第35章

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 howthick 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 (asdescribed 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 extentof 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.