THE IMPROVEMENT OF THE SOIL BY MECHANICAL PROCESSES.
Comparatively few uncultivated soils possess the physical properties or chemical composition required for the production of the most abundant crops. Either one or more of the substances essential to the growth of plants are absent, or, if present, they are deficient in quantity, or exist in some state in which they cannot be absorbed. Such defects, whether mechanical or chemical, admit of diminution, or even entire removal, by certain methods of treatment, the adaptation of which to particular cases is necessarily one of the most important branches of agricultural practice, as the elucidation of their mode of action is of its theory. The observations already made with regard to the characters of fertile soils must have prepared the reader for the statement that these defects may be removed, either by mechanical or chemical processes. The former method of improvement may at first sight appear to fall more strictly under the head of practical agriculture, of which the mechanical treatment of the soil forms so important a part, and that their improvement by chemical means should form the sole subject of our consideration in a treatise on agricultural chemistry. But the line of demarcation between the mechanical and the chemical, which seems so marked, disappears on more minute observation, and we find that the mechanical methods of improvement are frequently dependent on chemical principles; and those which, at first sight, appear to be entirely chemical, are also in reality partly mechanical. It will be necessary for us, therefore, to consider shortly the mechanical methods of improving the soil.
Draining.—By far the most important method of mechanically improving the soil is by draining—a practice the beneficial action of which is dependent on a great variety of circumstances. It is unnecessary to insist on the advantage derived from the rapid removal of moisture, which enables the soil to be worked at times when this used to be almost impossible, and other direct practical benefits. Of its more strictly chemical effects, the most important is probably that which it produces on the temperature of the soil. It has been already remarked that the germination of a seed is dependent on the soil in which it is sown acquiring a certain temperature, and the rapidity of the after-growth of the plant is, in part at least, dependent on the same circumstance. The necessary temperature is speedily attained by the heating action of the sun's rays, when the soil is dry; but when it is wet, the heat is expended in evaporating the moisture with which it is saturated; and it is only after this has been effected that it acquires a sufficiently high temperature to produce the rapid growth of the seeds committed to it.
The extent to which this effect occurs may be best illustrated by reference to some experiments made by Schübler, in which he determined the temperature attained by different soils, in the wet and dry state, when exposed to the sun's rays, from 11 till 3 o'clock, in the latter part of August, when the temperature in the shade varied from 73° to 77°.
| 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 silicates | 87·623 | 82·72 |
| Soluble silica | 0·393 | 0·12 |
| Alumina and peroxide of iron | 4·129 | 8·60 |
| Lime | 0·341 | 0·18 |
| Magnesia | 0·290 | 0·24 |
| Sulphuric acid | 0·027 | 0·03 |
| Phosphoric acid | 0·240 | trace |
| Potash | 0·052 | 0·12 |
| Soda | 0·050 | 0·04 |
| Water | 1·956 | 3·26 |
| Organic matter | 5·220 | 4·02 |
| ——— | —— | |
| 100·321 | 99·33 |
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.
Improving the Soil by Paring and Burning.—It has long been familiarly known, that a decided improvement has been produced on some soils by burning. Its advantages have chiefly been observed on two sorts, heavy clays and peat soils, on both of which it has been practised to a great extent. The action of heat on the heavy clays appears to be of a twofold character, depending partly on the change effected in its physical properties, and partly on a chemical decomposition produced by the heat. The operation of burning is effected by mixing the clay with brushwood and vegetable refuse, and allowing it to smoulder in small heaps for some time. It is a process of some nicety, and its success is greatly dependent on the care which has been taken to keep the temperature as low as possible during the whole course of the burning.
Experience has shown that burning is by no means equally advantageous to all clays, but is most beneficial on those containing a considerable quantity of calcareous matter, and of silicates of potash. In such clays heat operates by causing the lime to decompose the alkaline silicates, and liberate a quantity of the potash which was previously in an unavailable state. Its effect may be best illustrated by the following analyses by Dr. Voelcker of a soil, and the red ash produced in burning it.
| Soil. | Red Ash. | |
| Water | 0·93 | 1·18 |
| Organic matter | 10·67 | 3·32 |
| Oxides of iron and alumina | 13·40 | 18·42 |
| Carbonate of lime | 23·90 | 8·83 |
| Sulphate of lime | trace | 1·15 |
| Carbonate of magnesia | 1·10 | " |
| Magnesia | " | 1·76 |
| Phosphoric acid | trace | 0·71 |
| Potash | 0·38 | 1·08 |
| Soda | 0·13 | " |
| Chloride of sodium | " | 1·03 |
| Insoluble matter, chiefly clay | 49·66 | 62·52 |
| —— | —— | |
| 100·17 | 100·00 |
In this instance the quantity of burned soil amounted to about fifteen tons per acre, and it is obvious that the quantity of potash which had been liberated from the insoluble clay and the phosphoric acid are equal to that contained in a considerable manuring. In order to obtain these results, it is necessary, as has been already observed, to keep the temperature as low as possible during the process of burning, direct experiment having shown that when this precaution is not observed another change occurs, whereby the potash, which at low temperatures becomes soluble, passes again into an insoluble state. A part of the beneficial effect is no doubt also due to the change produced in the physical characters of the clay by burning, which makes it lighter and more friable, and by mixture with the unburnt clay ameliorates the whole. This improvement in the physical characters of the clay also requires that it shall be burnt with as low a heat as possible; for if it rises too high, the clay coheres into hard masses which cannot again be reduced to powder, and the success of the operation of burning may always be judged of by the readiness with which it falls into a uniform friable powder.
The improvement of peat by burning has been practised to some extent in Scotland, though less frequently of late years than formerly; but it is still the principal method of reclaiming peat soils in many countries, and particularly in Finland, where large breadths of land have been brought into profitable cultivation by means of it. The modus operandi of burning peat is very simple; it acts by diminishing the superabundant quantity of humus or other organic matters, which, in the previous section we have seen to be so injurious to the fertility of the soil. It may act also in the same way as it does on clay, by making part of the inorganic constituents more really soluble, although it is not probable that its effect in this way can be very marked. Its chief action is certainly by destroying the organic matters, and by thus improving the physical character of the peat, and causing it to absorb and retain a smaller quantity of water than it naturally does. For this reason it is that it proves successful only on thin peat bogs, for if they be deep, the inorganic matters soon sink into the lower part, and the surface relapses into its old state of infertility. It is probably for this reason that the practice has been so much abandoned in Scotland, more especially as other and more economical modes of treating peat soils have come into use.
Warping.—This name has been given to a method of improving soils by causing the water of rivers to deposit the mud it carries in suspension upon them, and which has been largely practised in the low lying lands of Lincoln and Yorkshire, where it was introduced about a century ago. It is most beneficial on sandy or peaty soil, and by its means large tracts of worthless land have been brought under profitable cultivation. It requires that the land to be so treated shall be under the level of the river at full tide, and it is managed by providing a sluice through which the river water is allowed to flood the land at high tide, and again to escape at ebb, leaving a layer of mud generally about a tenth of an inch in thickness, which it brought along with it. By the repetition of this process, a layer of several feet in thickness, of an excellent soil, is accumulated on the surface. Herapath, who has carefully examined this subject chemically, has shown that in one experiment where the water used contained 233 grains of mud per gallon, 210 were deposited during the warping. The following analyses will show the general nature of the matters deposited, and the change which they produce on the soil:—No. 1 is the mud from the Humber in its natural state, No. 2 a specimen of warp of average quality artificially dried, No. 3 a sandy soil before warping, and No. 4, the same fifteen years after having received a coating of 11 inches of mud.
| 1 | 2 | 3 | 4 | |
| Water | 47·49 | ... | 1·06 | 2·00 |
| Organic matter | 5·94 | 6·93 | 2·20 | 7·61 |
| Chloride of Calcium | ... | ... | } ... | ... |
| Magnesium | } ... | 0·10 | } ... | ... |
| Sodium | } 1·66 | } 0·94 | } 0·14 | 0·16 |
| Potassium | } | } | } | |
| Sulphate of Soda | } ... | 0·31 | } ... | ... |
| Magnesia | } ... | 1·18 | } ... | ... |
| Lime | trace | 1·10 | trace | trace |
| Carbonate of Magnesia | 2·60 | 0·31 | trace | 0·29 |
| Lime | 3·59 | 8·18 | trace | 0·46 |
| Potash and Soda | 0·18 | 0·47 | trace | 0·17 |
| Magnesia | 1·69 | 2·60 | trace | 0·26 |
| Lime | 0·39 | 0·68 | trace | 0·14 |
| Peroxide of Iron | } 6·63 | 5·05 | 0·08 | 1·17 |
| Alumina | } | 8·18 | 0·39 | 0·41 |
| Phosphate of Iron | 0·58 | 1·04 | trace | 0·28 |
| Silica | ... | 9·05 | 0·14 | 2·77 |
| Sand and Stones | 29·15 | 55·87 | 95·91 | 84·97 |
| ———— | ———— | ———— | ———— | |
| 100·00 | 100·00 | 100·00 | 100·00 |
It is easy to understand the importance of the effects produced by adding to any soil large quantities of a mud containing upwards of one per cent of phosphate of iron; and in point of fact, Herapath has calculated that in one particular instance the quantity of phosphoric acid brought by warping upon an acre of land, exceeded seven tons per acre. As, moreover, the matters are all in a high state of division, they must exist in a condition peculiarly favourable to the plant. The overflow of the Nile is only an instance of warping on the large scale, with this difference, that it is repeated once only in every year, whereas, in this country, the operation is repeated at every tide until a deposit sometimes of several feet in thickness is obtained, after which it is stopped, and the soil brought under ordinary cultivation.
An operation which is, in some respects, the converse of warping, has been carried out on Blair-Drummond Moss, where the peat has been dislodged and carried off by the action of water, leaving the subjacent soil in a state fitted for cropping. Of course both this and warping are restricted to special localities, but they are most important means of ameliorating the soil when circumstances admit of their being carried out.
Mixing of Soils.—When soils possess conspicuous defects in their physical, and even in their chemical properties, great advantages may, in some instances, be derived from their proper admixture. A light sandy soil, for instance, is greatly improved by the addition of clay, and vice versa; so that, when two soils of opposite properties occur near to one another, both may be improved by mixture. It has been applied to the improvement of heavy clay soil and of peat, the former being mixed with sand or marl so as to diminish its tenacity; the latter with clay or gravel to add to its inorganic matters, and in both instances it has proved successful.
The process of chalking, which has been carried out on a large scale in some parts of England, and which consists in bringing up the chalk from pits, penetrating through the overlying tenacious clay, and mixing it with the soil, operates, to some extent, in a similar manner, though no doubt the lime also exercises a strictly chemical action. It is probable that the mixing of soils might be advantageously extended, and it merits more minute study than it has yet obtained. Its use is obviously limited by the expense, because, of course, where good effects are to be obtained, it is necessary to remove large quantities of soil, in some instances as much as 50 or 100 tons per acre, but the expense might be much diminished if it were carried out methodically, and on a considerable scale. The admixture of highly fertile soils with others of inferior quality is also worthy of attention; indeed, it is understood that this has been done, to some extent, with the rich trap soils of some parts of Scotland, but the extent of the benefit derived from it has not been made public.