The Formation of Vegetable Mould through the Action of Worms

A figure of one of these castings from a photograph is here given (Fig. 3). The largest received by me was 34 inches in height and 1°35 inch in diameter; another A tower-like casting, probably ejected by a species of Pericheta, from the Botanic Garden, Calcutta: of natural size, engraved from a photograph. In the following year, Mr. Scott measured several of the largest; one was 6 inches in height and nearly 14 in diameter: two others were 5 inches in height and respectively 2 and rather more than 24 inches in diameter. The average weight of the 22 castings sent to me was 35 grammes (14 0z.); and one of them weighed 44°8 grammes (or 2 0z.). All these castings were thrown up either in one night or in two. Where the ground in Bengal is dry, as under large trees, castings of a different kind are found in vast numbers: these con- sist of little oval or conical bodies, from about the 3 to rather above 74, of an inch in length. They are obviously voided by a distinct species of worms.

The period during which worms neat Calcutta display such extraordinary activity lasts for only a little over two months, namely, during the cool season after the rains. - At this time they are generally found within about 10 inches beneath the surface. During the hot season they burrow to a greater depth, and are then found coiled up and apparently hybernating. Mr. Scott has never seen them at a greater depth than 24 feet, but has heard of their having been found at 4 feet. Within the forests, fresh castings may be found even during the hot season. The worms in the Botanic garden, during the cool and dry season, draw many leaves and little sticks into the mouths of their burrows, like our English worms; but they rarely act in this manner during the rainy season.

Mr. Scott saw worm-castings on the lofty mountains of Sikkim in North India. In South India Dr. King found in one place, on the plateau of the Nilgiris, at an elevation of 7000 feet, “a good many castings,’ which are interesting for their great size. The worms which eject them are seen only during the wet season, and are reported to be from 12 to 15 inches in length, and as thick as a man’s little finger. These castings were collected by Dr. King after a period of 110 days without any rain; and they must have been ejected either during the north-east or more probably during the previous south-west monsoon; for their surfaces had suffered some disintegration and they were penetrated by many fine roots. A drawing is here given (Fig. 4) of one which

seems to have best retained its original size and appearance. Notwithstanding some loss from disintegration, five of the largest of these castings (after having been well sun-dried) weighed each on an average 89°5 grammes, A casting from the Nilgiri Mountains in South India; of natural size, engraved from a photograph. or above 3 oz.; and the largest weighed 123°14 grammes, or 41 oz.,—that is above a quarter of a pound! The largest convolutions were rather more than one inch in diameter;

whilst soft,and that their diameters had thus been increased. Some had flowed so much that they now consisted of a pile of almost flat confluent cakes. All were formed of fine, rather light-coloured earth, and were surpris- ingly hard and compact, owing no doubt te the animal matter by which the particles of earth had been cemented together. They did not disintegrate, even when left for some hours in water. Although they had been cast up on the surface of gravelly soil, they contained extremely few bits of rock, the largest of which was only ‘15 inch in diameter.

Dr. King saw in Ceylon a worm about 2 feet in length and 4 inch in diameter; and he was told that it was a very common species during the wet season, These worms must throw up castings at least as large as those on the Nilgiri Mountains; but Dr. King saw none during his short visit to Ceylon. Sufh- cient facts have now been given, showing that worms do much work in bringing up fine earth to the surface in most or all parts of the world, and under the most different climates. ‘

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 worras 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 Maer 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, 4 inch in thickness, beneath which, at a depth of 24 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 appear ance 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

einders had been strewed, only about half-a- year before, and these either still Jay on the surface or were entangled among the roots of the grasses; and I here saw the commence- ment of the burying process, for worm-cast- ings had been heaped on several of the smaller fragments. After an interval of 43 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 3? of an inch. ‘Therefore mould to an average thickness of *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

marl had been strewed several times at un- known dates, holes were dug in 1842; and a layer of cinders could be traced at a depth of 33 inches, beneath which at a depth of 94 inches from the surface there was a line of cinders together with burnt marl. On the sides of one hole there were two layers ot cinders, at 2 and 34 inches beneath the sur- face; and below them at a depth in parts of 94, and in other parts of 104 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 mar] 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 3 inch thick, beneath which there was a layer of vegetable mould 24 inches

thick. This layer did not contain fragments of any kind; but beneath it there was a layer of mould, 1} inch in thickness, full of fragments Section, reduced to half the natural scale, of the vegetable mould in a field, drained and reclaimed fifteen years previously; A, turf; B, vegetable mould without any stones; C, mould with fragments of burnt marl, coal-cinders and quartz pebbles ; D, sub-soil of black, peaty sand with quartz pebbles. of burnt marl, conspicuous from their red eolour, 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 Ad inches from the surface, the original black, peaty, sandy soil with a few quartz pebbles was encountered. Here therefore the frag- ments of burnt marl and cinders had been covered in the course of 15 years by a layer of fine vegetable mould, only 24 inches in thickness, excluding the turf. Six and a haif 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 63 years, about 14 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 213 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 thick- ness for the whole period is ‘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 shghtly 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} years, the sand formed a layer at a depth of # 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 11 inch beneath the turf; so that on an average, *21 inches of mould had been annu- ally 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,* * This case is given in a postscript to my paper in the ‘Transact. Geolog. Soc.’ (Vol. v. p. 505), and coutains a serious error, as in the account received I mistook the figure 30 for 80, ‘he tenant, moreover, formerly said that he had maried the field

thirty years before, but was now positive that this was dune in and a layer of the marl fragments could be traced at a depth, carefully measured, of 12 inches in some parts, and of J4 inches in other parts. This difference in depth de- pended 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 un- broken 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 other- wise 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

1809, that is twenty-eight years before the first examination of the field by my friend. The error, as far as the figure 80 is concerned, was corrected in an article by me, in the ‘ Gardeners’ Chronicle,’ 1844, p. 218. beneath the general level of the field. I+ should, however, be observed that the thick- ness of the blackish sandy soil, which had been thrown up by the worms above the marl- fragments in the course of 324 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 sub-stratum 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 fes- tooned and penetrated by many deep well- like cavities.* During the dissolution of the * These pits or pipes are still in process of formation. During the last forty years I have seen or heard of five cases, in which a

chalk, the insoluble matter, including a vast number of unrolled flints of all sizes, has circular space, several feet in diameter, suddenly fell in, leaving on the field an open hole with perpendicular sides, some fect in depth. This occurred in one of my own fields, whilst it was being rolled, and the hinder quarters of the shaft horse fell in; two or three cart-loads of rubbish were required to fill up the hole. The subsidence occurred where there was a broad depression, as if the surface had fallen in at several former periods. I heard of a hole which must have been suddenly formed at the bottom of a small shallow pool, where sheep had been washed during many years, and into which a man thus occupied fell to his great terror. The rain-water over this whole district sinks perpen- dicularly into the ground, but the chalk is more porous in certain places than in others, Thus the drainage from the overlying clay is directed to certain points, where a greater amount of cal- careous matter is dissolved than elsewhere. Even narrow open channels are sometimes formed in the solid chalk, As the chalk is slowly dissolved over the whole country, but more in some parts than in others, the undissolved residue—that is the over- lying mass of red clay with flints,—likewise sinks slowly down, and tends to fill up the pipes or cavities. But the upper part of the red clay holds together, aided probably by the roots of plants, for a longer time than the lower parts, aud thus forms a roof, which sooner or later falls in, as in the above mentioned five cases. The downward movement of the clay may be com- pared with that of a glacier, but is incomparably slower ; and this movement accounts for a singular fact, namely that the much elongated flints which are embedded in the chalk in a nearly horizontal position, are commonly found standing nearly or quite upright in the red clay. This fact is so common that the work- men assured me that this was their natural position.

I roughly measured one which stood vertically, and it was of the same length and of the same relative thickness as one of my arms, These elongated flints must get placed in their upright pusition, 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 80, 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

on the same principle that a trunk of a tree left on a glacier assumes a position parallel to the line of motion. The flints in the clay which form almost half its bulk, are very often broken, though not rolled or abraded; and this may be ac- counted for by their mutual pressure, whilst the whole mass is subsiding. I may add that the chalk here appears to have been originally covered in parts by a thin bed of fine sand with some perfectly rounded flint pebbles, probably of Tertiary age; for such sand oiten partly fills up the deeper pits or cavities in the chalk,

here been thrown up at an average rate of ‘22 inches 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, 24 inches in thickness. In this latter case the mould was altogether 9% 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 ex- amining 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 trom them. It was evident that the corners of the original fragments of chalk had been wholly dissolved, from pre- senting a large surface to the carbonic acid dissolved in the rain-water and to that gener-

ated in soil containing vegetable matter, as well as to the humus-acids.* The projecting corners would also, relatively to the other parts, have been embraced by a larger num- ber 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 53 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 41 to 428 inches in thickness. In another and undisturbed place, the mould varied much in thickness, namely from 64 to 84 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 °2 to °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 thick- ness, 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 de- scribed, 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 transfor- mation 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 accur- ately. The turf was rather less than half an inch, and the mould, which did not contain any stones, 24 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 (1.e., nearly one inch in twelve years); but the rate must have been much slower at first, and after- wards 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 sur- face indicated that the land had been cul- tivated 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 flag- stones, set edgeways; but worms threw up many castings and weeds grew thickly be- tween 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 over- lying 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 sub- stances 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, fora length of 60 yards, “a distinct, very ‘even, narrow line of coal-ashes, mixed with “small coal, perfectly parallel with the top- sward.”* This parallelism and the length of the section gives interest to the case. Secondly, Mr. Dancer states} 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.” Worms appear to act in the same manner in New Zealand as in Europe; for Professor J.