SOIL. The comparatively soft and loose upper layer of the earth's crust, upon which plants depend for their nourishment. The various kinds of Soils, and their modes of origin, will be found described below. Soils should be carefully examined as regards their composition and physical properties, in order to ascertain their capabilities for cultivation, the kinds of plants for which they are naturally best fitted, and the means by which they can be rendered more fertile. ANALYSIS. Soils may be examined in the following way, as regards their general composition; and much valuable information can be obtained from such an analysis: The Soil is first thoroughly dried at 212deg. Fahr., and a given weight, say 1/4lb., is boiled in distilled water till the particles of which it is made up fall thoroughly apart. The substances in the soil that are soluble in pure water will be dissolved in this way; and the solution is carefully filtered through paper into a vessel, and kept for chemical analysis. The solid residue is carefully washed twice or thrice with distilled water, on a filter, to remove the whole of the soluble substances, and the washings are added to the solution. The residue is then thoroughly dried at 212deg. Fahr., and weighed; and the loss, as compared with the previous weight or 1/4lb., gives the amount of substances in the soil that are soluble in pure water. The solid material is then again washed, and the water is poured off, carrying with it the lighter particles. This is repeated till only the sand and gravel are left behind: these are dried and separated, by sifting through gauze. The washings, also, are collected and dried. The gravel, sand, and fine particles, which form the clay or mud of soils, are weighed separately, and the relative weight of each is thus determined. Each is then examined with a good lens, and the proportions of pure quartz sand (silica), mica, volcanic rocks, limestone, or other minerals, are noted. This examination is facilitated if a little Hydrochloric Acid (Spirit of Salt) is poured over the material under examination; since quartz sand remains unchanged, limestone is dissolved with the formation of bubbles of Carbonic Acid gas, ironstone is slowly dissolved, and the acid turns brown, and gives the very characteristic test for iron by turning blue when a solution of Prussiate of Potash is mixed with it. Other minerals in Soils give less conspicuous results with the acid. The chemical analysis of the portion soluble in water, and the complete analysis of the solid residue, require a considerable knowledge of chemistry for the attainment of success, and should be entrusted to a professional analyst. The amount of organic matter, i.e., remains of animals and of plants, in Soils, very greatly affects their value. The fresh Soil must be thoroughly dried, as already stated, to drive off the water as completely as possible. A given weight of it is then burned in a platinum dish, over a lamp, in the open air; and the burning is continued till all the blackness is got rid of, i.e., till the Carbon is entirely burned away. The residue is then carefully weighed again, and the loss of weight represents the amount of organic matter destroyed. It is desirable to ascertain the conditions in which the latter is present in the fresh Soil; but exact analysis demands more experience of chemical manipulation than is usually met with, except among chemists. Organic matter is usually present as Humic and Ulmic Acids (along with small quantities of some other organic acids), and insoluble vegetable matter, including often a good deal of tannin. Nitrates, also, are formed from organic remains. Those who desire fuller details on the methods of analysis, will find them in most works on Agricultural Chemistry, such as Johnston's "Analysis of Soils," or Johnston and Cameron's "Elements of Agricultural Chemistry and Geology." PHYSICAL PROPERTIES. Not less valuable than the knowledge of the chemical composition of Soils, is that of certain properties grouped under the term heading this paragraph. Of these, the chief are the capacity for absorbing and retaining water, various chemical compounds, and heat; the density and power of cohesion of the particles of Soil, and the mode of shrinkage in dry weather. These vary greatly, according to the composition of the Soil; but their general characters may now be indicated. Absorption and Retention of Water. This is a quality of great importance in fitting the Soil to supply the moisture required by plants. Soils absorb rain; though when the rainfall is very heavy, they cannot absorb more than a part, and the rest flows off the surface into streams. The capacity for absorption of rain-water, and for keeping it stored within reach of the roots of plants, varies much in Soils, according to their composition and the size of the particles of which they are made up. Sandy Soils rapidly absorb the water; but they allow it to drain away almost as quickly; so that plants growing in sand are liable to suffer from drought. Fine sand is about twice as retentive of water as coarse. Clay retains twice or thrice as much as sand. Vegetable earth, or humus, absorbs and retains about twice or thrice as much as clay, and becomes about two and a-half or three times as heavy when soaked in water as when dry. The more retentive Soils lose less by evaporation than do the others. Almost all Soils are more or less full of water, in the liquid form, at a few feet below the surface, the depth varying with the nature of the Soil and Subsoil and with the season of the year and the climate. This subterranean water is brought within reach of the roots of plants by what is known as capillary attraction, the water rising in the fine crevices between and in the particles of the Soil. If the subterranean water is stagnant near the surface, substances are apt to be formed in the Soil that are injurious to plants, and that thus diminish its fertility. Drainage is usually necessary in clays and peat Soils, to prevent water from accumulating and doing harm in this way. Certain Soils also have the property of absorbing a good deal of vapour from the atmosphere, and condensing it in their particles, the amount increasing in proportion to the moistness of the air. Experiments have shown that humus can absorb about half its own weight of water from air saturated with vapour; and clays absorb one-tenth to one-fifth of their own weight; but quartz sand absorbs little, if any, moisture in that way. This source of moisture becomes less productive as the temperature rises. During the night, and in cold weather, it is probable that Soils absorb a good deal of moisture from the atmosphere; but when the air is warmer and drier than the soil, the latter loses water by evaporation, and what is lost in this way is replaced from below by capillary attraction. Evaporation cools the surface from which it is going on; hence, wet land is colder than that which is well-drained. Humus gives up least by evaporation, and quartz sand most, of all Soils under similar conditions. The Power of Absorbing and Retaining Chemical Compounds is one possessed to a greater or less extent by all Soils, and is of the utmost importance in the nutrition of plants. If solutions of various kinds (e.g., of Potassic Nitrate or Ammonium Nitrate, or of Phosphates of Potassium, Calcium, &c.) are allowed to trickle through a moderately thick layer of Soil, it is found that the water flowing off contains little of these substances for a time; but at last the Soil becomes saturated with them, and allows the solution to pass through unchanged. By this property of retaining certain compounds, Soils are enabled to store up soluble manures, as well as Ammonia and Nitrates from the atmosphere, and various substances formed in the changes due to weathering of the rocks and Soils; and from this store plants can draw, as they need these substances in their food. Owing to this process, filtering impure water through, or over, a sufficient extent of earth is a very efficient mode of removing impurities, and is frequently resorted to for purifying sewage-water before discharging it into streams. The Capacity for Absorbing and Retaining Heat varies with colour and texture, with the amount of moisture in, and of evaporation from, the Soil, and with the angle of exposure to the rays of the sun, direct or reflected. Of course, the actual temperature depends also upon the amount of protection afforded by buildings or other objects in the neighbourhood. Bottom-heat is of great use in stimulating the action of roots, but in Great Britain it can be given only in hotbeds or hothouses. Exposure to the sun's rays is, therefore, the only source of warmth that need be discussed here. The more directly the rays fall, the greater is their effect. Evaporation keeps down the temperature, and it has been found that wet Soils are usually from 10deg. to 15deg. Fahr. colder than dry ones of the same composition. Hence, draining wet Soils renders them warmer, and hastens the ripening of the crops on them. Dark grey Soils absorb most heat; next come black Soils, then brown and dark red. Pale sands, marls, and clays absorb least. The temperature of the Soil exercises a marked influence on the growth of plants, since they suffer if the buds and leaves are stimulated by a warm atmosphere while the soil is too cold to permit of the roots supplying the necessary sap to the other organs. The Density and Power of Cohesion of the particles of Soils, and the Mode of Shrinkage, are of interest and importance chiefly on account of their influence on the capacities of Soils for moisture, chemical substances, and heat. Pure sands show little cohesion, and change little in bulk or form during dry weather. Clays are very coherent, and may lose as much as one-fifth of their bulk by shrinkage during droughts; and peaty Soils shrink even more than clays. Cracks form in these Soils to a considerable depth, and allow evaporation to continue; and roots are broken across, and exposed to the air in the cracks, or are crushed by the shrinkage. Soils can frequently be much improved by a judicious mixture with others: e.g., sand should be added to clay; clay or peat to sand; lime, clay, and sand to peat. ORIGIN. All Soils are formed in one or other of two ways, either from the weathering and breaking down of rocks, or from the decay of plants or animals, and most of them are produced more or less in both ways; though the former has been the source of by far the greater bulk of all Soils except peat. In some localities, it is not difficult to recognise that the Soils are of the same composition as the rocks on which they lie, and that they are formed by the action of the weather in breaking up the rocks and reducing them to fragments. In course of time, also, the decay of successive generations of plants gives origin to an admixture of humus in almost all Soils. But, in most parts of Great Britain, the Soils are different in composition from the rocks on which they lie, and must have been brought into their present situations from a greater or less distance. That this should be the case in valleys, is easily understood, for the Soil on the slopes is constantly being carried down by showers and streams, and spread over the lower ground. This also occurs on fields or meadows along rivers in the lowlands liable to be overflowed, as the finer particles of mud in the water are deposited on the flooded ground, where free from the action of currents. Frost splits pieces of stone from exposed rocks and cliffs, and causes them to roll down the slopes, and it is also a powerful agent in reducing rocks and stones into the finer particles of which Soils are composed. But a layer of a few inches of earth will protect the rocks very greatly, if not entirely, from the action of the air, rain, and frost; and these agents are not sufficient to account for the amount and depth of soil, or for the relations of the soil to the subjacent rocks, now prevalent over the country, especially in Scotland and in the northern half of England. A far more powerful force than any now existing in Britain was, however, at work during a comparatively recent geological period. This was ice, which, for a long time, covered the land (much as Greenland is covered now) with a continuous sheet. Formed and renewed on the higher tracts of country, it spread down the valleys, and over all but the highest peaks, extended over the lowlands of Scotland and Northern England, and met ice pushing its way down from the Norwegian mountains into the ocean. Probably, the present German Ocean was then blocked with ice; and ice from Scotland extended over the Isle of Man, the North of Ireland, and the Hebrides, and a considerable distance into the Atlantic Ocean. As the ice pressed down from the hills, its enormous weight ground down the looser and more prominent rocks, and pushed the Soil, loose rocks, and stones, before it in the direction in which it travelled, until they were deposited in hollows, protected behind hills or ridges. As the climate became warmer, the ice diminished, till it did not reach the sea; then it gradually receded in the lowlands, till it became restricted to the mountain valleys; and, finally, it disappeared completely, even from our mountains, leaving its traces only in markings on rocks over which it had passed, and in the mounds of stones and earth (moraines) left behind as the glaciers receded. The Soils formed before the Ice Age were pushed by the ice from the situations in which they had originated, over rocks of a different kind, and were so mixed together as to frequently render it very difficult to trace their sources; but, by this mixing, the Soils have often been much improved--clays, sands, and limestones being mingled, so as to combine their valuable properties. Vegetable Earths, or Humus, are formed in great part of the remains of plants, and, to a slight extent, of animals; and along with these is a varying amount of Soils of purely mineral origin (as described above). Cultivated Soil usually contains a good deal of Humus, which gives a darker hue the greater the amount of it in the Soil. When the organisms decay in comparatively dry earth, the resulting Soil is known as Mould; and any Soil containing more than 6 per cent. of organic remains, is called a Vegetable Mould; but the proportion of organic matter is often much higher. These Moulds are known as sandy, clayey, or loamy, according to the nature of the mineral Soil mixed with them. Peat is the name given to Vegetable Earths formed in the temperate zones, under water, or in swamps saturated with water. It is frequently from 5ft. to 6ft. deep, and in some Irish Peat-bogs it even reaches 40ft. in depth. It is formed by the decay of aquatic and marsh plants. Peat-bogs seem frequently to have originated in forests, where fallen trees obstructed the drainage of the surface-water. The ground became saturated with water, but the surface of the swamps then, as now, bore a luxuriant vegetation of marsh-plants and Bog-mosses (Sphagna), which decayed below, and formed new Peat, while they continued to grow upwards. The Peat-mosses, so plentiful in many districts, have been formed chiefly, if not almost wholly, since the Ice Age; they seem to be now wasting away from natural causes, apart from human agency, more rapidly than they are renewed. Newly-formed Peat yields in general only 1 or 2 per cent. of mineral substances derived from the plants, and is brown, light, porous, and fibrous. Deeper down, the Peat becomes much darker and denser, and gradually loses all traces of its vegetable origin: the ash may reach as much as from 10 to 30 per cent. of its weight. Peat-bogs are not adapted for cultivation in their natural condition, although they are the favourite habitat of certain plants, such as many of the Heaths, Rhododendrons, and allied plants, and although Peat is most useful material in gardening operations, being employed in the Soils prepared for the cultivation of many plants from the Cape of Good Hope and Australia, and many Monocotyledons. Peat is also an excellent material for steeping in liquid farmyard manure, either in tanks, or as litter, to prevent the manure running to waste, the Peat being afterwards used as manure, alone or in composts. To render Peat-bogs fertile, the excess of water must be drained off, and lime, sand, and clay added The effect of this treatment is that the Soil is warmed, and the hurtful organic acids are destroyed by the lime; while the lime, sand, and clay, together, render the Soil more open and pervious to gases, and prevent stagnation of water in it. By such treatment, Peat-bogs may, in time, be converted into fields or gardens, capable of yielding a good return; but the labour and expense incurred in improvements of this kind are usually considerable. CLASSIFICATION. The chief kinds of Soils have been incidentally mentioned above, but it will be well to recount their more important differences. They may be classed according to their composition as follows: Sandy Soils, with not less than 80 per cent. of pure quartz sand; such as may be met with among the dunes or sandhills, along our coasts. These contain little nourishment for plants, are very liable to suffer from drought, have little cohesion, and are blown about by the winds; they produce light, but early, crops. Sandy Soils can be improved by the addition of clay, and lime in the form of marl or of chalk. Turnips often do well in the better class of Sandy soils; and, in wet years, these Soils yield a very fair produce. Clay Soils, chiefly composed of clay (Aluminum Silicate), result from the breaking down of felspars in granites and in rocks of similar composition. They are heavy, dense, and very coherent, and are very retentive of water; but the water is apt to stagnate in them, and to render them late and cold by the amount of evaporation from the surface, near which it remains, there being no crevices for it to pass down. In droughts, plants on Clay Soils are apt to suffer, as roots cannot penetrate into stiff clays, or obtain water from the Sub-soil. These Soils contain abundance of mineral food for plants; but it is rendered unavailable by their stiff texture. When this is corrected, by the addition of sand, lime, ashes, or suitable manures, and when the stagnant water is removed, by drainage, clays become very fertile. Calcareous Soils contain above 20 per cent. of Carbonate of Lime, in the form of chalk, or mixed with clay to form marls. These may be dry and friable, or (e.g., marl) may approach the clays in texture. In productiveness they vary greatly. They are less frequent than the two soils already described. Peaty Soils have been discussed above at sufficient length; as have been also the methods of improving them. Vegetable Moulds (Humus) vary much in percentages of organic matter contained in them. They belong to the more fertile kinds of Soils, and are retentive of water. Gravelly Soils may belong to any one of the first three classes, seldom to the fourth. The term applies only to the presence of gravel or stones in Soils, without reference to their composition. Loamy Soils are intimate mixtures of all the first four, in which the clay is under 50 per cent., and the lime under 5 per cent. Loams are productive and excellent Soils, being easily cultivated and fertile.