The difference of depth and richness of vegetable mould, may nearly always be referred to existing causes, such as the original surface (diluvium, or decayed rock), being a combination of earths favourable to vegetation; occupying a genial situation; being favourably placed with regard to moisture, that is, less or more moist, according as the original surface has been clayey or sandy, or open or close bottomed; and is in no way connected with those flood torrents to which we owe the diluvium deposits themselves—tills, sand and gravel, in which we have never found any vegetable matter, excepting in the coaly or mineralized state.
Unless in the case of alluvium, or of drift sand, or where surface earth has been rolled down from {323} heights, or been forced by man[62], soil is seldom found to exceed 6 feet in depth, and that only in warm moist situations, propitious to vegetation. In Scotland we never have seen it exceed 3 or 4 feet in depth where its accumulation had not been aided by the above causes. The most common depth is from 6 inches to 2 feet; but, in many of our sterile districts, the surface hardly deserves the name of mould, containing very little vegetable matter, or that matter being unavailable from the presence of tannin.
It is a well known fact, that summer-fallowing always dissipates a portion of the vegetable matter in the soil, although it may, at the same time, tend to fertility, especially in adhesive soils, and where the climate is not very arid and warm, overbalancing the loss from dissipation by the advantage resulting from aëration and absorption of gases and heat, and the sun’s rays; by the mechanical disposition and comminution from being thoroughly dried and then moistened; and, probably, by the formation of salts, {324} stimulative to vegetation; or, as it has been thought, by the resting for a season. In the case of any tannin or inert vegetable matter existing in the soil, the heat and drying will tend to reduce these to a condition suitable for vegetable food. In the West Indies, when a summer fallowing is resorted to in order to get clear of the weeds, the fertility of the ground is considerably lessened, from the evaporation or burning out of the putrescent or carbonaceous matter. Were the fallowing continued for several successive seasons, there is no doubt that the whole matter, which, combined with earth, forms mould, would be dissipated.
About a century ago, it was the practice, in our neighbourhood (an alluvial clay district), to build up the soil of the fallow division, furrow deep, into thin dikes, or walls, about 5 feet high. This was done in early summer. After being dried and aërated by the summer’s drought, the dikes were levelled down in the autumn and sown with wheat. This system was considered so fertilizing as to counterbalance the labour and the loss of a crop.
Our own practice has proven that there is scarcely any manure more effective for one crop, particularly of spring sowing, than the clay of old mud walls of {325} houses, though applied in no larger quantity than is usually given of farm-yard manure, and though the clay appear quite free from vegetable matter. It is improbable that the resting of the clay from production could have any effect to occasion this fertility. We considered it to arise chiefly from a quantity of nitre having been formed in, or deposited about, the walls from their long proximity to animal effluvia and to atmospheric air. The fertilizing effect of the dike system of summer-fallow, and even of the present system, may also depend in part on the formation of nitre, well known to be a powerful manure or stimulant in this country. In dry seasons we have scraped together handfuls of salts, partly nitre, from the exposed surface of clay-banks. Should a considerable part of the fertilizing effect of fallowing arise from the formation of nitre, the application of lime and putrescent manures to fallows, in the early part of summer, will be advantageous, as the presence of both are favourable to the formation of nitre. Of course, the utility of encouraging the formation[63] of {326} nitre or other salts, combinations of potassa or of soda, will depend on the climate, whether much or little rain falls, and whether the rain water goes off by evaporation or by drainage. In the case of little rain, or the rain-water being nearly all evaporated, nitre and other salts will accumulate in the soil, so as, from their excess, to be injurious to vegetation; whereas, should much rain fall, or the rain-water be chiefly carried off by drainage, vegetation may languish from deficiency of these salts, there being less deposition of the salts, or the salts as they form being washed away. The same will apply to the graminivorous animals. Sea-salt, perhaps also nitre and other salts, will be serviceable in a moist country, or far from the sea, where the plants and water contain little saline matter, and probably pernicious in a dry climate, where the plants and water generally contain much saline matter.
In the portion of the earth from the Atlantic eastward, through Numidia, Libya, Egypt, Nubia, Arabia, Persia, as far as the Indus, from the enormous ruins, and other vestiges of dense population, as well as from ancient records, there must have {327} existed a considerable depth of vegetable mould covering, where now little is left but pure sand, baked clay, bare rock, and saline encrustations. From the footing which an industrious and brave nation has recently so honourably acquired in this territory, may we not hope that the tide of arid sterility, dissipating the vegetable covering, will be turned, and that through European enterprise and mechanical science, by means of steam and wind power, a system of irrigation will be introduced which will reanimate this dead portion of the earth—spreading forth again perpetual spring, strewing the desert all over with herbs, and fruits, and flowers, converting the sirocco into a breeze loaded with fragrance, and reproducing, in profusion, all the delights of the gardens of Hesperus? From the carbonaceous or soil-matter being burned out, and from the quantity of saline deposit, a very considerable time will, however, elapse before production be generally extended, and the desert so far circumscribed, and the ground cooled so much, as to condense a sufficiency of rain and dew, that a new vegetable mould cover may be formed.
But to return from our wide exclusion, we observe, that Mr Cruickshank states, page 25, “that any land that is proper for Scots fir will be found to answer well with the larch.” This observation, with {328} what he says of larch “being heavier in proportion to its bulk” than Scots fir, and that “spruce is very easily wrought, and tries the carpenter’s tools less than any other kind of wood used in building,” would lead us to suspect that our author has had a very limited acquaintance with his subject. A number of different soils will produce large Scots fir where larches will be generally rotted and hollow in the heart, by twenty years of age[64]. This ignorance of our author is the more glaring, as it is coupled with {329} some severe strictures on planters in general for their ignorance of the proper location of trees. He says, “Scots fir, on soils of a fertile character, is short lived, and the excellence of its timber is in proportion to the slowness of its growth.” This is erroneous. We would rather say it is short-lived in bad soil: Memel fir (Pinus sylvestris), is of very superior quality, very large growthed, and of great age. He also asserts “elm prefers a strong clay soil, and it is perhaps impossible to bring this tree to the utmost size it is capable of attaining in land of a different quality.” This is also erroneous. We have seen very beautiful large Scots elms grubbed out from a soil of pure gravel, and we can show thousands of instances where Scots elms do not thrive well in clay—in rich as well as poor clay. We are aware that in every volume treating of numerous facts, such as Mr Cruickshank’s, many inaccuracies may always be picked out, but the above are rather too prominent.
Mr Cruickshank censures the practice of covering fir seeds one-half inch deep in England, referring the {330} demand there for Scots plants to the seeds being thus buried in place of being sown, and states that they should only be covered one-fourth of an inch, as is the practice in Aberdeenshire. He also reprehends the author of the Encyclopædia of Gardening, on account of some directions which this author has given, to form, by forcing, a fine friable soil, suitable for the delicate seeds of trees, where this does not previously exist. Now, we should consider that the difference of climate between the neighbourhood of London and Aberdeen would require a difference of cover nearly equal to this; and that forcing a friable earth for seed-beds was absolutely necessary, in the very adhesive clays around London, and so general in the more recent formations of the south and middle of England, although superfluous in the north of Scotland, where sandy or light soil is sufficiently abundant. Seeds, under a moist cloudy atmosphere, will vegetate without cover at all; but in situations where the air is arid in spring, with much sunshine, a covering of some depth is necessary, and that covering, where the rudiments of the plant spring out weak and delicate, is required to be soft and friable, a good absorber and retainer of moisture, and not disposed to run together with rain, or crack with drought.
Mr Cruickshank gives an account of our different {331} forest trees, neither very accurate nor interesting, but, luckily, not very tedious. He then proceeds to treat of nursery, sowing, transplanting, and choosing of plants, where many sensible, though some of them common-place, observations occur, of much use to the generality of planters. His views, however, of the proper manner of planting seedlings in the nursery, are defective. The best method of planting these—neither by laying, nor by dibbling—is first to stretch the line and make a furrow, level in the bottom, as broad as the roots may stretch, with the inner side straight and steep. One person then holds the plant erect in its berth, from two to four inches from the perpendicular side, according to the general size of the horizontal roots, so that the fibres may be regularly spread; and another person throws on the earth from the place of the next furrow; the placer of the plants footing the earth to the roots as he proceeds, or after the row is completed.
The following observations of Mr Cruickshank are worthy the attention of planters: