第34章

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 aheight 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.