Description of Soil. Wet. Degs. Dry. Degs.
Siliceous sand 99·1 112·6
Calcareous sand 99·3 112·1
Sandy clay 98·2 111·4
Loamy clay 99·1 112·1
Stiff clay 99·3 112·3
Fine bluish-grey clay 99·5 113·0
Garden mould 99·5 113·5
Arable soil 97·7 111·7
Slaty marl 101·8 115·3

In a soil which is naturally dry or has been drained, the superfluous moisture escapes by the drains, and only that comparatively small quantity which is retained by capillary attraction is evaporated, and hence the soil is more frequently and for a longer period in a condition to take advantage of the heating effect of the sun's rays, and in this way the period of germination, and, by consequence also, that of ripening is advanced. The extent of this influence is necessarily variable, but it is generally considerable, and in some districts of Scotland the extensive introduction of draining has made the harvest, on the average of years, from ten to fourteen days earlier than it was before. It is unnecessary to insist on the importance of such a change, which in upland districts may make cultivation successful when it was previously almost impossible. The removal of moisture by drainage affects the physical characters of the soil in another manner; it makes it lighter, more friable, and more easily worked; and this change is occasioned by the downward flow of the water carrying with it to the lower part of the soil the finer argillaceous particles, leaving the coarser and sandy matters above, and in this way a marked improvement is produced on heavy and retentive clays. The access of air to the soil is also greatly promoted by draining. In wet soils the pores are filled with water, and hence the air, which is so important an agent in their amelioration, is excluded; but so soon as this is removed, the air is enabled to reach and act upon the organic matters and other decomposable constituents present. In this way also provision is made for the frequent change of the air which permeates the soil; for every shower that falls expels from it a quantity of that which it contains, and as the moisture flows off by the drains, a new supply enters to take its place, and thus the important changes which the atmospheric oxygen produces on the soil are promoted in a high degree. The air which thus enters acts on the organic matters of the soil, producing carbonic acid, which we have already seen is so intimately connected with many of its chemical changes. In its absence the organic matters undergo different decompositions, and pass into states in which they are slowly acted on, and are incapable of supplying a sufficient quantity of carbonic acid to the soil; and they thus exercise an action on the peroxide of iron, contained in all soils, reduce it to the state of protoxide, or, with the simultaneous reduction of the sulphuric acid, they produce sulphuret of iron, forms of combination which are well known to be most injurious to vegetation.

The removal of water from the lower part of the soil, and the admission of air, which is the consequence of draining, submits that part of it to the same changes which take place in its upper portion, and has the effect of practically deepening the soil to the extent to which it is thus laid dry. The roots of the plants growing on the soil, which stop as soon as they reach the moist part, now descend to a lower level, and derive from that part of it supplies of nourishment formerly unavailable. The deepening of the soil has further the effect of making the plants which grow upon it less liable to be burned up in seasons of drought, a somewhat unexpected result of making a soil drier, but which manifestly depends on its permitting the roots to penetrate to a greater depth, and so to get beyond the surface portion, which is rapidly dried up, and to which they were formerly confined.

It may be added also that the abundant escape of water from the drains acts chemically by removing any noxious matters the soil may contain, and by diminishing the amount of soluble saline matters, which sometimes produce injurious effects. It thus prevents the saline incrustation frequently seen in dry seasons on soils which are naturally wet, and which is produced by the water rising to the surface by capillary attraction, and, as it evaporates, depositing the soluble substances it contained, as a hard crust which prevents the access of air to the interior of the soil.

It is thus obvious that the drainage of the soil modifies its properties both mechanically and chemically. It exerts also various other actions in particular cases which we cannot here stop to particularize. It ameliorates the climate of districts in which it is extensively carried out, and even affects the health of the population in a favourable manner. The sum of its effects must necessarily differ greatly in different soils, and in different districts; but a competent authority[J] has estimated, that, on the average, land which has been drained produces a quarter more grain per acre than that which is undrained. But this by no means exhausts the benefits derived from it, draining being merely the precursor of further improvement. It is only after it has been carried out that the farmer derives the full benefit of the manures which he applies. He gains also by the increased facility of working the soil, and by the rapidity with which it dries after continued rain, thus enabling him to proceed at their proper season with agricultural operations, which would otherwise have to be postponed for a considerable time.

It would be out of place to enlarge here upon the mode in which draining ought to be carried out; it may be remarked, however, that much inconvenience and loss has occasionally been produced by too close adherence to particular systems. No rules can be laid down as to the depth or distance between the drains which can be universally applicable, but the intelligent drainer will seek to modify his practice according to the circumstances of the case. As a general rule, the drains ought to be as deep as possible, but in numerous instances it may be more advantageous to curtail their depth and increase their number. If, for instance, a thick impervious pan resting on a clay were found at the depth of three feet below the surface, it would serve no good purpose to make the drains deeper; but if the pan were thin, and the subjacent layer readily permeable by water, it might be advantageous to go down to the depth of four feet, trusting to the possible action of the air which would thus be admitted, gradually to disintegrate the pan, and increase the depth of soil above it. It is a common opinion that if we reach, at a moderate depth, a tenacious and little permeable clay, no advantage is obtained by sinking the drains into it; but this is an opinion which should be adopted with caution, both because no clay is absolutely impermeable, even the most tenacious permitting to a certain extent the passage of water, and because the clay may have been brought down by water from the upper part of the soil, and may have stopped there merely for want of some deeper escape for the water, and which drains at a lower level might supply. In some cases it may even be advisable to vary the depth of the drains in different parts of the same field, and the judicious drainer may sometimes save a considerable sum by a careful observation of the peculiarities of the different parts of the ground to be drained.

Subsoil and Deep Ploughing.—It frequently happens, when a soil is drained, that the subsoil is so stiff as to permit the passage of water imperfectly, and to prevent the tender roots of the plant from penetrating it, and reaching the new supplies of nourishment which are laid open to them. In such cases the benefits of subsoil ploughing and deep ploughing are conspicuous. The mode of action of these two methods of treatment is similar but not identical. The subsoil plough merely stirs and opens the subsoil, and permits the more ready passage of water and the access of air and of the roots of plants—the former to effect the necessary decompositions, the latter to avail themselves of the valuable matters set free. But deep ploughing produces more extensive changes; it raises new soil to the surface, mixes it with the original soil, and thus not only brings up fresh supplies of valuable matters to it, but frequently changes its chemical and mechanical characters, rendering a heavy soil lighter by the admixture of a light subsoil, and vice versa. Both are operations which are useless unless they are combined with draining, for it must manifestly serve no good purpose to attempt to open up a soil unless the water which lies in it be previously removed. In fact, subsoiling is useless unless the subsoil has been made thoroughly dry; and it has been found by experience that no good effects are obtained if it be attempted immediately after draining, but that a sufficient time must elapse, in order to permit the escape of the accumulated moisture, which often takes place very slowly. Without this precaution, the subsoil, after being opened by the plough, soon sinks together, and the good effects anticipated are not realized. The necessity for allowing some time to elapse between draining and further operations is still more apparent in deep ploughing, when the soil is actually brought to the surface. In that case it requires to be left for a longer period after draining, in order that the air may produce the necessary changes on the subsoil; for if it be brought up after having been for a long time saturated with moisture, and containing its iron as protoxide, and the organic matter in a state in which it is not readily acted upon by the air, the immediate effect of the operation is frequently injurious in place of being advantageous. One of the best methods of treating a soil in this way is to make the operation a gradual one, and by deepening an inch or two every year gradually to mix the soil and subsoil; as in this way from a small quantity being brought up at a time no injurious effects are produced. Deep ploughing may be said to act in two ways, firstly, by again bringing to the surface the manures which have a tendency to sink to the lower part of the soil, and, secondly, by bringing up a soil which has not been exhausted by previous cropping—in fact a virgin soil.

The success which attends the operation of subsoiling or deep ploughing must manifestly be greatly dependent on the character of the subsoil, and good effects can only be obtained when its chemical composition is such as to supply in increased quantity the essential constituents of the plant; and it is no doubt owing to this that the opinions entertained by practical men, each of whom speaks from the results of his own experience, are so varied. The effects produced by deep ploughing on the estates of the Marquis of Tweeddale, are familiarly known to most Scottish agriculturists, and they are at once explained by the analyses of the soil and subsoil here given, which show that the latter, though poor in some important constituents, contains more than twice as much potash as the soil.

Soil.Subsoil.
Insoluble silicates87·62382·72
Soluble silica0·3930·12
Alumina and peroxide of iron4·1298·60
Lime0·3410·18
Magnesia0·2900·24
Sulphuric acid0·0270·03
Phosphoric acid0·240trace
Potash0·0520·12
Soda0·0500·04
Water1·9563·26
Organic matter5·2204·02
—————
100·32199·33

In addition to the difference in the amount of potash, something is probably due to the large proportion of alumina and oxide of iron in the subsoil, which for this reason must be more tenacious than the soil itself, which appears to be rather light. In other instances, the use of the subsoil plough has occasioned much disappointment, and has led to its being decried by many practical men; but of late years its use having become better understood, its merits are more generally admitted. We believe, that in all cases in which the soil is deep, more or less marked good effects must be produced by its use, but of course there must be cases in which, from the defective composition of the subsoil or other causes, it must fail. It may sometimes be possible a priori to detect these cases, but in a large majority of them our knowledge is still too limited to admit of satisfactory conclusions being arrived at.