Imatges de pàgina
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423. Good vegetable soil is indicated by Trifolia, Vicia, and Lathyrus pratensis. Thymus serpyllum, wild thyme, indicates a vegetable mould of great thinness; and ragweed, Senecio jacobæa, one of depth; and when the ragweed prevails, it indicates the absence of sheep, which are very fond of, and eat down its young leaves.

424. Purge flax, Linum catharticum; Autumn apargia, Apargia autumnalis ; and mouse-eared hawk-weed, Hieracium pilosella, indicate a dry soil;-the Galium verum, hollow bed-straw, a very dry

one.

425. Yellow iris, Iris pseud-acorus ; the sharp-flowered rush, Juncus acutiflorus; lady's smock, Cardamine pratensis; and ragged robbin, Lychnis flos-cuculi, assure us of a supply of moisture below.

426. The purple dead-nettle, Lamium purpureum; and smooth naked horsetail, Equisetum limosum, indicate a retentive subsoil.

427. The broom, Genista scoparia, indicates a pernicious, and the whin, Ulex Europaus, a more favourable subsoil.

428. The common nettle, Urtica dioica; common dock, Rumex obtusifolius; mugwort, Artemisia vulgaris; annual poa, Poa annua; field poa, Poa pratensis; and common tansy, Tanacetum vulgare, grow near the dwellings of man, on the bare soil, irrespective of its kind; while in the same locality the white clover, Trifolium repens; red clover, Trifolium pratense; annual poa; hoary plantain, Plantago media; ribwort, Plantago lanceolata; purple meadow vetch, Vicia cracca; and common daisy, Bellis perennis, are found in the pasture around his house.

429. Common chickweed, Stellaria media; and common fumitory, Fumaria officinalis, indicate a rich condition of soil.

430. The great ox-eye, Chrysanthemum leucanthemum, points out a soil in a state of poverty; and its poverty from want of

manure is indicated by the parsley-pest, Alchemilla arvensis.

431. Wild mustard, Sinapis arvensis, tells of the application of manure derived from towns.

432. The common corn thistle, Cnicus arvensis, indicates that the land is not well farmed.

433. Wherever the least admixture of peat is found in the soil, the Erica or Calluna and spotted-bearded orchis, Orchis maculata, are sure to be there.

434. Taking a more extended view of the indications of the condition of soils by plants, these observations of Dr Singer seem well founded:-" Green mountains, like those of Cheviot and Ettrick Forest, abounding in grass without heath, indicate a strong soil, which is rendered productive, though frequently steep and elevated, by a retentive subsoil. This quality, and the frequent mists and showers that visit rather elevated sheep-walks, render them productive in strong grasses (Agrostis.)

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. . Dark mountains, clothed with a mixture of heath and grass, indicate a drier soil on a less retentive bottom. are many of the Highland mountains, and such also are some of those which appear occasionally among the green mountains of the southern pastoral district, in which the light soil is incumbent commonly on gravel or porous rock. On these darkcoloured mountains, a green and grassy part often appears where there is no heath, and the subsoil is retentive; and if the upper edge of such a spot appears well defined, this is occasioned by the regular approach of a stratum of clay or other substance impervious to water towards the surface, and the green hue disappears below, when the subsoil again becomes open. On any of the mountains, whether dark or green, when the fern or bracken, Pteris aquilina, appears in quantities, it indicates a deep soil and a dry subsoil."* A stunted growth of heath indicates a part having been bared by the paring-spade; and when vegetation becomes of a brown colour in summer, the subjacent rock is only a little way under the surface.

* Prize Essays of the Highland and Agricultural Society, vol. vii. p. 264.

435. Viewing the connexion of plants to the soil on the great scale, we cannot but be forcibly impressed with the conviction, that "the grand principle of vegetation is simple in its design; but view it in detail, and its complication astonishes and bewilders." And yet, as Professor Macgillivray justly observes, "it is the same sun that calls forth, and, thus elicited, gives vigour to the vegetation, the same earth that supports it, the same moisture that swells its vessels, the same air that furnishes the medium in which it lives; but amid all these systems of general, how multiple the variations of particular constituent causes, and how infinitely diversified their results!"

436. Mechanical structure of soils.-It is now time to take a closer view of soils, their structure and composition; their structure is mechanical, and their composition chemical. We shall first consider their mechanical structure; and I shall describe these in the words of Dr Henry Madden of Brighton. "Soil, considered scientifically," he observes, "may be described to be essentially a mixture of an impalpable powder with a greater or smaller quantity of visible particles of all sizes and shapes. Careful examination will prove to us, that although the visible particles have several indirect effects, of so great importance that they are absolutely necessary to soil, still the impalpable powder is the only portion which directly exerts any influence upon vegetation. This impalpable powder consists of two distinct classes of substances, viz., inorganic or mineral matters, and animal and vegetable substances, in all the various stages of decomposition.

437. "A very simple method may be employed to separate these two classes of particles from each other, viz., the impalpable powder and the visible particles; and, in so doing, we obtain a very useful index to the real value of the soil. Indeed all soils, except stiff clays, can be discriminated in this manner. The greater the proportion of the impalpable matter, the greater, cæteris paribus, will be the fertility of the soil.

438. "To effect this separation, the

following easy experiment may be performed. Take a glass-tube about 2 feet long, closed at one end; fill it about half full of water, and shake into it a sufficient quantity of the soil to be examined, to fill the tube about 2 inches from the bottom; then put in a cork, and having shaken the tube well to mix the earth and water thoroughly, set the tube in an upright position, for the soil to settle down. Now, as the larger particles are of course the heavier, they fall first, and form the undermost layer of the deposite, and so on in regular gradation, the impalpable powder being the last to subside, and hence occupying the uppermost portion. Then by examining the relative thickness of the various layers, and calculating their proportions, a very accurate mechanical analysis of the soil may be made.

439. "The stones which we meet with in soil have in general the same composition as the soil itself, and hence, by gradually crumbling down under the action of air and moisture, they are continually adding new impalpable matter to the soil, and as a large quantity of this impalpable mineral matter is annually removed by the crops, it will at once be perceived that this constant addition must be of great value to the soil. This, therefore, is one important function performed by the stones of soil,-viz., their affording a continually renewed supply of impalpable mineral matter.

440. "On considering the nourishment of plants, we find that their food undergoes various preliminary changes in the soil previous to its being made use of by the plants, and the aid of chemistry will prove to us that the effect is produced by the joint action of air and water; it follows, therefore, that soil must be porous. Now, this porosity of the soil is in part produced by the presence of the larger particles of matter, which, being of all varieties of shape, can never fit closely together, but always leave a multitude of pores between them; and in this manner permit of the free circulation of air and water through the soil.

441 "As the porous nature of soil may,

* Prize Essays of the Highland and Agricultural Society, vol. vii. p. 117.

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to a certain extent, be taken as an index of its power of retaining moisture, it is advisable to determine its amount. This is effected in the following way:-Instead of putting the water first into the tube, as directed above, (438,) and shaking the soil into it, take a portion of soil dried by a heat of about 200° Fahr., and shake it into the dry tube, and by tapping the closed end frequently on the table, make the soil lie compactly at the bottom; when this has been fully effected, that is, when further tapping produces no reduction of bulk, measure accurately the column of soil, cork the tube, shake it till the soil becomes again quite loose, and then pour in the water as directed above (438.) After the soil has fully subsided, tap the tube as before, and re-measure the column of soil, and the increase of bulk is dependent upon the swelling of each particle by the absorption of water, and hence shows the amount of porosity. In very fertile soil, I have seen this amount to one-sixth of the whole bulk.

442. "The functions of the impalpable matter are far more complicated, and will require a somewhat detailed description. In this portion of the soil, the mineral and organic matter are so completely united, that it is quite impossible to separate them from each other; indeed, there are weighty reasons for believing that they are chemically combined. It is from this portion of the soil that plants obtain all their mineral ingredients, and likewise all their organic portions, in so far as these are obtained by the roots; in fact, plants receive nothing from the soil except water, which has been associated with that portion which is at present engaging our attention.

443. "The particles forming the impalpable matter are in such close apposition, that the whole acts in the same way as a sponge, and is hence capable of absorbing liquids and retaining them. It is in this way that soil remains moist so near the surface, even after a long continued drought; and I need not say how valuable this property must be to the plants, since by this means they are supplied with moisture from below, induced by the capillary action of the soil during the heat of sum

mer, when otherwise, unless artificially watered, they would very soon wither."

444. On the important results arising from the action of the mechanical property of the soil, of the capillary power, and of its mode of action, Professor Johnston has these observations :-- "When warm weather comes, and the surface soil dries rapidly, then by capillary action the water rises from beneath, bringing with it the soluble substances that exist in the subsoil through which it ascends, for water is never pure. Successive portions of the water evaporate from the surface, leaving their saline matter behind them. And as the ascent and evaporation go on as long as the dry weather continues, the saline matter accumulates about the roots of plants, so as to put within their reach an ample supply of any soluble substance which is really not defective in the soil. I believe that in sandy soils, and generally in all light soils, of which the particles are very fine, this capillary action is of great importance, and is intimately connected with their power of producing remunerating crops. They absorb the falling rains with great rapidity, and these carry down the soluble matters as they descend, so that when the soil becomes soaked, and the water begins to flow over its surface, the saline matter being already deep, is in little danger of being washed away. On the return of dry weather, the water reascends from beneath, and again diffuses the soluble ingredients through the upper soil."*

445. "Another most useful function of this impalpable portion," continues Dr Madden, "is its power of separating organic matter from water in which it has been dissolved. Thus, for example, if the dark brown liquid which flows from a dunghill is taken and poured on the surface of some earth in a flower-pot, and a sufficient quantity added to soak the whole earth, so that a portion flows out through the bottom of the pot, this latter liquid will be found much lighter in colour than before it was poured upon the earth, and this effect will be increased the nearer the soil approaches in its nature to subsoil. Now, as the colour was entirely owing to the organic matter dissolved in it, it follows

* Johnston's Lectures on Agricultural Chemistry and Geology, 2d edit. p. 535.

that the loss of colour is dependent upon an equivalent loss of organic matter, or, in other words, a portion of the organic matter has entered into chemical combination with the impalpable mineral matter, and has thus become insoluble in water. The advantage of this is, that when soluble organic matter is applied to soil, it does not all soak through with the water, and escape beyond the reach of the roots of the plants, but is retained by the impalpable portions in a condition not liable to injury from rain, but still capable of becoming food for plants when it is required.

446. "Hitherto I have pointed out merely the mechanical relations of the various constituents of soil, with but little reference to their chemical constitution; this branch of the subject, although by far the most important and interesting, is nevertheless so difficult and complex, that I cannot hope for the practical farmer doing much more than making himself familiar with the names of the various chemical ingredients, learning their relative value as respects the fertility of the soil, and acquiring a knowledge of the quantities of each requisite to be applied to particular crops; for, as to his attempting to prove their existence in his own soil by analysis, I fear that is far too difficult a subject for him to grapple with, unless regularly educated as an analytical chemist."

447. Chemical composition of soils."Soil, to be useful to the British agriculturalist, must contain no less than 12 different chemical substances, viz., silica, alumina, oxide of iron, oxide of manganese, lime, magnesia, potass, soda, phosphoric acid, sulphuric acid, chlorine, and organic matter. I shall confine my observations almost solely to their relative importance to plants, and their amount in the soil.

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450. "Oxide of Iron.-There are two oxides of iron found in soils-namely, the protoxide and peroxide; one of which, the protoxide, is frequently very injurious to vegetation: indeed, so much so, that per cent of a soluble salt of this oxide is sufficient to render soil almost barren. The peroxide, however, is often found in small quantities in the ashes of plants. The two oxides together constitute from to 10 per cent of soil. The blue, yellow, red, and brown colours of soil, are more or less dependent upon the presence of iron.

451. "Oxide of Manganese.-This oxide exists in nearly all soils, and is occasionally found in plants. It does not, however, appear to exert any important influence either mechanically or chemically. Its amount varies from a mere trace to about 1 per cent. It assists in giving the black colour to soil.

452. "These four substances constitute by far the greatest bulk of every soil, except the chalky and peaty varieties, but, nevertheless, chemically speaking, are of trifling importance to plants; whereas the remaining eight are so absolutely essential that no soil can be cultivated with any success, unless provided with them, either naturally or artificially. And yet, when it is considered that scarcely any of them constitute 1 per cent of the soil, their value will no doubt excite surprise. The sole cause of their utility lies in the fact, that they constitute the ashes of the plants ; and as no plant can, by possibility, thrive

without its inorganic constituents, (its ashes,) hence no soil can be fertile which does not contain the ingredients of which these are made up. The very small percentage of these ingredients in any soil necessitates a minute analysis of every soil before it can be ascertained whether or not it contains any, or what proportion of these ingredients. But the reason for such minuteness in analysis becomes obvious when we consider the immense weights which have to be dealt with in practical agriculture; for example, every imperial acre of soil, considered as only 8 inches deep, will weigh 1884 tons, so that 0.002 per cent, that is, only a two-thousandth per cent the amount of sulphuric acid in

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a barren soil-amounts to 80.64 lbs. in the imperial acre!

453. "Potass and soda exist in variable quantities in many of the more abundant minerals, and hence it follows that their proportion in soil will vary according to the mineral which produced it. For the sake of reference, I have subjoined the following table, which shows the amount per cent of alkalies in some of these minerals, and likewise a rough calculation of the whole amount per imperial acre, on the supposition of a soil composed solely of these rocks, and of a depth of 10 inches; and the amount is abundant beyond conjecture::

Amount per Imperial Acre in a soil 10 inches deep.

Felspar

Clinkstone

Clay-slate Basalt

Name of Alkali.

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454. "One acquainted with chemistry will naturally ask the question, How is it that these alkalies have not been long ago washed away by the rain, since they are both so very soluble in water? The reason of their not having been dissolved is the following-and it may in justice be taken as an example of those wise provisions of nature, whereby what is useful is never wasted, and yet is at all times ready to be abundantly supplied :

455. "These alkalies exist in combination with the various other ingredients of the rock in which they occur, and in this way have such a powerful attraction for these ingredients, that they are capable of completely resisting the solvent action of water as long as the integrity of the mass is retained. When, however, it is reduced to a perfectly impalpable powder, this attraction is diminished to a considerable extent, and then the alkali is much more easily dissolved. Now this is the case in soil, and, consequently, while the stony portions of soil contain a vast supply of these valuable ingredients in a condition in which water can do them no injury, the impalpable powder is supplied with them in a soluble state, and hence in a condition available to the wants of vegetation.

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