Mr.A.A.Julien has lately collected all the extant information about the acids generated in humus, which, according to some chemists, amount to more than a dozen different kinds.These acids, as well as their acid salts (i.e., in combination with potash, soda, and ammonia), act energetically on carbonate of lime and on the oxides of iron.It is also known that some of these acids, which were called long ago by Thenard azohumic, are enabled to dissolve colloid silica in proportion to the nitrogen which they contain. In the formation of these latter acids worms probably afford some aid, for Dr.H.Johnson informs me that by Nessler's test he found 0.018 per cent.of ammonia in their castings.
It may be here added that I have recently been informed by Dr.Gilbert "that several square yards on his lawn were swept clean, and after two or three weeks all the worm-castings on the space were collected and dried.These were found to contain 0.35 of nitrogen.This is from two to three times as much as we find in our ordinary arable surface-soil; more than in our ordinary pasture surface-soil; but less than in rich kitchen-garden mould.Supposing a quantity of castings equal to 10 tons in the dry state were annually deposited on an acre, this would represent a manuring of 78 lbs.of nitrogen per acre per annum; and this is very much more than the amount of nitrogen in the annual yield of hay per acre, if raised without any nitrogenous manure.Obviously, so far as the nitrogen in the castings is derived from surface-growth or from surface-soil, it is not a gain to the latter; but so far as it is derived from below, it is a gain."The several humus-acids, which appear, as we have just seen, to be generated within the bodies of worms during the digestive process, and their acid salts, play a highly important part, according to the recent observations of Mr.Julien, in the disintegration of various kinds of rocks.It has long been known that the carbonic acid, and no doubt nitric and nitrous acids, which are present in rain-water, act in like manner.There is, also, a great excess of carbonic acid in all soils, especially in rich soils, and this is dissolved by the water in the ground.The living roots of plants, moreover, as Sachs and others have shown, quickly corrode and leave their impressions on polished slabs of marble, dolomite and phosphate oflime.They will attack even basalt and sandstone. But we are not here concerned with agencies which are wholly independent of the action of worms.
The combination of any acid with a base is much facilitated by agitation, as fresh surfaces are thus continually brought into contact.This will be thoroughly effected with the particles of stone and earth in the intestines of worms, during the digestive process; and it should be remembered that the entire mass of the mould over every field, passes, in the course of a few years, through their alimentary canals.Moreover as the old burrows slowly collapse, and as fresh castings are continually brought to the surface, the whole superficial layer of mould slowly revolves or circulates; and the friction of the particles one with another will rub off the finest films of disintegrated matter as soon as they are formed.Through these several means, minute fragments of rocks of many kinds and mere particles in the soil will be continually exposed to chemical decomposition; and thus the amount of soil will tend to increase.
As worms line their burrows with their castings, and as the burrows penetrate to a depth of 5 or 6, or even more feet, some small amount of the humus-acids will be carried far down, and will there act on the underlying rocks and fragments of rock.Thus the thickness of the soil, if none be removed from the surface, will steadily though slowly tend to increase; but the accumulation will after a time delay the disintegration of the underlying rocks and of the more deeply seated particles.For the humus- acids which are generated chiefly in the upper layer of vegetable mould, are extremely unstable compounds, and are liable to decomposition before they reach any considerable depth. A thick bed of overlying soil will also check the downward extension of great fluctuations of temperature, and in cold countries will check the powerful action of frost.The free access of air will likewise be excluded.From these several causes disintegration would be almost arrested, if the overlying mould were to increase much in thickness, owing to none or little being removed from the surface. In my own immediate neighbourhood we have a curious proof how effectually a few feet of clay checks some change which goes on in flints, lying freely exposed; for the large ones which have lain forsome time on the surface of ploughed fields cannot be used for building; they will not cleave properly, and are said by the workmen to be rotten.
It is therefore necessary to obtain flints for building purposes from the bed of red clay overlying the chalk (the residue of its dissolution by rain-water) or from the chalk itself.