Elements of agricultural chemistry and geology

by Jas. F. W. Johnston

On this subject I shall add one important practical remark, which will readily suggest itself to the geologist who has studied the action of air and water on the various clay beds that occur here and there as members of the series of stratified rocks. There are no clays which do not gradually soften under the united influence of air and of running water. It is false economy, therefore, to lay down tiles without soles—however hard and stiff the clay subsoil may appear to be. In the course of ten or fifteen years the stiffest clays will soften, so as to allow the tile to sink; and many very much sooner. The passage for the water is thus gradually narrowed; and when the tile has sunk a couple of inches, the whole must be taken up. Thousands of miles of drains have been thus laid down, both in the low country of Scotland and in the southern counties of England, which have now become nearly useless; and yet the system still goes on. It would appear even as if the farmers and proprietors of each district—unwilling to believe in or to be benefitted by the experience of others—were determined to prove the matter in their own case also, before they will consent to adopt that surer system which, though demanding a slightly greater outlay at first, will return upon the drainer with no after-calls for either time or capital. If my reader live in a district where this practice is now exploded, and if he be inclined to doubt if other counties be farther behind the advance of knowledge than his own, I would invite him to spend a week in crossing the county of Durham, where he may find opportunities not only of satisfying his own doubts, but of scattering here and there a few words of useful advice among the more intelligent of our practical farmers.

2. Subsoiling.—The subsoil plough is an auxiliary to the drain. Though there are few subsoils through which the water will not at length make its way, yet there are some so stiff either naturally or from long consolidation, that the good effect of a well-arranged line of drains is lessened by the slowness with which they allow the superfluous rains to pass through them. In such cases, the use of the subsoil plough is most advantageous in loosening the under layers of clay, and allowing the water to find a ready escape downwards and to either side until it reach the drains.

It is well known that if a piece of stiff clay be cut into the shape of a brick, and then allowed to dry, it will contract and harden—it will form an air-dried brick, almost impervious to any kind of gas—wet it again, it will swell and become still more impervious. Cut up while wet, it will only be divided into so many pieces, each of which will harden when dry, or the whole of which will again attach themselves and stick together if exposed to pressure. But tear it asunder when dry, and it will fall into many pieces, will more or less crumble, and will readily admit the air into its inner parts. So it is with a clay subsoil.

After the land is provided with drains, the subsoil being very retentive, the subsoil plough is used to open it up—to let out the water and to let in the air. If this is not done, the stiff under-clay will contract and bake as it dries, but it will neither sufficiently admit the air nor open a free passage for the roots. But let this operation be performed when the clay is still too wet, a good effect will follow, in the first instance; but after a while, the cut clay will again cohere, and the former will pronounce subsoiling to be a useless expense on his land. Defer the use of the subsoil plough till the clay is dry—it will then tear and break instead of cutting, and its openness will remain. Once give the air free access, and it, after a time, so modifies the drained clay, that it no longer has an equal tendency to cohere.

Mr. Smith of Deanston very judiciously recommends that the subsoil plough should never be used till at least a year after the land has been thoroughly drained. This in many cases will be a sufficient safeguard—will allow a sufficient time for the clay to dry; in other cases two years may not be too much. But this precaution has by some been neglected, and subsoiling being with them a failure, they have sought, in some supposed chemical or other quality of their soil, for the cause of a want of success which is to be found in their own neglect of a most necessary precaution. Let not the practical man be too hasty in desiring to attain those benefits which attend the adoption of improved modes of culture; let him give every method a fair trial; and above all, let him make his trial in the way and with the precautions recommended by the author of the method, before he pronounce its condemnation.

3. Deep-ploughing, like subsoiling, aids the effect of the drains, and so far, and where it goes nearly as deep, more completely effects the same object. But independent of this, it has other uses and merits, and where it has been successfully applied, has improved the land by the operation of other causes.

Subsoiling only lets out the water, and allows access to the air and a free passage to the roots. Deep-ploughing, in addition to these, brings new earth to the surface, forms thus a deeper soil, and more or less alters both its physical qualities and its chemical constitution.

If the plough be made to bring up two inches of clay or sand, it will stiffen or loosen the soil, as the case may be, or it may affect its colour or density. It is clear and simple enough, therefore, that by deep-ploughing the physical properties of the soil may be altered.

But there are certain substances contained in every soil, whether in pasture or under the plough, which gradually make their way down towards the subsoil. They sink till they reach at last that point beyond which the plough does not usually penetrate. Every farmer knows that lime thus sinks. In peat-soils top-dressed with clay, the clay thus sinks. In sandy soils also which have been clayed, the clay sinks; and in all these cases, I believe, the sinking takes place more rapidly when the land is laid down to grass. Where soils are marled, the marl sinks; and the rains, in like manner, gradually wash out that which gives their fertilizing virtue to the under chalk-soils ([see page 88]), and render necessary a new application from beneath, to renovate its productive powers.

If this be the case with earthy substances such as those now mentioned, which are insoluble in water, it will be readily believed that those saline ingredients of the soil which are readily soluble will be still sooner washed out of the upper and conveyed to the under soil. Thus the subsoil may gradually become rich in those substances of which the surface-soil has been robbed. Bring up a portion of this subsoil by deep-ploughing, and you restore to the land a portion of what it has lost—substances, perhaps, which may render it much more fruitful than before. Such is an outline of the theory of deep-ploughing, and it is entirely unexceptionable.