, which did not contain any stones, 2.5
inches in thickness. Beneath this lay coarse clayey earth full of
flints, like that in any of the neighbouring ploughed fields. This
coarse earth easily fell apart from the overlying mould when a spit
was lifted up. The average rate of accumulation of the mould
during the whole thirty years was only .083 inch per year (i.e.,
nearly one inch in twelve years); but the rate must have been much
slower at first, and afterwards considerably quicker.
The transformation in the appearance of this field, which had been
effected beneath my eyes, was afterwards rendered the more
striking, when I examined in Knole Park a dense forest of lofty
beech-trees, beneath which nothing grew. Here the ground was
thickly strewed with large naked stones, and worm-castings were
almost wholly absent. Obscure lines and irregularities on the
surface indicated that the land had been cultivated some centuries
ago. It is probable that a thick wood of young beech-trees sprung
up so quickly, that time enough was not allowed for worms to cover
up the stones with their castings, before the site became unfitted
for their existence. Anyhow the contrast between the state of the
now miscalled "stony field," well stocked with worms, and the
present state of the ground beneath the old beech-trees in Knole
Park, where worms appeared to be absent, was striking.
A narrow path running across part of my lawn was paved in 1843 with
small flagstones, set edgeways; but worms threw up many castings
and weeds grew thickly between them. During several years the path
was weeded and swept; but ultimately the weeds and worms prevailed,
and the gardener ceased to sweep, merely mowing off the weeds, as
often as the lawn was mowed. The path soon became almost covered
up, and after several years no trace of it was left. On removing,
in 1877, the thin overlying layer of turf, the small flag-stones,
all in their proper places, were found covered by an inch of fine
mould.
Two recently published accounts of substances strewed on the
surface of pasture-land, having become buried through the action of
worms, may be here noticed. The Rev. H. C. Key had a ditch cut in
a field, over which coal-ashes had been spread, as it was believed,
eighteen years before; and on the clean-cut perpendicular sides of
the ditch, at a depth of at least seven inches, there could be
seen, for a length of 60 yards, "a distinct, very even, narrow line
of coal-ashes, mixed with small coal, perfectly parallel with the
top-sward." {45} This parallelism and the length of the section
give interest to the case. Secondly, Mr. Dancer states {46} that
crushed bones had been thickly strewed over a field; and "some
years afterwards" these were found "several inches below the
surface, at a uniform depth."
The Rev. Mr. Zincke informs me that he has lately had an orchard
dug to the unusual depth of 4 feet. The upper 18 inches consisted
of dark-coloured vegetable mould, and the next 18 inches of sandy
loam, containing in the lower part many rolled pieces of sandstone,
with some bits of brick and tile, probably of Roman origin, as
remains of this period have been found close by. The sandy loam
rested on an indurated ferruginous pan of yellow clay, on the
surface of which two perfect celts were found. If, as seems
probable, the celts were originally left on the surface of the
land, they have since been covered up with earth 3 feet in
thickness, all of which has probably passed through the bodies of
worms, excepting the stones which may have been scattered on the
surface at different times, together with manure or by other means.
It is difficult otherwise to understand the source of the 18 inches
of sandy loam, which differed from the overlying dark vegetable
mould, after both had been burnt, only in being of a brighter red
colour, and in not being quite so fine-grained. But on this view
we must suppose that the carbon in vegetable mould, when it lies at
some little depth beneath the surface and does not continually
receive decaying vegetable matter from above, loses its dark colour
in the course of centuries; but whether this is probable I do not
know.
Worms appear to act in the same manner in New Zealand as in Europe;
for Professor J. von Haast has described {47} a section near the
coast, consisting of mica-schist, "covered by 5 or 6 feet of loess,
above which about 12 inches of vegetable soil had accumulated."
Between the loess and the mould there was a layer from 3 to 6
inches in thickness, consisting of "cores, implements, flakes, and
chips, all manufactured from hard basaltic rock." It is therefore
probable that the aborigines, at some former period, had left these
objects on the surface, and that they had afterwards been slowly
covered up by the castings of worms.
Farmers in England are well aware that objects of all kinds, left
on the surface of pasture-land, after a time disappear, or, as they
say, work themselves downwards. How powdered lime, cinders, and
heavy stones, can work down, and at the same rate, through the
matted roots of a grass-covered surface, is a question which has
probably never occurred to them. {48}
The Sinking of great Stones through the Action of Worms.--When a
stone of large size and of irregular shape is left on the surface
of the ground, it rests, of course, on the more protuberant parts;
but worms soon fill up with their castings all the hollow spaces on
the lower side; for, as Hensen remarks, they like the shelter of
stones. As soon as the hollows are filled up, the worms eject the
earth which they have swallowed beyond the circumference of the
stones; and thus the surface of the ground is raised all round the
stone. As the burrows excavated directly beneath the stone after a
time collapse, the stone sinks a little. {49} Hence it is, that
boulders which at some ancient period have rolled down from a rocky
mountain or cliff on to a meadow at its base, are always somewhat
imbedded in the soil; and, when removed, leave an exact impression
of their lower surfaces in the underlying fine mould. If, however,
a boulder is of such huge dimensions, that the earth beneath is
kept dry, such earth will not be inhabited by worms, and the
boulder will not sink into the ground.
A lime-kiln formerly stood in a grass-field near Leith Hill Place
in Surrey, and was pulled down 35 years before my visit; all the
loose rubbish had been carted away, excepting three large stones of
quartzose sandstone, which it was thought might hereafter be of
some use. An old workman remembered that they had been left on a
bare surface of broken bricks and mortar, close to the foundations
of the kiln; but the whole surrounding surface is now covered with
turf and mould. The two largest of these stones had never since
been moved; nor could this easily have been done, as, when I had
them removed, it was the work of two men with levers. One of these
stones, and not the largest, was 64 inches long, 17 inches broad,
and from 9 to 10 inches in thickness. Its lower surface was
somewhat protuberant in the middle; and this part still rested on
broken bricks and mortar, showing the truth of the old workman's
account. Beneath the brick rubbish the natural sandy soil, full of
fragments of sandstone was found; and this could have yielded very
little, if at all, to the weight of the stone, as might have been
expected if the sub-soil had been clay. The surface of the field,
for a distance of about 9 inches round the stone, gradually sloped
up to it, and close to the stone stood in most places about 4
inches above the surrounding ground. The base of the stone was
buried from 1 to 2 inches beneath the general level, and the upper
surface projected about 8 inches above this level, or about 4
inches above the sloping border of turf. After the removal of the
stone it became evident that one of its pointed ends must at first
have stood clear above the ground by some inches, but its upper
surface was now on a level with the surrounding turf. When the
stone was removed, an exact cast of its lower side, forming a
shallow crateriform hollow, was left, the inner surface of which
consisted of fine black mould, excepting where the more protuberant
parts rested on the brick-rubbish. A transverse section of this
stone, together with its bed, drawn from measurements made after it
had been displaced, is here given on a scale of 0.5 inch to a foot
(Fig. 6). The turf-covered border which sloped up to the stone,
consisted of fine vegetable mould, in one part 7 inches in
thickness. This evidently consisted of worm-castings, several of
which had been recently ejected. The whole stone had sunk in the
thirty-five years, as far as I could judge, about 1.5 inch; and
this must have been due to the brick-rubbish beneath the more
protuberant parts having been undermined by worms. At this rate
the upper surface of the stone, if it had been left undisturbed,
would have sunk to the general level of the field in 247 years; but
before this could have occurred, some earth would have been washed
down by heavy rain from the castings on the raised border of turf
over the upper surface of the stone.
The second stone was larger that the one just described, viz., 67
inches in length, 39 in breadth, and 15 in thickness. The lower
surface was nearly flat, so that the worms must soon have been
compelled to eject their castings beyond its circumference. The
stone as a whole had sunk about 2 inches into the ground. At this
rate it would have required 262 years for its upper surface to have
sunk to the general level of the field. The upwardly sloping,
turf-covered border round the stone was broader than in the last
case, viz., from 14 to 16 inches; and why this should be so, I
could see no reason. In most parts this border was not so high as
in the last case, viz., from 2 to 2.5 inches, but in one place it
was as much as 5.5. Its average height close to the stone was
probably about 3 inches, and it thinned out to nothing. If so, a
layer of fine earth, 15 inches in breadth and 1.5 inch in average
thickness, of sufficient length to surround the whole of the much
elongated slab, must have been brought up by the worms in chief
part from beneath the stone in the course of 35 years. This amount
would be amply sufficient to account for its having sunk about 2
inches into the ground; more especially if we bear in mind that a
good deal of the finest earth would have been washed by heavy rain
from the castings ejected on the sloping border down to the level
of the field. Some fresh castings were seen close to the stone.
Nevertheless, on digging a large hole to a depth of 18 inches where
the stone had lain, only two worms and a few burrows were seen,
although the soil was damp and seemed favourable for worms. There
were some large colonies of ants beneath the stone, and possibly
since their establishment the worms had decreased in number.
The third stone was only about half as large as the others; and two
strong boys could together have rolled it over. I have no doubt
that it had been rolled over at a moderately recent time, for it
now lay at some distance from the two other stones at the bottom of
a little adjoining slope. It rested also on fine earth, instead of
partly on brick-rubbish. In agreement with this conclusion, the
raised surrounding border of turf was only 1 inch high in some
parts, and 2 inches in other parts. There were no colonies of ants
beneath this stone, and on digging a hole where it had lain,
several burrows and worms were found.
At Stonehenge, some of the outer Druidical stones are now
prostrate, having fallen at a remote but unknown period; and these
have become buried to a moderate depth in the ground. They are
surrounded by sloping borders of turf, on which recent castings
were seen. Close to one of these fallen stones, which was 17 ft
long, 6 ft. broad, and 28.5 inches thick, a hole was dug; and here
the vegetable mould was at least 9.5 inches in thickness. At this
depth a flint was found, and a little higher up on one side of the
hole a fragment of glass. The base of the stone lay about 9.5
inches beneath the level of the surrounding ground, and its upper
surface 19 inches above the ground.
A hole was also dug close to a second huge stone, which in falling
had broken into two pieces; and this must have happened long ago,
judging from the weathered aspect of the fractured ends. The base
was buried to a depth of 10 inches, as was ascertained by driving
an iron skewer horizontally into the ground beneath it. The
vegetable mould forming the turf-covered sloping border round the
stone, on which many castings had recently been ejected, was 10
inches in thickness; and most of this mould must have been brought
up by worms from beneath its base. At a distance of 8 yards from
the stone, the mould was only 5.5 inches in thickness (with a piece
of tobacco pipe at a depth of 4 inches), and this rested on broken
flint and chalk which could not have easily yielded to the pressure
or weight of the stone.
A straight rod was fixed horizontally (by the aid of a spirit-
level) across a third fallen stone, which was 7 feet 9 inches long;
and the contour of the projecting parts and of the adjoining
ground, which was not quite level, was thus ascertained, as shown
in the accompanying diagram (Fig. 7) on a scale of 0.5 inch to a
foot. The turf-covered border sloped up to the stone on one side
to a height of 4 inches, and on the opposite side to only 2.5
inches above the general level. A hole was dug on the eastern
side, and the base of the stone was here found to lie at a depth of
4 inches beneath the general level of the ground, and of 8 inches
beneath the top of the sloping turf-covered border.
Sufficient evidence has now been given showing that small objects
left on the surface of the land where worms abound soon get buried,
and that large stones sink slowly downwards through the same means.
Every step of the process could be followed, from the accidental
deposition of a single casting on a small object lying loose on the
surface, to its being entangled amidst the matted roots of the
turf, and lastly to its being embedded in the mould at various
depths beneath the surface. When the same field was re-examined
after the interval of a few years, such objects were found at a
greater depth than before. The straightness and regularity of the
lines formed by the imbedded objects, and their parallelism with
the surface of the land, are the most striking features of the
case; for this parallelism shows how equably the worms must have
worked; the result being, however, partly the effect of the washing
down of the fresh castings by rain. The specific gravity of the
objects does not affect their rate of sinking, as could be seen by
porous cinders, burnt marl, chalk and quartz pebbles, having all
sunk to the same depth within the same time. Considering the
nature of the substratum, which at Leith Hill Place was sandy soil
including many bits of rock, and at Stonehenge, chalk-rubble with
broken flints; considering, also, the presence of the turf-covered
sloping border of mould round the great fragments of stone at both
these places, their sinking does not appear to have been sensibly
aided by their weight, though this was considerable. {50}
On the number of worms which live within a given space.--We will
now show, firstly, what a vast number of worms live unseen by us
beneath our feet, and, secondly, the actual weight of the earth
which they bring up to the surface within a given space and within
a given time. Hensen, who has published so full and interesting an
account of the habits of worms, {51} calculates, from the number
which he found in a measured space, that there must exist 133,000
living worms in a hectare of land, or 53,767 in an acre. This
latter number of worms would weigh 356 pounds, taking Hensen's
standard of the weight of a single worm, namely, three grams. It
should, however, be noted that this calculation is founded on the
numbers found in a garden, and Hensen believes that worms are here
twice as numerous as in corn-fields. The above result, astonishing
though it be, seems to me credible, judging from the number of
worms which I have sometimes seen, and from the number daily
destroyed by birds without the species being exterminated. Some
barrels of bad ale were left on Mr. Miller's land, {52} in the hope
of making vinegar, but the vinegar proved bad, and the barrels were
upset. It should be premised that acetic acid is so deadly a
poison to worms that Perrier found that a glass rod dipped into
this acid and then into a considerable body of water in which worms
were immersed, invariably killed them quickly. On the morning
after the barrels had been upset, "the heaps of worms which lay
dead on the ground were so amazing, that if Mr. Miller had not seen
them, he could not have thought it possible for such numbers to
have existed in the space." As further evidence of the large
number of worms which live in the ground, Hensen states that he
found in a garden sixty-four open burrows in a space of 14.5 square
feet, that is, nine in 2 square feet. But the burrows are
sometimes much more numerous, for when digging in a grass-field
near Maer Hall, I found a cake of dry earth, as large as my two
open hands, which was penetrated by seven burrows, as large as
goose-quills.
Weight of the earth ejected from a single burrow, and from all the
burrows within a given space.--With respect to the weight of the
earth daily ejected by worms, Hensen found that it amounted, in the
case of some worms which he kept in confinement, and which he
appears to have fed with leaves, to only 0.5 gram, or less than 8
grains per diem. But a very much larger amount must be ejected by
worms in their natural state, at the periods when they consume
earth as food instead of leaves, and when they are making deep
burrows. This is rendered almost certain by the following weights
of the castings thrown up at the mouths of single burrows; the
whole of which appeared to have been ejected within no long time,
as was certainly the case in several instances. The castings were
dried (excepting in one specified instance) by exposure during many
days to the sun or before a hot fire.
WEIGHT OF THE CASTINGS ACCUMULATED AT THE MOUTH OF A SINGLE BURROW.
(Weight in ounces given in parenthesis--DP.)
(1.) Down, Kent (sub-soil red clay, full of flints, over-lying the
chalk). The largest casting which I could find on the flanks of a
steep valley, the sub-soil being here shallow. In this one case,
the casting was not well dried (3.98)
(2.) Down.--Largest casting which I could find (consisting chiefly
of calcareous matter), on extremely poor pasture land at the bottom
of the valley mentioned under (1.) (3.87)
(3.) Down.--A large casting, but not of unusual size, from a
nearly level field, poor pasture, laid down in a grass about 35
years before (1.22)
(4.) Down. Average weight of 11 not large castings ejected on a
sloping surface on my lawn, after they had suffered some loss of
weight from being exposed during a considerable length of time to
rain (0.7)
(5.) Near Nice in France.--Average weight of 12 castings of
ordinary dimensions, collected by Dr. King on land which had not
been mown for a long time and where worms abounded, viz., a lawn
protected by shrubberies near the sea; soil sandy and calcareous;
these castings had been exposed for some time to rain, before being
collected, and must have lost some weight by disintegration, but
they still retained their form (1.37)
(6.) The heaviest of the above twelve castings (1.76)
(7.) Lower Bengal.--Average weight of 22 castings, collected by
Mr. J. Scott, and stated by him to have been thrown up in the
course of one or two nights (1.24)
(8.) The heaviest of the above 22 castings (2.09)
(9.) Nilgiri Mountains, S. India; average weight of the 5 largest
castings collected by Dr. King. They had been exposed to the rain
of the last monsoon, and must have lost some weight (3.15)
(10.) The heaviest of the above 5 castings (4.34)
In this table we see that castings which had been ejected at the
mouth of the same burrow, and which in most cases appeared fresh
and always retained their vermiform configuration, generally
exceeded an ounce in weight after being dried, and sometimes nearly
equalled a quarter of a pound. On the Nilgiri mountains one
casting even exceeded this latter weight. The largest castings in
England were found on extremely poor pasture-land; and these, as
far as I have seen, are generally larger than those on land
producing a rich vegetation. It would appear that worms have to
swallow a greater amount of earth on poor than on rich land, in
order to obtain sufficient nutriment.
With respect to the tower-like castings near Nice (Nos. 5 and 6 in
the above table), Dr. King often found five or six of them on a
square foot of surface; and these, judging from their average
weight, would have weighed together 7.5 ounces; so that the weight
of those on a square yard would have been 4 lb. 3.5 oz. Dr. King
collected, near the close of the year 1872, all the castings which
still retained their vermiform shape, whether broken down or not,
from a square foot, in a place abounding with worms, on the summit
of a bank, where no castings could have rolled down from above.
These castings must have been ejected, as he judged from their
appearance in reference to the rainy and dry periods near Nice,
within the previous five or six months; they weighed 9.5 oz., or 5
lb. 5.5 oz. per square yard. After an interval of four months, Dr.
King collected all the castings subsequently ejected on the same
square foot of surface, and they weighed 2.5 oz., or 1 lb. 6.5 oz.
per square yard. Therefore within about ten months, or we will say
for safety's sake within a year, 12 oz. of castings were thrown up
on this one square foot, or 6.75 pounds on the square yard; and
this would give 14.58 tons per acre.
In a field at the bottom of a valley in the chalk (see No. 2 in the
foregoing table), a square yard was measured at a spot where very
large castings abounded; they appeared, however, almost equally
numerous in a few other places. These castings, which retained
perfectly their vermiform shape, were collected; and they weighed
when partially dried, 1 lb. 13.5 oz. This field had been rolled
with a heavy agricultural roller fifty-two days before, and this
would certainly have flattened every single casting on the land.
The weather had been very dry for two or three weeks before the day
of collection, so that not one casting appeared fresh or had been
recently ejected. We may therefore assume that those which were
weighed had been ejected within, we will say, forty days from the
time when the field was rolled,--that is, twelve days short of the
whole intervening period. I had examined the same part of the
field shortly before it was rolled, and it then abounded with fresh
castings. Worms do not work in dry weather during the summer, or
in winter during severe frosts. If we assume that they work for
only half the year--though this is too low an estimate--then the
worms in this field would eject during the year, 8.387 pounds per
square yard; or 18.12 tons per acre, assuming the whole surface to
be equally productive in castings.
In the foregoing cases some of the necessary data had to be
estimated, but in the two following cases the results are much more
trustworthy. A lady, on whose accuracy I can implicitly rely,
offered to collect during a year all the castings thrown up on two
separate square yards, near Leith Hill Place, in Surrey. The
amount collected was, however, somewhat less than that originally
ejected by the worms; for, as I have repeatedly observed, a good
deal of the finest earth is washed away, whenever castings are
thrown up during or shortly before heavy rain. Small portions also
adhered to the surrounding blades of grass, and it required too
much time to detach every one of them.
On sandy soil, as in the present instance, castings are liable to
crumble after dry weather, and particles were thus often lost. The
lady also occasionally left home for a week or two, and at such
times the castings must have suffered still greater loss from
exposure to the weather. These losses were, however, compensated
to some extent by the collections having been made on one of the
squares for four days, and on the other square for two days more
than the year.
A space was selected (October 9th, 1870) for one of the squares on
a broad, grass-covered terrace, which had been mowed and swept
during many years. It faced the south, but was shaded during part
of the day by trees. It had been formed at least a century ago by
a great accumulation of small and large fragments of sandstone,
together with some sandy earth, rammed down level. It is probable
that it was at first protected by being covered with turf. This
terrace, judging from the number of castings on it, was rather
unfavourable for the existence of worms, in comparison with the
neighbouring fields and an upper terrace. It was indeed surprising
that as many worms could live here as were seen; for on digging a
hole in this terrace, the black vegetable mould together with the
turf was only four inches in thickness, beneath which lay the level
surface of light-coloured sandy soil, with many fragments of
sandstone. Before any castings were collected all the previously
existing ones were carefully removed. The last day's collection
was on October 14th, 1871. The castings were then well dried
before a fire; and they weighed exactly 3.5 lbs. This would give
for an acre of similar land 7.56 tons of dry earth annually ejected
by worms.
The second square was marked on unenclosed common land, at a height
of about 700 ft. above the sea, at some little distance from Leith
Hill Tower. The surface was clothed with short, fine turf, and had
never been disturbed by the hand of man. The spot selected
appeared neither particularly favourable nor the reverse for worms;
but I have often noticed that castings are especially abundant on
common land, and this may, perhaps, be attributed to the poorness
of the soil. The vegetable mould was here between three and four
inches in thickness. As this spot was at some distance from the
house where the lady lived, the castings were not collected at such
short intervals of time as those on the terrace; consequently the
loss of fine earth during rainy weather must have been greater in
this than in the last case. The castings moreover were more sandy,
and in collecting them during dry weather they sometimes crumbled
into dust, and much was thus lost. Therefore it is certain that
the worms brought up to the surface considerably more earth than
that which was collected. The last collection was made on October
27th, 1871; i.e., 367 days after the square had been marked out and
the surface cleared of all pre-existing castings. The collected
castings, after being well dried, weighed 7.453 pounds; and this
would give, for an acre of the same kind of land, 16.1 tons of
annually ejected dry earth.
SUMMARY OF THE FOUR FOREGOING CASES.
(1.) Castings ejected near Nice within about a year, collected by
Dr. King on a square foot of surface, calculated to yield per acre
14.58 tons.
(2.) Castings ejected during about 40 days on a square yard, in a
field of poor pasture at the bottom of a large valley in the Chalk,
calculated to yield annually per acre 18.12 tons.
(3.) Castings collected from a square yard on an old terrace at
Leith Hill Place, during 369 days, calculated to yield annually per
acre 7.56 tons.
(4.) Castings collected from a square yard on Leith Hill Common
during 367 days, calculated to yield annually per acre 16.1 tons.
The thickness of the layer of mould, which castings ejected during
a year would form if uniformly spread out.--As we know, from the
two last cases in the above summary, the weight of the dried
castings ejected by worms during a year on a square yard of
surface, I wished to learn how thick a layer of ordinary mould this
amount would form if spread uniformly over a square yard. The dry
castings were therefore broken into small particles, and whilst
being placed in a measure were well shaken and pressed down. Those
collected on the Terrace amounted to 124.77 cubic inches; and this
amount, if spread out over a square yard, would make a layer 0.9627
inch in thickness. Those collected on the Common amounted to
197.56 cubic inches, and would make a similar layer 0.1524 inch in
thickness,
These thicknesses must, however, be corrected, for the triturated
castings, after being well shaken down and pressed, did not make
nearly so compact a mass as vegetable mould, though each separate
particle was very compact. Yet mould is far from being compact, as
is shown by the number of air-bubbles which rise up when the
surface is flooded with water. It is moreover penetrated by many
fine roots. To ascertain approximately by how much ordinary
vegetable mould would be increased in bulk by being broken up into
small particles and then dried, a thin oblong block of somewhat
argillaceous mould (with the turf pared off) was measured before
being broken up, was well dried and again measured. The drying
caused it to shrink by 1/7 of its original bulk, judging from
exterior measurements alone. It was then triturated and partly
reduced to powder, in the same manner as the castings had been
treated, and its bulk now exceeded (notwithstanding shrinkage from
drying) by 1/16 that of the original block of damp mould.
Therefore the above calculated thickness of the layer, formed by
the castings from the Terrace, after being damped and spread over a
square yard, would have to be reduced by 1/16; and this will reduce
the layer to 0.09 of an inch, so that a layer 0.9 inch in thickness
would be formed in the course of ten years. On the same principle
the castings from the Common would make in the course of a single
year a layer 0.1429 inch, or in the course of 10 years 1.429 inch,
in thickness. We may say in round numbers that the thickness in
the former case would amount to nearly 1 inch, and in the second
case to nearly 1.5 inch in 10 years.
In order to compare these results with those deduced from the rates
at which small objects left on the surfaces of grass-fields become
buried (as described in the early part of this chapter), we will
give the following summary:-
SUMMARY OF THE THICKNESS OF THE MOULD ACCUMULATED OVER OBJECTS LEFT
STREWED ON THE SURFACE, IN THE COURSE OF TEN YEARS.
The accumulation of mould during 14.75 years on the surface of a
dry, sandy, grass-field near Maer Hall, amounted to 2.2 inches in
10 years.
The accumulation during 21.5 years on a swampy field near Maer
Hall, amounted to nearly 1.9 inch in 10 years.
The accumulation during 7 years on a very swampy field near Maer
Hall amounted to 2.1 inches in 10 years.
The accumulation during 29 years, on good, argillaceous pasture-
land over the Chalk at Down, amounted to 2.2 inches in 10 years.
The accumulation during 30 years on the side of a valley over the
Chalk at Down, the soil being argillaceous, very poor, and only
just converted into pasture (so that it was for some years
unfavourable for worms), amounted to 0.83 inch in 10 years.
In these cases (excepting the last) it may be seen that the amount
of earth brought to the surface during 10 years is somewhat greater
than that calculated from the castings which were actually weighed.
This excess may be partly accounted for by the loss which the
weighed castings had previously undergone through being washed by
rain, by the adhesion of particles to the blades of the surrounding
grass, and by their crumbling when dry. Nor must we overlook other
agencies which in all ordinary cases add to the amount of mould,
and which would not be included in the castings that were
collected, namely, the fine earth brought up to the surface by
burrowing larvae and insects, especially by ants. The earth
brought up by moles generally has a somewhat different appearance
from vegetable mould; but after a time would not be distinguishable
from it. In dry countries, moreover, the wind plays an important
part in carrying dust from one place to another, and even in
England it must add to the mould on fields near great roads. But
in our country these latter several agencies appear to be of quite
subordinate importance in comparison with the action of worms.
We have no means of judging how great a weight of earth a single
full-sized worm ejects during a year. Hensen estimates that 53,767
worms exist in an acre of land; but this is founded on the number
found in gardens, and he believes that only about half as many live
in corn-fields. How many live in old pasture land is unknown; but
if we assume that half the above number, or 26,886 worms live on
such land, then taking from the previous summary 15 tons as the
weight of the castings annually thrown up on an acre of land, each
worm must annually eject 20 ounces. A full-sized casting at the
mouth of a single burrow often exceeds, as we have seen, an ounce
in weight; and it is probable that worms eject more than 20 full-
sized castings during a year. If they eject annually more than 20
ounces, we may infer that the worms which live in an acre of
pasture land must be less than 26,886 in number.
Worms live chiefly in the superficial mould, which is usually from
4 or 5 to 10 and even 12 inches in thickness; and it is this mould
which passes over and over again through their bodies and is
brought to the surface. But worms occasionally burrow into the
subsoil to a much greater depth, and on such occasions they bring
up earth from this greater depth; and this process has gone on for
countless ages. Therefore the superficial layer of mould would
ultimately attain, though at a slower and slower rate, a thickness
equal to the depth to which worms ever burrow, were there not other
opposing agencies at work which carry away to a lower level some of
the finest earth which is continually being brought to the surface
by worms. How great a thickness vegetable mould ever attains, I
have not had good opportunities for observing; but in the next
chapter, when we consider the burial of ancient buildings, some
facts will be given on this head. In the two last chapters we
shall see that the soil is actually increased, though only to a
small degree, through the agency of worms; but their chief work is
to sift the finer from the coarser particles, to mingle the whole
with vegetable debris, and to saturate it with their intestinal
secretions.
Finally, no one who considers the facts given in this chapter--on
the burying of small objects and on the sinking of great stones
left on the surface--on the vast number of worms which live within
a moderate extent of ground on the weight of the castings ejected
from the mouth of the same burrow--on the weight of all the
castings ejected within a known time on a measured space--will
hereafter, as I believe, doubt that worms play an important part in
nature.
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