CHAPTER VII.
Soils of the Granitic and Trap Rocks.—Accumulations of transported Sands, Gravels, and Clays.—Use of Geological Maps in reference to Agriculture. —Physical characters and Chemical constitution of Soils.—Relation between the nature of the Soil and the kind of Plants that naturally grow upon it.
Soils of the Granitic and Trap Rocks.—Accumulations of transported Sands, Gravels, and Clays.—Use of Geological Maps in reference to Agriculture. —Physical characters and Chemical constitution of Soils.—Relation between the nature of the Soil and the kind of Plants that naturally grow upon it.
It was stated, in the preceding lecture, (see p. 82,) that rocks are divided by geologists into the stratified and theunstratified.[14]The stratified rocks cover by far the largest portion of the globe, and thus form a variety of soils, of which a general description has just been given. The unstratified rocks are oftwo kinds—thegranitesand thetraprocks; and as a considerable portion of the area of our island is covered by them, it will be proper shortly to consider the peculiar characters of each, and the differences of the soils produced from them.
1. Thegranitesconsist of a mixture, in different proportions, of three minerals, known by the names ofquartz,felspar, andmica. The latter, however, is generally present in such small quantity, that in our general description it may be safely left out of view. Granites, therefore, consist chiefly of quartz and felspar, in proportions which vary very much, but the former, on an average, constitutes perhaps from one-third to one-half of the whole.
Quartzhas already been described—(see p. 51)—as the substance of flint, the silica of the chemist. When the granite decays, this portion of it forms a more or less coarse siliceous sand.
Felsparis a white, greenish, or flesh-coloured mineral, often more or less earthy in its appearance, but generally hard and brittle, and sometimes glassy. It is scratched by, and thus is readily distinguished from, quartz. When it decays, it forms an exceedinglyfine clay.
A remarkable difference appears thus to exist, in chemical constitution, between these two minerals—a difference which mustaffect also the soils produced from them. A granite soil, in addition to the siliceous sand, will consist chiefly of silica, alumina, and potash; a hornblende soil, in addition to silica and alumina, of much lime, magnesia, and oxide of iron—of nearly 2½ cwt. of each of these latter for every ton of decayed rock. A hornblende soil, therefore,contains more of those inorganic constituents which the plants require for their healthy sustenance, and therefore will prove more generally productive than a soil of decayed felspar. But when the two are mixed, as in the greenstones, the soil must be still more favourable to vegetable life. The potash and soda, of which the hornblende is nearly destitute, the felspar is able abundantly to supply; while, by the hornblende are yielded lime and magnesia, which are known to exercise a remarkable influence on the progress of vegetation.
2. Thetraprocks, comprising the greenstones and basalts, consist essentially[15]of felspar andhornblendeoraugite. In contrasting the trap rocks with the granites, it may be statedgenerally, that while the granites consist of felspar andquartz, the traps consist of felspar and hornblende (or augite). In the traps, both the felspar and the hornblende are reduced, by the action of the weather to a more or less fine powder, affording materials for a soil; in the granites the felspar is the principal source of all the earthy matter they are capable of yielding. If we compare together, therefore, the chemical composition of the two minerals (hornblende and felspar), we shall see in what respect these two varieties of soil ought to differ. Thus they consist of
Thus theory shews, that while granite soils may be eminently unfruitful, trap soils may be eminently fertile. And such is actually the result of observation and experience in every part of the globe. Unproductive granite soils cover nearly the whole of Scotland north of the Grampians, and large tracts of land in Devon and Cornwall, and on the east and west of Ireland; while fertile trap soils extend over thousands of square miles in the lowlands of Scotland, and in the north of Ireland; and where in Cornwall they occasionally mix with the granite soils, they are found to redeem them from their natural barrenness.
While such is the general rule in regard to these two classes of soils, it happens on some spots that the presence of other minerals in thegranites, or of hornblende or mica in larger quantity than usual, give rise to a granitic soil of average fertility, as is the case in the Scilly isles; while, in like manner, the trap rocks are sometimes, as in parts of the isle of Skye, so peculiar in constitution as to condemn the land to almost hopeless infertility.
In some districts the decayed traps are dug up, and applied with advantage, as a top-dressing, to other kinds of land; and as by admixture with the decayed trap, the granitic soils are known to be improved in quality, so an admixture of decayed granite with many trap soils, were it readily accessible, might add to their fertility also.
It is necessary to guard the reader against disappointment, when he proceeds to examine the existing relation between the soils and the rocks on which they lie, or to infer the quality of the soil from the known nature of the rock in conformity with what has been above laid down,—by explaining another class of geological appearances whichpresent themselves not only in our own country but in almost every other part of the globe.
The unlearned reader of the preceding section and chapter may say—I know excellent land resting upon the granites, fine turnip soils on the Oxford or London clays, tracts of fertile fields on the coal measures, and poor, gravelly farms on the boasted new red sandstone: I have no faith in theory—I can have none in theories which are so obviously contradicted by natural appearances. Such, it is to be feared, is the hasty mode of reasoning among too manylocally[16]excellent practical men, familiar, it may be, with many useful and important facts, but untaught to look through and beyond isolated facts to the principles on which they depend.
Every one who has lived long, on the more exposed shores of our island, has seen, that when the weather is dry, and the sea winds blow strong, the sands of the beach are carried inland and spread over the soil,sometimes to a considerable distance from the coast. In some countries this sand-drift takes place to a very great extent, and gradually swallows up large tracts of fertile land.
Again, most people are familiar with the fact, that during periods of long continued rain, when the rivers are flooded and overflow their banks, they not unfrequently bear with them loads of sand and gravel, which they carry far and wide, and strew at intervals over the surface soil.
So the annual overflowings of the Nile, the Ganges, and the river of Amazons, gradually deposit accumulations of soil over surfaces of great extent;—and so also the bottoms of most lakes are covered with thick beds of sand, gravel, and clay, which have been conveyed into them from the higher grounds by the rivers through which they are fed.
To these and similar agencies, a large portion of the existing dry land of the globe has been, and is still exposed. Hence in many places, the rocks, and the soils naturally derived from them, are buried beneath accumulated heaps or layers of sand, gravel, and clay, which have been brought from a greater or less distance, and which have not unfrequently been derived from rocks of a totally different kind from those of the district in which they are now found. On theseaccumulations of transported materials, a soil is produced which often has no relation in its characters to the rocks which cover the country, and the nature of which a familiar acquaintance with these rocks would not enable us to predict.
To this cause is due that discordance between the first indications of geology, as to the origin of soils from the rocks on which they rest, and the actually observed character of those soils in certain districts—of which discordance mention has been made as likely to awaken doubt and distrust in the mind of the less instructed student in regard to the predictions of agricultural geology. There are several circumstances, however, by which the careful observer is materially aided in endeavouring to understand what the nature of the soils is likely to be, and how they ought to be treated, even when the subjacent rocks are thus overlaid by masses of drifted materials. Thus—
1. It not unfrequently happens, that the materials brought from a distance are more or less mixed up with the fragments and decayed matter of the rocks which are native to the spot, so that though modified in quality, the soil, nevertheless, retains the general characters of that which is formed on other spots from the decay of these rocks alone.
2. Where the formation is extensive, or covers a large area, as the new red sandstones and coal measures do in this country,—the mountain limestones in Ireland, and the granites in the north of Scotland—the transported sand, gravel, or clay, strewed over one part of the formation, has not unfrequently been derived from the rocks of another part of thesameformation, so that, after all, the soils may be said to be produced from the rocks on which they rest, and may be judged of from the known constitution of these rocks.
3. Or if not from the rocks of the same formation, they have most frequently been derived from those of a neighbouring formation—from rocks which are to be found atno great distance, and generally on higher ground. Thus the ruins of the millstone grit rocks are often spread over the surface of the coal measures—of these, again, over the magnesian limestone,—of the latter, over the new red sandstone, and so on. The effect of this kind of transport upon the soils, is merely to overlap, as it were, the edges of one formation with the proper soils of the formations that adjoin it in the particular direction from which the drifted materials are known to have come.
It appears, therefore, that the occurrence on certain spots, or tractsof country, of soils that have no apparent relation to the rocks on which they immediately rest, tends in no way to throw doubt upon, to discredit or to disprove, the conclusions drawn from the more general facts and principles of geology. It is still generally true that soilsarederived from the rocks on which they rest. The exceptions are local, and the difficulties which these local exceptions present, require only from the agricultural geologist a more careful study of the structure of each district, before he pronounces a decided opinion as to the degree of fertility it either naturally possesses, or by skilful cultivation may be made to attain.
Geologicalmapspoint out with more or less precision the extent of country over which the chalk, the red sandstone, the granites, &c., are found immediately beneath the loose materials on the surface; and these maps are of great value in indicating also the general quality of the soils over the same districts. It may be true, that here and there thenaturalsoils are masked or buried by transported materials, yet thepolitical economistmay, nevertheless, with safety estimate the general agricultural capabilities and resources of a country by the study of its geological structure—thecapitalistjudge in what part of it he is likely to meet with an agreeableinvestment—and thepractical farmerin what country he may expect to find land that will best reward his labours—that will admit of the kind of culture to which he is most accustomed, or, by the application of better methods, will manifest the greatest agricultural improvement.
The influence of climate on the fertility of a soil is often very great. This influence depends very much upon what are called thephysicalproperties of soils.
1. Some soils are heavier and denser than others, sand and marls being the heaviest, and peaty soils the lightest. In reclaiming peat lands, it is found to be highly beneficial to increase their density by a covering of clay, sand, or limestone gravel.
2. Again, some soils absorb the rains that fall, and retain them in larger quantity and for a longer period than others. Strong clays absorb and retain nearly three times as much water as sandy soils do, while peaty soils absorb a still larger proportion. Hence the more frequent necessity for draining clayey than sandy soils; hence also the reason why, in peaty lands, the drains must be kept carefully open, inorder that the access of springs and of other water from beneath, may be as much as possible prevented.
3. When dry weather comes, soils lose water by evaporation with different degrees of rapidity. In this way a siliceous sand will give off the same weight of water in the form of vapour, in one-third of the time necessary to evaporate it from a stiff clay, a peat, or a rich garden mould, when all are equally exposed to the air. Hence the reason why plants are so soon burned up in a sandy soil. Not only do such soilsretainless of the rain that falls, but that which is retained is also more speedily dissipated by evaporation. When rains abound, however, or in very moist seasons, these same properties of sandy soils enable them to sustain a luxuriant vegetation, when plants will perish on clay lands from excess of moisture.
4. In drying under the influence of the sun, soils contract and diminish in bulk in proportion to the quantity of clay or of peaty matter they contain. Sand does not at all diminish in bulk in drying, but peat shrinks in one-fifth, and agricultural clay nearly as much. The roots are thus compressed, and air is excluded, especially from the hardened clays, and thus the plant is placed in a condition unfavourable to its growth. Hence the value of proper admixtures ofsand and clay. By the latter (the clay), a sufficient quantity of moisture is retained, and for a sufficient length of time; while, by the former, the roots are preserved from compression, and a free access of air is permitted.
5. In the hottest and most drying weather, the soil has seasons of respite from the scorching influence of the sun. During the cooler season of the night, even when no perceptible dew falls, it has the power of again extracting from the air a portion of the moisture it had lost during the day. Perfectly pure sand possesses this power in the least degree; it absorbs little or no moisture from the air. A stiff clay, on the other hand, will in a single night absorb sometimes as much as a 30th part of its own weight, and a dry peat as much as a 12th of its weight; and, generally, the quantity thus drunk in by soils of various qualities, is dependent upon the proportions of clay and vegetable matter they severally contain. We cannot fail to perceive from these facts, how much of the productive capabilities of a soil is dependent upon the proportions in which its different earthy and vegetable constituents are mixed together.
6. The temperature of a soil, or the degree of warmth it is capable of attaining under the influence of the sun’s rays, materially affects the progress of vegetation. Every gardener knows how muchbottomheat forces the growth, especially of young plants; and wherever a natural warmth exists in the soil, independent of the sun, as in the neighbourhood of volcanoes, there it exhibits the most exuberant fertility. One main influence of the sun in spring and summer is dependent upon its power of thus warming the soil around the young roots, and thus rendering it propitious to their rapid growth. But the sun does not warm all soils alike: some become much hotter than others, though exposed to the same sunshine. When the temperature of the air in the shade is no higher than 60° to 70°, adrysoil may become so warm as to raise the thermometer to 90° or 100°. Mrs. Ellis states, that among the Pyrenees the rocks actually smoke after rain under the influence of the summer sun, and become so hot, that you cannot sit down upon them. Inwetsoils the temperature rises more slowly, and never attains the same height as in a dry soil by 10° or 15°. Hence it is strictly correct to say, that wet soils arecold; and it is easy to understand how this coldness is removed by perfect drainage. Dry sands and clays, and blackish garden mould, become warmed to nearly an equal degree under the same sun; brownish red soils are heated somewhat more, and dark-coloured peat the most of all. It is probable, therefore, that the presence of dark-coloured vegetable matter renders the soil more absorbent of heat from the sun, while the colour of the dark-red marls of the new and old red sandstones may, in some degree, aid the other causes of fertility in the soils which they produce.
Granite generally forms hills and sometimes entire ridges of mountains. When it decays, the rains and streams wash out and carry down the fine felspar clay, and leave the (quartz) sand on the sides of the hills. Hence the soil in the bottoms and flats of granite countries consists of a cold, stiff, wet, more or less impervious clay, which often bears only heath, bog, or a poor and unnutritive pasture. The hill sides are either bare or covered with a thin, sandy, and ungrateful soil, of which little can be made by the aid even of skill and industry. Yet the opposite sides of the same mountains often present a remarkable difference in this respect, those which are most beaten by the rains having the light clay most thoroughly washed from their surfaces, and being therefore the most barren.
In reading the above observations, the practical reader can hardly fail to have been struck with the remarkable similarity in physical properties between stiff clay and peaty soils. Both retain much of the water that falls in rain, and both part with it slowly by evaporation. Both contract much in drying; and both absorb moisture readily from the air in the absence of the sun. In this similarity of properties, we see not only why the first steps in improving both kinds of soil must be very nearly the same; but why, also, a mixture either of clay or of vegetable matter will equally impart to a sandy soil many of those aids to, or elements of, fertility—of which they are alike possessed.
Soils perform at least three functions, in reference to vegetation.They serve as a basis in which plants may fix their roots and sustain themselves in their erect position,—they supply inorganic food to vegetables at every period of their growth,—and they are the medium in which many chemical changes take place, that are essential to a right preparation of the various kinds of food which the soil is destined to yield to the growing plant.
We have spoken of soils as consisting chiefly of sand, lime, and clay, with certain saline and organic substances in smaller and variable proportions. But the study of the ash of plants (see chap. iv.) shews us, that a fertile soil must of necessity contain an appreciable quantity of at least eleven different substances, which in most cases exist in greater or less relative abundance in the ash both of wild and of cultivated plants.
Two well known geological facts lead to precisely the same conclusion. We have seen that the soils formed from the unstratified rocks,—the granites and the traps,—while they each contain certain earthy substances in proportions peculiar to themselves, yet contain also in general atraceof most of those different kinds of matter which are found in the ash of plants. And when to this fact is added theother, that the stratified rocks appear to be only the long accumulating fragments and ruins of more ancient unstratified masses—which, under various agencies, have gradually crumbled to dust, been strewed over the surface in alternate layers, and afterwards again consolidated,—the reader will readily grant, that in all rocks, and consequently in all soils,tracesof every one of these substances may generally be presumed to exist.
Actualchemical analysisconfirms these deductions in regard to the constitution of soils. It shews that, in most soils, the presence of the several constituents of the ash of plants may be detected, though in very variable proportions. And following up its investigations, in regard to the effect of this difference in the proportion of the generally less abundant constituents of the soil, it establishes certain other points of the greatest possible importance to agricultural practice. Thus, it has found, for example,
1. That as a proper adjustment of the proportions of clay and sand is necessary, in order that a soil may possess the most favourablephysicalproperties—so that the mere presence of the various kinds of inorganic food in a soil is not sufficient to make it productive of a given crop, but that they must be so adjusted inquantity that the plant shall be able readily and at the proper time to obtain an adequate supply of each.
2. That when a soil is particularly poor in certain of these substances, the valuable, cultivated corn crops, grasses, and trees, refuse to grow upon them in a healthy manner, and to yield remunerating returns. And,
3. That when certain other substances are present in too great abundance, the soil is rendered equally unpropitious to the most important crops.
In these facts the intelligent reader will perceive the foundation of the varied applications to the soil which are everywhere made under the direction of a skilful practice, and of the difficulties which, in so many localities, lie in the way of bringing the land into such a state as shall fit it readily to supply all the wants of those kinds of vegetables which it is the special object of artificial culture easily and abundantly to raise.
Chemical analysis is a difficult art,—one which demands much chemical knowledge, and skill in chemical practice (manipulation, as it is called), and calls for both time and perseverance—if valuable, trustworthy, andminutely correctresults are to be obtained. I believe it is only by aiming after such minutely correct results thatchemical analysis is likely to throw light on the peculiar properties of those soils which, while they possess much general similarity in composition and in physical properties, are yet found in practice to possess very different agricultural capabilities. Many such cases occur in every country, and they are the kind of difficulties in regard to which agriculture has a right to say to chemistry—“These are matters which I hope and expect you will satisfactorily clear up.” But while agriculture has a right to use such language, she has herself preliminary duties to perform. She has no right in one breath to deny the value of chemical theory to agricultural practice, and in another to ask the sacrifice of time and labour in doing her chemical work. Chemistry is a wide field, and many zealous lives may be spent in the prosecution of it without at all entering upon the domain of practical agriculture. It may be that here and there it may fall in with the humour or natural bias of some one chemist to apply his knowledge to this most important art; but hitherto the appreciation of such efforts has, in general, been so small—the reception of scientific results and suggestions by the agricultural body so ungracious—that little wonder can exist that so many have quitted the field in disgust—that the majority of capable men should studiously avoid it.
Hence it has happened that, in England, the analysis of soils has rarely been undertaken, except as a matter of professional business, where so much time was, by a fair calculation, given for so much money, and an analysis made, of that degree of accuracy only which the time allotted to it permitted the analyst to attain.
In order, therefore, to illustrate the deductions which, as above stated, may be drawn from an accurate chemical analysis, I shall exhibit the constitution of three different soils as determined by Sprengel, a German chemist, now at the head of the Prussian Agricultural school, and whose own taste, as well as his professional function, have long directed his attention, and with much success, to scientific agriculture.
No. 1is a very fertile alluvial soil from East Friesland, formerly overflowed by the sea, but for 60 years cultivated with corn and pulse cropswithout manure.No. 2is a fertile soil near Göttingen, which produces excellent crops of clover, pulse, rape, potatoes, and turnips, the two last more especiallywhen manured with gypsum.No. 3is a very barren soil from Lunenburg.When washed with water in the manner described in pages 70 to 73, they gave, respectively, from 1000 parts of soil—
No. 1is a very fertile alluvial soil from East Friesland, formerly overflowed by the sea, but for 60 years cultivated with corn and pulse cropswithout manure.
No. 2is a fertile soil near Göttingen, which produces excellent crops of clover, pulse, rape, potatoes, and turnips, the two last more especiallywhen manured with gypsum.
No. 3is a very barren soil from Lunenburg.
When washed with water in the manner described in pages 70 to 73, they gave, respectively, from 1000 parts of soil—
The most striking distinction presented by these numbers is the large quantity of saline matter in No. 1. This soluble matter consisted of common salt, chloride of potassium, sulphate of potash and gypsum, with a trace of sulphate of magnesia, sulphate of iron, and phosphate of soda. The presence of this comparatively large quantity of these different saline substances,—originally derived, no doubt, in great part from the sea,—was probably one reason why it could be so long cropped without manure.
The unfruitful soil is much the lightest of the three, containing 40 per cent. of sand; but this is not enough to account for its barrenness—many light soils containing a larger proportion of sand, and yet being sufficiently fertile.
The finer portions separated from the sand, and soluble matter, consisted in 1000 parts of
1. The composition of No. 1 illustrates the first of those general deductions above stated, that a considerable supply ofallthe species of inorganic food is necessary to render a soil eminently fertile. Not only does this soil contain a comparatively large quantity of soluble saline matter, but it contains also nearly 10 per cent. of organic matter, and, what in connection with this is of great importance, 6 per cent. of lime. The potash and soda, and the several acids, are also present in sufficient abundance.
2. In the second,—a fertile soil, but one whichcannot dispense with manure,—there is little soluble saline matter, and in the insoluble portion we see that there are meretracesof potash, soda, and the important acids. It contains also 5 per cent. only of organic matter, and about 2 per cent. of lime, which smaller proportions, together with the deficiencies above stated, remove this soil from the mostnaturallyfertile class to that class which is susceptible, in hands of ordinary skill, of beingbrought to, andkept in, a very productive condition.
3. In the fine part of the third soil, we observe that there are many more substances deficient than in No. 2. The organic matter amounts apparently to 4 per cent., and there seems to be nearly half a per cent. of lime. But it will be recollected, that this soil contains 40 per cent. of sand, so that in every hundred of soil there are only 60 of the fine matter, of which the composition is presented in the table, or 100 lbs. of the native soil contain only 2½ lbs. of organic matter and ¼ lb. of lime.
But all thesewantswould not condemn the soil to hopeless barrenness, because in favourable circumstances, and where it was worth the cost, they might all be supplied. But the oxide of iron amounts to 8per cent. of this fine matter, a proportion of this substance which, in a soil containing so little organic matter, appears, from practical experience, to be incompatible with the healthy growth of cultivated crops. To this soil, therefore, there requires to be added not only those substances of which it is destitute, but such other substances also as shall prevent the injurious effect of the large proportion of oxide of iron.
In these three soils, then, we have examples,first, of one which contains within itself all the elements of fertility;second, of a soil which is destitute, or nearly so, of certain substances,—which, however, can be readily added by the ordinary manures in general use,—and to which the elements of gypsum are especially useful, in aiding it to feed the potato and the turnip; and,third, of a soil not only poor in many of the necessary species of the inorganic food of plants, but too rich in one which, when present in excess, is prejudicial to vegetable life.
This illustration, therefore, will aid the general reader in comprehending how far rigid chemical analysis is fitted to throw light upon the capabilities of soils, and todirectagricultural practice.
The importance of this study of the chemical constitution of soils will, perhaps, be most readily appreciated by a glance at the very different kinds of vegetables which, under the same circumstances, different soils naturally produce.
There are none so little skilled in regard to the capabilities of the soil, as not to be aware that some lands naturally produce abundant herbage or rich crops, while others refuse to yield a nourishing pasture, and are deaf to the often repeated solicitations of the diligent husbandman. There exists, therefore, a universally understood connection between the kind of soil and the kind of plants that naturally grow upon it. It is interesting to observe how close this relation in many cases is.
1. The sands of the sea-shore, and the margins of salt-lakes, are distinguished by their peculiar tribes of salt-loving plants;—the drifted sands more remote from the beach produce their own long waving coarser grass,—while further inland again, other vegetable races appear.
2. Peaty soils laid down to grass, or existing as natural meadows,produce one woolly soft grass almost exclusively (theHolcus lanatus); when limed, again, these same soils become propitious to green crops and produce much straw, but refuse to fill the ear.
3. On the margins of water-courses, in which silica abounds, the mare’s-tail (Equisetum) springs up in abundance; while, if the stream contain much carbonate of lime, the water-cress appears and lines its sides, and the bottom of its shallow bed, sometimes for many miles from its source.
4. The Cornish heath (Erica vagans) shews itself only above the serpentine rocks; the red clover and the vetch delight in the presence of gypsum; and white clover, of alkaline matter in the soil.
5. Then, again, plants seem to alternate with each other on the same soil. Burn down a forest of pines in Sweden, and one of birch takes its placefor a while. The pines after a time again spring up and ultimately supersede the birch. The same takes place naturally. On the shores of the Rhine are seen ancient forests of oak from two to four centuries old,—gradually giving place to a natural growth of beech; and others where the pine is succeeding to both. In the Palatinate, the ancient oak woods are followed by natural pines; and in the Jura, the Tyrol, and Bohemia, the pine alternates with the beech.
These and other similar differences depend upon the chemical constitution of the soil. The slug may live well, and therefore infest a field almost deficient in lime; the common land snail will abound at the roots of the hedges only where lime is plentiful, and can easily be obtained for the construction of its shell. So it is with plants. Each grows spontaneously where its wants can be most fully and most easily supplied. If they cannot move from place to place like the living animal, yet their seeds can lie dormant, until either the hand of man or the operation of natural causes produces such a change in the constitution of the soil as to fit it for ministering to their most important wants.
And such changes do naturally come over the soil. The oak, after thriving for long generations on a particular spot, gradually sickens; its entire race dies out,—and other races succeed it. The operation of natural causes has gradually removed from the soil that which favoured the oak, and has introduced or given the predominance to those substances which favour the beech or the pine.
In the hands of the farmer the land grows sick of this crop,—it becomes tired of that. These facts are generally indications of a change in the chemical constitution of the soil. This alteration mayproceed slowly and for many years, and the same crops may still grow upon it for a succession of rotations. At length the change is too great for the plant to bear; it sickens, yields an unhealthy crop, and becomes ultimately extinct.
The plants we raise for food have similar likes and dislikes with those that are naturally produced. On some kinds of food they thrive,—fed with others, they sicken or die. The soil must therefore be prepared for their special growth.
In an artificial rotation of crops, we only follow nature. One crop extracts from the soil a certain quantity of all the inorganic constituents of plants; but some of these in much larger proportions than others. A second crop carries off in preference a larger quantity of those substances which the former had left; and thus it is clearly seen, both why an abundant manuring may so alter the constitution of the soil, as to enable it to grow almost any crop; and why, at the same time, this soil may in succession yield more abundant crops and in greater number, if the kinds of plant sown and reaped be so varied as to extract from the soil, one after the other, the several different substances which the manure we have originally added is known to contain.
The management and tilling of the soil, in fact, is a branch of practical chemistry, which, like the art of dyeing or of lead smelting, may advance to a certain degree of perfection, without the aid of pure science; but which can only have its processes explained, and be led on to shorter,—more simple,—more economical,—and more perfect processes, by the aid of scientific principles.