Section I.—of Mechanical Methods of Improving the Soil.

1. Draining.—The first step to be taken, in order to increase the fertility of nearly all the improveable lands of Great Britain, is to drain them. So long as they remain wet, they will continue to be cold. The heat of the sun’s rays, which is intended by nature to warm the soil, will be expended in evaporating the water from its surface; and thus the plants will never receive that genial warmth about their roots which so much favours their rapid growth. Where too much water is present in the soil also, that food of the plant which the soil supplies is so much diluted, that either a much greater quantity of fluid must be taken in by the roots,—much more work done,—or the plant will be scantily nourished. The presence of so much water in the stem and leaf keeps down their temperature likewise, when the sunshine appears; an increased evaporation takes place from their surfaces, a lower natural heat, in consequence, prevails in the interior of the plant, and the chemical changes on which its growth depends proceed with less rapidity.

By the removal of the water, the physical properties of the soil also are in a remarkable degree improved. Dry pipe-clay can be easily reduced to a fine powder, but it naturally, and of its own accord, runs together when water is poured upon it. So it is with clays in the field. The soil expands, becomes close and adhesive, and excludes the air from the roots of the growing plant,—the access of which air appears to be almost an essential element in the healthy growth of the most important vegetable productions.

Open an outlet for the water below, and as it trickles away, the air from above will follow it and take its place among the pores of the soil, carrying to every root the salutary influences it is appointed to bear with it wherever it penetrates. When freed from water also, the stiff soil becomes more mellow; and when once stirred up to a considerable depth, more universally porous,—so that air can make its way everywhere, and the roots can find their easy way in every direction. The presence of vegetable matter,—whether existing naturally in a soil thus physically altered, or artificially added to it,—becomes of double value. When drenched with water, this vegetable matter either decomposes very slowly, or produces acid compounds more or less unwholesome to the plant, and even exerts injurious chemical reactions upon the earthy and saline constituents of the soil. In the presence of air, on the contrary, this vegetable matter decomposes rapidly, produces carbonic acid in large quantity, as well as other compounds fit for food, and even renders the inorganic constituents of the soil more fitted to enter the roots, and thus to supply more rapidly what the several parts of the plant require.

Nor is it only stiff and clayey soils to which draining can with advantage be applied. It will be obvious to every one, that when springs rise to the surface in sandy soils, a drain must be made to carry off the water,—it will also readily occur, that where a sandy soil rests upon a hard or clayey bottom, drains may also be necessary; but it is not unfrequently supposed, that when the subsoil is sand or gravel, that drains can only in special cases be necessary.

Every one, however, is familiar with the fact, that when water is applied to the bottom of a flower-pot full of soil, it will gradually find its way to the surface, however light the soil may be. So it is in sandy soils or subsoils in the open field. If water abound at the depth of a few feet, or if it so abound at certain seasons of the year, that water will rise to the surface; and as the sun’s heat dries it off by evaporation, more water will follow to supply its place. This attraction from beneath will always go on when the air is dry and warm, and thus a double evil will ensue—the soil will be kept moist and cold, and instead of a constant circulation of air downwards, there will be a constant current of water upwards. Thus will the roots, the under soil, and the organic matter it contains, be all deprived of the benefits which the access of the air is fitted to confer. The remedy for these evils is to be found in an efficient system of drainage.

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.

But suppose the land to have originally contained something noxious to vegetation, which in process of time has been washed down into the subsoil, then to bring this again to the surface would be materially to injure the land. This also is true, and a sound discretion must no doubt be employed, in judging when and where such evil effects are likely to follow.

Such cases, however, are more rare than many suppose. There are few subsoils which a full and fair exposure to a winter’s frost will not in a great degree deprive of all their noxious qualities, and render fit to ameliorate the general surface of the poorer lands. If the reader doubt this fact, let him visit Yester, and give a calm consideration to the efforts produced by the use of deep-ploughing on the home-farm of the Marquis of Tweeddale.

In many cases the farmer fears, as he does in the county of Durham, to bring up a single inch of the yellow clay that lies beneath his soil. In the first inch lodges, among other substances, the iron worn from his plough, which in some soils, and after a lapse of years, amounts to a considerable quantity. Till it is exposed to the air, this iron is hurtful to vegetation, and one of the benefits of a winter’s exposure of such subsoils to the air, is the effect produced upon the iron it contains.

It is the want of drainage, however, and of the free access of air, that most frequently renders subsoils for a time injurious to vegetation. Let the lands be well drained—let the subsoils be washed for a few years by the rain-water passing through them,—and there are few of those which are clayey in their nature that may not ultimately be brought to the surface, not only with safety, but with advantage to the soil.

4. Ploughing.—Other benefits, again, attend upon the ordinary ploughings, hoeings, and workings of the land. Its parts are more minutely divided—the air gets access to every particle—it is rendered lighter, more open, more permeable to the roots. The vegetable matter it contains decomposes more rapidly by a constant turning of the soil, so that wherever the fibres of the roots penetrate, they find organic food provided for them, and an abundant supply of the oxygen of the atmosphere to aid in preparing it. The production of ammonia and of nitric acid also (see pages 33 to 36), and the absorption of one or both from the air, take place to a greater extent, the finer the soil is pulverised, and the more it has been exposed to the action of the atmosphere. The general advantage, indeed, to be derived from the constant working of the soil, may be inferred from the fact, that Tull reaped twelve successive crops of wheat from the same land by the repeated use of the plough and the horse-hoe. There are few soils so stubborn as not to shew themselves grateful in proportion to the amount of this kind of labour that may be bestowed upon them.

5. Mixing.—It has been shewn (page 114), that the physical properties of the soil have an important influence upon its average fertility. The admixture of pure sand with clay soils produces an alteration which is often beneficial, and which is wholly physical. The sand merely opens the pores of the clay, and makes it more permeable to the air.

The admixture of clay with sandy or peaty soils, however, produces both a physical and a chemical alteration. The clay not only consolidates and gives body to the sand or peat, but it also mixes with them certain earthy and saline substances useful or necessary to the plant, which neither the sand nor peat might originally contain in sufficient abundance. It thus alters its chemical constitution, and fits it for nourishing new races of plants.

Such is the case also with admixtures of marl, of shell-sand, and of lime. They slightly consolidate the sands and open the clays, and thus improve the mechanical texture of both kinds of soil, but their main operation is chemical; and the almost universal benefit they produce depends upon the new chemical element they introduce into the constitution of the soil.

It is a matter of almost universal remark, that in our climate soils are fertile—clayey or loamy soils, that is—only when they contain an appreciable quantity of lime. In whatever way it acts, therefore, the mixing of lime in any of the forms above mentioned, with a soil in which little or no lime exists, is one of the surest practical methods of bringing it nearer in composition to those soils from which the largest returns of agriculture produce are usually obtained. Some of the chemical effects of the lime upon the soil will be explained in a subsequent section. (See page 195.)