the formation of vegetable mould-第17章
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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 yearthough this is too low an estimatethen 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
