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, ½ inch in thickness, beneath which, at a
depth of 2½ 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 4¾ 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 ¾ 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 3½ inches, beneath which
at a depth of 9½ 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 3½ inches beneath the surface; and below them at a
depth in parts of 9½, and in other parts of 10½ 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.
[Picture: Fig. 5: Section of the vegetable mould in a field. Fig. 6:
Traverse section across a large stone]
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 ½ inch thick, beneath which
there was a layer of vegetable mould 2½ inches thick. This layer did not
contain fragments of any kind; but beneath it there was a layer of mould,
1½ 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 4½ 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 2½ 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 6½ years, about 1½ 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 21½ 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 2½ years, the sand formed a layer at a
depth of ¾ 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 1½ 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, {126} 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 32½
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. {128} 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, 2¼ inches
in thickness. In this latter case the mould was altogether 9¼ 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. {131} 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 5½ 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 4⅛ to 4⅜ inches in thickness. In another and undisturbed
place, the mould varied much in thickness, namely, from 6½ to 8½ 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° to 15°); 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,
2½ 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.” {136a} This parallelism and the
length of the section give interest to the case. Secondly, Mr. Dancer
states {136b} 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 {138a} 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. {138b}
_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. {139} 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 ½ 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 1½ 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 2½ inches, but in one place
it was as much as 5½. 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 1½ 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 28½ inches
thick, a hole was dug; and here the vegetable mould was at least 9½
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 9½ 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 5½ 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 ½ 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 2½ 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.
{147}
_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,
{148} 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, {149} 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 14½ 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.
(1.) Down, Kent (sub-soil red clay, full of flints, 3.98
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
(2.) Down.—Largest casting which I could find (consisting 3.87
chiefly of calcareous matter), on extremely poor pasture
land at the bottom of the valley mentioned under (1.)
(3.) Down.—A large casting, but not of unusual size, from 1.22
a nearly level field, poor pasture, laid down in a grass
about 35 years before
(4.) Down. Average weight of 11 not large castings 0.7
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
(5.) Near Nice in France.—Average weight of 12 castings of 1.37
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
(6.) The heaviest of the above twelve castings 1.76
(7.) Lower Bengal.—Average weight of 22 castings, 1.24
collected by Mr. J. Scott, and stated by him to have been
thrown up in the course of one or two nights
(8.) The heaviest of the above 22 castings 2.09
(9.) Nilgiri Mountains, S. India; average weight of the 5 3.15
largest castings collected by Dr. King. They had been
exposed to the rain of the last monsoon, and must have lost
some weight
(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 7½ ounces; so that the weight of those on a square yard
would have been 4 lb. 3½ 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 9½ oz., or 5 lb. 5½
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 2½ oz., or 1 lb. 6½ 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. 13½ 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
3½ 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 1½ 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 14¾ years on the surface of a dry,
sandy, grass-field near Maer Hall, amounted to 2.2 inches in 10 years.
The accumulation during 21½ 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 larvæ 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 débris, 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.