(Weight in ounces given in parenthesis--DP.)(1.) Down, Kent (sub-soil red clay, full of flints, 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 (3.98)(2.) Down.--Largest casting which I could find (consisting chiefly of calcareous matter), on extremely poor pasture land at the bottom of thevalley mentioned under (1.) (3.87)
(3.) Down.--A large casting, but not of unusual size, from a nearly level field, poor pasture, laid down in a grass about 35 years before (1.22)(4.) Down.Average weight of 11 not large castings 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 (0.7)(5.) Near Nice in France.--Average weight of 12 castings of 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 (1.37)(6.) The heaviest of the above twelve castings (1.76)(7.) Lower Bengal.--Average weight of 22 castings, collected by Mr.J.Scott, and stated by him to have been thrown up in the course of one or two nights (1.24)(8.) The heaviest of the above 22 castings (2.09)(9.) Nilgiri Mountains, S.India; average weight of the 5 largest castings collected by Dr.King.They had been exposed to the rain of the last monsoon, and must have lost some weight (3.15)(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 haveweighed together 7.5 ounces; so that the weight of those on a square yard would have been 4 lb.3.5 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 9.5 oz., or 5 lb.5.5 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 2.5 oz., or 1 lb.6.5 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.13.5 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.