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THE SOILS WE CULTIVATE.
UNIVERSALITY Or SOILS—ALL BOCKS WEATHER INTO SOILS—SOILS PARTLY OF MECHANICAL, PARTLY OF CHEMICAL, AND PARTLY OF ORGANIC ORIGIN—EFFECTS OF VEGETABLE AND ANIMAL AGENCY —SOILS FORMED BY WIND-DRIFT—BY WORM-CASTINGS—SUBSOILS, THEIR NATURE AND COMPOSITION — ANTIQUITY OF CULTIVABLE SOILS.
Generally speaking, the soils and subsoils of a country receive but scanty consideration from geologists, yet fewthings are more curious in their formation, none so indispensable in the cosmical purposes they subserve. It is true they are recent, and insignificant in mass, compared with the rocky crust which forms the great theme of geology • but considering how intimately they are associated with the manifestations of vegetable and animal life, as well as with the food-supplies of man, they are deserving of a closer inquiry, whether as regards their origin, their qualities, or their artificial amelioration. They meet us at every turn on the terrestrial surface, and, Like the outer covering of plants and animals, give beauty and unity of outline to the stony skeleton that lies below—here levelling inequalities, there smoothing over asperities, and everywhere conferring on the harder elements softer and more attractive features. According to depth and quality they are wet or dry, cold or warm; here teeming with luxuriant plantgrowth, there scantily covered with the lowest forms of vegetable existence; here capable of amelioration by human ingenuity and industry, there defying all change, and doomed to everlasting sterility. There are few things, indeed, more varied in character, and none that tells so directly on the aspects, the fertility, the industry, or the general wellbeing of a country. Let us note a few facts relating to their origin and more conspicuous characteristics; and, first, of the origin of soils, and their slow and gradual formation.
Take the surface of the hardest and most refractory rock —say granite or porphyry; strip it of every loose and incohering particle, sweep it bare by the conjoint agencies of wind and water, and then let it remain exposed to the influences that naturally surround it. In a few years the gases of the atmosphere will have weathered and wasted the surface, the frosts disintegrated, and the winds and rains blown and washed the disintegrated fragments into the sheltered hollows and depressions. Insects, the droppings of passing animals, the drift of wind-borne debris, and the like, will mingle with the rocky fragments—and thus imperceptibly the naked surface receives the first beginnings of a soil. In a few years more, the air-borne spores and seeds of plants will germinate and take root, and, however feeble the growth of the lichen and moss, or other lowly plant, its annual decay is ever adding to and altering the character of the scanty mould; and thus, in the course of generations, the naked rock is covered with soil, partly through mechanical, partly through chemical, and partly through vital agencies. And as this soil is ever thickening and increasing, higher plants and higher animals will gradually find a new home—each in turn preparing the way for another—till at length, in the course of ages, the flinty rock is clothed with a fertile and life-sustaining envelope.
Take, again, the surface of the recent lava-stream, bare, blistered, and vitrified, and which looks as if it would resist for ever the powers of disintegration and decay; and yet see how it yields to the combined influences of chemical and vital action, and breaks down at last into soil and fertility. The vine, olive, and myrtle clad slopes of Vesuvius, a fewhundred years ago, may have been as rough and slaggy as the overflows of 1868; and these, in time and under the same agencies, will be covered with a similar mantle of softness and greenery. "The decomposition of lava," says Professor Phillips, in his recent work on Vesuvius, "is effected by the action of air and water, aided by the growth, and decay of vegetation. According to the nature of the particular current, its order of silication, the state of its aggregation, and the presence of iron, alumina, soda, potash, magnesia, or lime, changes in the mineral substance are more or less easy. The carbonic acid of the atmosphere, with that derived from the decay of plants, operates slowly but effectually in breaking the chemical bond of union among the elements, and making new arrangements. The iron oxide becomes a hydrate or a carbonate, the alkalies are separated, and the rock is reduced to soil, on which, plants operate farther changes. Even on the solid lava, the almost unobserved lichen—itself a sort of living fibre of stone—fixes its unfriendly hold, breaks up the firmest union of grains, and admits the farther action of other vegetable growth. Nor must we omit the supply of gaseous agents: sulphuretted hydrogen, productive of sulphurous acids, and carbonic acid, which is the long-enduring follower of irruptions, and ascends through innumerable fissures, to perform its almost universal work of disintegrating the not everlasting rocks."
Or take the sand of the sea-shore blown by the winds beyond tide-mark, and left high and dry in its barren purity. Spotless and glittering in the sunshine as it may remain for years, the time will come when some drifted seed of maritime plant will take root and bind the shifting surface into clumps. Other plants will gradually follow, and their annual decay, together with fragments of seaweed, the droppings of sea-fowl, the wasting of the shells incorporated in the sand, and the like, will prepare the way for the stronger sand-reed with its conservative roots, and then the first beginnings of turf or surface-soil is established. We have watched the process by the sandy shores of the Tay and Eden, where acres of barren shifting sands are now clothed with a luxuriant growth of sand-reed, mosses, sedums, grasses, rest-harrow, trefoils, and other plants, whose annual growth and decay, incorporated with the remains of insects, the droppings of rabbits and sheep, and other animal exuviae, are gradually accumulating a layer of dark and composite soil. And in this way, and none other, have millions of acres of sea-formed sands been converted into dry, rolling, pleasant, and profitable pasturelands.
Or still farther, let us take the oft-repeated instance of the coral islet, and note, as it emerges from the sea, how its own fragments pounded into sand by the surf, together with broken shells, sea - urchins, dead fishes, and other marine exuviae, are gradually forming a receptacle for the nuts and seeds that may be drifted from some older land. These seeds come at last; the cocoa-palm strikes its roots, and waves its feathery fronds along the shores of the solitary atoll—the first harbinger of other vegetable forms, whose growth and decay will in a few generations clothe the sunny islet with a soil of the richest fertility. This conversion of the coral-reef into a fertile island is well illustrated by Pratas Island, about 170 miles from the mainland of China, and 250 from Formosa. "This islet," says Dr Collingwood, " is about a mile and a half long, and half a mile wide, and is only visible at a distance of eight or nine miles in clear weather—not rising in its highest part more than twenty or thirty feet above the level of the sea, though the bushes that cover some parts give it an additional elevation of ten feet or so. It is formed entirely of coarse coral sand or debris, generally shelving gradually, but in some parts having a steep bank about three feet high. The interior is rough and irregular from accumulations of similar 'white sand blown up from the shore; and so overgrown is it with shrubs as to be in some parts almost impenetrable, though the soil might be supposed to be anything but favourable to vegetable growth, nothing but sand being everywhere visible, and that of the coarsest and loosest description."
Even the stagnant pool, the quaking morass, and the long reach of sea-mud, gradually acquire by accumulation and drainage a surface consistency; and on that surface, where the green scum formerly mantled, now spring up terrestrial plants, whose growth and decay, associated with animal exuvias and the castings of the earth-worm, are season after season converting the whole into rich and exuberant soil. This effect of the earth-worm was long since noticed by White in his ' Natural History of Selborne,' who remarks, that "by perforating and loosening the soil they render it pervious to the rain and to the fibres of plants; and by drawing straws and stalks of leaves into it, and especially by throwing it up into coils called worm-casts, they fertilise it—for that matter, being their excrement, is a fine manure for grain and grasses." More recently, Mr Darwin has shown that worm-casts, which so annoy the gardener by defacing his smooth-shaven lawns, are of no small importance to the agriculturist; and that this despised creature is not only of great service in loosening the earth, but is also a most active and powerful agent in adding to the depth of the soil, and in covering comparatively barren tracts with a superficial layer of fertile mould. His attention had been directed to several fields at Maer Hall in