On Naval Timber and Arboriculture / With Critical Notes on Authors who have Recently Treated the Subject of Planting

by Patrick Matthew

As the causes which promote or retard the formation, or which tend to dissipate the earth’s covering of vegetable mould—a covering, on the richness or thickness of which the fertility of ground, as well for most kinds of naval timber as for other products, is so much dependent, though of the greatest importance—have never, that we are aware of, been generally brought into view, we shall devote some space to their consideration.

In the first place, to give a fair specimen of our author, we shall transcribe several pages where he {312} has treated this subject with some ingenuity, and on which he appears to have bestowed considerable care.

“Those who have never had an opportunity of seeing old woodlands brought into cultivation, will scarce credit what has now been advanced, that the soil should be enriched by the production of wood, when the experience of ages has proved that it is always exhausted by other crops.”—“Trees draw their nourishment from a much greater depth than any of the grasses, roots, or different kinds of grain raised by the agriculturist. Most of the latter derive the whole of their subsistence from the part of the soil that lies within a few inches of the surface; but the former, from the superior strength and magnitude of their roots, are enabled to penetrate much farther, and extract food from the very rock which forms the substratum of a great portion, both of our cultivated and uncultivated grounds. This, though it does not account for lands being positively enriched by wood, makes it, at the same time, far less surprizing that trees should grow to a large size, and yet not exhaust the upper part of the soil in so great a degree as most of the crops cultivated by the farmer.

“There is another circumstance which gives ground in wood a great advantage over that in {313} tillage, which is, that the leaves of the trees are suffered to decay and rot where they fall, and, by this means, an annual addition is made to the depth of the vegetable mould. Now, the leaves of a tree may be considered as bearing the same proportion to the trunk and branches, in respect to the nourishment which they require, as the straw of corn bears to the grain. But the manure which cultivated land receives, is, in general, little more than the straw which grows on it after it has served for food or litter to cattle. Ground in wood, then, actually receives, in the annual fall of the leaves, as much enrichment as the farmer bestows on his land under tillage.

“Ground employed in agriculture is exposed at almost every season of the year to the full action of the atmosphere; and in the drought and heat of summer, much of its strength is evaporated. In land covered with wood, the case is entirely different, as from the shade afforded by the leaves and branches, very little evaporation takes place. This, then, is another reason that serves in some measure, at least, to explain the seemingly paradoxical fact in question. For, that evaporation has a very powerful tendency to exhaust land, by drawing off and dissipating the more volatile part of the matter which {314} assists in the process of vegetation, there can be no doubt, when we consider that any kind of dung may be deprived of the greater part of its strength by being long exposed to a dry atmosphere. Nor is it merely by preserving its own original substance that land in wood has the advantage of cultivated ground. Whatever is extracted from the latter in the form of vapour, falls again, when condensed, in the shape of rain or dew; but, instead of descending wholly on the same spots from whence it rose, it is, of course, diffused over the whole space which the clouds, containing it, may happen to cover, and woods and moors have as good a chance of receiving it on its return to the earth, as the ground in tillage. The part of it which falls, either on the cultivated fields or the naked wastes, may be again evaporated before it has time to be productive of any benefit; but the portion of it which the woodlands imbibe is retained to enrich the soil; for, the umbrage excluding the rays of the sun, there is no possibility of its being extracted a second time. Land covered with trees, therefore, while it never loses any thing, receives, with every fall of rain, or of dew, a tribute from the riches of the cultivated part of the country. The advantage derived from this source is greater than will be credited by those who are not aware how much {315} of the substances proper for vegetable nutriment are exhaled from the land in a gaseous state during the dry season of the year.

“But the principal way in which wood becomes instrumental in enriching land still remains to be noticed. When trees attain a certain size, they attract multitudes of birds, which build their nests and seek shelter among the branches. The dung of these animals is the very richest kind of manure which can be applied to land, and possesses, at least, three times the strength of that commonly used in agriculture. The quantity of it produced during the long series of years which trees require to reach maturity is, especially where large colonies of crows take up their abode, very considerable, and must have a powerful influence in improving and fertilizing the soil.

“I ought not to omit here to mention, among the causes why ground is improved by producing wood—the minuteness into which its particles are divided by the roots and their fibres. On taking up a young tree, or even a gooseberry bush, and shaking the earth from its roots, we find the mould that falls from it as completely reduced to powder, as if it had been passed through a fine sieve. Now, the fact {316} seems undoubted, that land is much increased in fertility by being brought to this state.”

Whether a greater accumulation of vegetable mould or enriching of the soil, would take place under a system of rotation of crops, stirring of the ground, and manuring, or under Nature’s own system of management—whether, under forest, or under the rich leafy grasses depastured by cattle, is a question of the greatest intricacy, and only admits of local decision, being dependent upon climate, soil, and circumstance. From our author’s statements, it would appear that his mind had only ranged along the surface of the subject. He has not taken into account the quantity of root which herbaceous vegetables annually leave in the ground—in some kinds little inferior in bulk to the portion above ground. We have traced oat and wheat roots running down into clay five and six feet (as deep as those of many kinds of trees), extremely numerous, and fine as human hair. He seems not aware that the bulk of yearly vegetable produce is much increased by culture, alternate cropping, and extraneous manure, such as lime, mixture of earths, sea-ware, bones. He has not considered that the annual dead roots within the soil, and the vegetable and animal manure, and the {317} sward and the stubble ploughed down, conduce much more to enrich and thicken the soil than the tree leaves, blown about by the winds, and nearly dissipated into air, before the residuum fixes as a part of the soil; and also that ploughing is often beneficial to shallow soils, by mixing the thin covering of mould with the pure earth of the subsoil,—the vegetable soil-matter, from consequent deeper cover, and more equable moisture, not losing so much by evaporation, and at the same time being more efficacious as nutriment to the vegetation. He seems unacquainted with the fact, that the matter of wood and tree-leaves, especially of the resinous kinds, and those containing much tannin, if not actually pernicious, have very little fertilizing effect—saw-dust has generally no manuring influence, but turns into peat. He also appears to be ignorant, that some kinds of vegetables draw more from the air and water, and others more from the earth; and, especially, that vegetables in a moist climate, depastured or cut before maturity, exhaust the soil much less than when allowed to seed. In Britain, soils, particularly those of good quality, become richer, and thicken more under pasturage, than under any other common vegetation. This is owing to the manuring of the cattle—to the natural grasses not being what is termed scourging plants, especially {318} when not allowed to seed—to the complete cover of the ground by the leaves—to the quantity of root which dies annually—and to the mould thrown up by the red earth-worm, renovating the surface, and partly covering the moss and decayed leaves and old bulbs. It is a curious fact, that, under pasturage, fertility should increase in Britain and diminish in Australia. An uncropt deep cover of grass appears necessary to shelter the vegetable soil-matter during the arid heat, and even to protect the roots from being burned out, in the latter country. And the manure of cattle, instead of being covered by the luxuriant herbage before it is desiccated, and enriching the soil as in England, is, in New South Wales, under the powerful sun and arid air, quickly reduced to dust and dissipated.

The fertility of soils may also be quickly increased, and the vegetable cover thickened almost to any extent under tillage, by first rearing a quantity of large growing annual vegetables, and when nearly full extended, burying this green vegetable produce in drills, resowing the ground immediately with another fast growing kind, and proceeding thus continuedly.

The influence of birds in enriching forest soil, is exceedingly limited, and is chiefly perceptible, not in continued forest, but in some detached portions or {319} clumps of park trees, which colonies of rooks or other large birds frequent.

Of the natural grass which Mr Cruickshank states succeeds in woods to the original heaths, and which he describes as affording such excellent tender food for cattle, we can only say, that either the woods must have been unprofitably thin, and the trees naked, or that he has completely mistaken the quality of the herbage. The grass of woods is unhealthy food for cattle, and generally not relished, being rendered unpalatable and noxious by the resinous and bitter droppings from the tree leaves, and by the bitter and nauseous juices generated in the soil by the roots of trees, which the herbage roots draw up. In dry soils, there is sometimes an accumulation of whitish substance within the ground, around the roots of trees, which some refer to excrementitious deposit[61], but which, we think, is rather the produce of a subterraneous vegetable, of the nature of a fungus or mould. Wherever this has increased to a considerable extent, we believe old forest ground will be found of great fertility. {320}

The friability and minute division of the soil to which Mr Cruickshanks refers, existing around the bulbs of trees, can only be of utility where the soil is too adhesive. Light soil is often injured by being cropped by plants which tend greatly to reduce adhesion—what the farmer styles being driven: besides, all luxuriant annual crops render adhesive soils friable; and, remaining for a time under natural grass, gives what is termed a turfiness to soils, which continues for several years, and which renders both adhesive and light soils more productive, preventing the adhesive from sinking down into mortar under cultivation, and the light from losing all adhesion or granular arrangement.

There is, no doubt, a disposition to accumulate vegetable deposit in forests, from the moistness, coolness of the ground, and shade, not tending so much as the sunshine and exposure of open country to dissipate or volatilize the residuum of the decayed leaves and roots. In a lower latitude, beyond the line of peat formation, this will have some influence to increase the depth and richness of the vegetable mould; but, in Scotland, where cold till bottom prevails, more injury will result from forest tending to throw the debri of vegetation into combinations unfavourable to the nourishment of plants (such as peat {321} and compounds in which iron forms a part), than advantage, from the dead vegetable matter not being so much dissipated by aration and exposure to the sun. We have often observed the effect of remaining for a length of time in a state of considerable dryness, dissipating the vegetable part of the soil, in some of the old infield clays, where the crown of the large ridges are raised up a foot or two above the original surface-level. At the crown of the ridge, the vegetable clay mould often only extends down about nine inches from the surface, the subsoil immediately under being nearly void of vegetable matter, and extremely close tenacious clay,—a solid foot of it, though of equal moistness, being nearly double the weight of the same bulk of the vegetable clay mould above it. From this clay, almost purely mineral, being a little above the original surface-level, there can be no doubt, that at one time it consisted of the vegetable surface mould of the country, heaped up by repeated ploughings, and that it has gradually lost the vegetable part. The depth of vegetable soil, near the furrows of the ridges, is generally found to be greater than at the ridge crown.

The same dissipation of vegetable matter takes place when a ditch has been dug in clay ground, and {322} the excavated earth thrown up to form a dike on one side. On removal of the dike, the original surface, which no doubt, at the time the dike was formed, consisted of vegetable clay mould similar to the surface around, is always found to be close, heavy, poor clay, containing little or no carbonaceous or vegetable matter. In this case, from the draining effect of the ditch, the original surface under the dike must have been drier than the subsoil of the crowns of the ridges.