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Transactions of the Highland and Agricultural Society of Scotland
On the Soils and Subsoils suited for Planting

By William Gilchrist, Cluny Castle, Aberdeen. [Premium—Five Sovereigns.]

As a general rule, soils and subsoils that are considered suitable for growing the various species of forest trees are those that are unsuitable for agricultural purposes, consequently trees are often planted in soils and subsoils quite uncongenial for their growth and development. Although there is no doubt that soil and subsoil exercise a considerable influence on the development of trees it is not altogether paramount, as, however suitable they may be, if the exposure is too severe, or the altitude too high, the trees will not succeed. Altitude and exposure must, therefore, be taken in connection with soil and subsoil, as suited for the growth of timber trees. Altitudes are generally described as low, medium, and high—low being under 350 feet, medium from 350 to 800 feet, and high from 800 feet upward.

In describing the different soils and subsoils we lay no claim to a scientific knowledge of the subject, but simply base our report on practical experience and observation. Relying on this basis, the chief difficulty is to get examples alike in every respect, as almost every situation has some local peculiarity that, in some way or other, tends to stimulate the growth of trees. In some parts of the country the soils are so variable that it is often difficult, without a chemical analysis, to distinguish between peat and loam, the one running so much into the other; and there is no doubt that many of the inaccurate statements, regarding the suitableness of certain soils and subsoils for the development of forest trees, may be traced to this cause.

1. Peat.—This is generally understood to be the natural accumulation of vegetable matter on the surface of waste lands, and is always more or less saturated with water. Considered as suitable for the growth of wood, it must be divided into two distinct classes—first, the natural accumulation of wooded vegetable matter; and second, the natural accumulation of succulent vegetable matter. The former of these is generally suitable for growing wood, while the latter is only occasionally, and under special circumstances, fitted for that purpose.

1st, Brown peat, with an open subsoil on the granitic formation. This is the prevalent soil on hill sides and moorland that is covered with heath. It is well suited for the growth of Scots fir at low and medium altitudes. In many cases the Scots fir produces good timber on this sort of soil at altitudes ranging above 1000 feet; but these are exceptional cases, and are generally found to be in sheltered and otherwise favourable situations. The quality of the timber on this sort of soil depends entirely on the depth of the soil and the openness of the subsoil. If the peat soil is deep the young trees succeed very slowly for a number of years, until their rootlets get through into the substratum, after which they grow more rapidly; but at the best the wood is soft and rough in the grain. When the trees are cut down the concentric rings for the first fifteen or twenty years are found to be comparatively small, after that period they gradually increase. When the soil is shallower the trees grow more equally throughout, provided always that the subsoil is open, and are smaller in the grain, and the timber of superior quality. When in a young state they generally grow with a good deal of taper; but with a medium altitude and a suitable situation they will, at the age of sixty, have from 30 to 35 feet of bool, and average about 10 cubic feet. Some of them will be considerably larger, but in all plantations of that age there is a certain amount of spar-wood, and sometimes even props, which take down the average size.

This variety of peat is also very suitable for birch and plane, especially the former, which in many parts of Scotland grows naturally on soil of this description. Still, although the birch is a graceful tree, and a rapid grower at low and medium altitudes, and on soil and subsoil most suitable for its growth, it is not to be compared, as a profitable tree, with the Scots fir, and should never be planted for profit except at high altitudes and hill sides, where, owing to the severe exposure, Scots fir does not succeed. We have never met with any large quantity of plane trees growing on this sort of soil, but, judging from what we have seen of it in mixed plantation, as also in groups and single trees, there is no doubt that the soil is suitable for its growth, especially over whinstone formation at medium altitudes.

At low altitudes this soil is also suitable for the growth of spruce in masses, and where a good demand exists for spruce it is a remunerative crop. When grown singly or wide apart they are, in general, very rough and branchy; but when grown in masses they are the reverse, and are very suitable for many country purposes. Willows and alders may also be grown, but the former almost invariably gets stagheaded when about forty years old, and the latter is not a profitable tree at the best. It may, however, be grown as copse wood with advantage.

On low altitudes this sort of peat generally requires to be regularly drained from 2 to 2½ feet deep; but on medium and high altitudes it seldom requires to be close drained, except where the formation is flat.

2d, Peat containing a proportion of sand, and resting on a clayey bottom. This, at low and medium altitudes, will grow oak, lime, and elm to fair sizes, but not so profitably as it would Scots fir, spruce, and larch. The latter often succeeds very well and grows to good sizes on light soil, provided it has a cool subsoil such as referred to. What a number of contract planters call "fern ground" comes very near to this description, and it is invariably selected by them as a soil suitable for the growth of larch. No doubt, it is also quite suitable for the growth of the other trees mentioned, but larch is the most profitable and remunerative crop when the situation is suitable. The timber, when matured, is generally clean grown, with very little taper; tough, but only a small proportion of heart-wood, as compared with larch grown on heavier soils. Elm grown on this soil is soft and open in the texture, and more brittle than when grown on stronger soil. The same remarks apply to the ash; and a strange coincidence is, that both shed their foliage from ten to fourteen days earlier, when grown on peat soil, than they do on loam,—especially is this the case if the bottom is damp. On peat soil the timber of the oak—even the heart-wood—is brittle, soft, and open, with a comparative small proportion of heart-wood. The bark is also more corky, and does not contain the same percentage of tanning properties. However, the soil is quite suitable for its growth; and where it is desirable to grow oak as an ornamental tree, or for variety, a peat soil with a proportion of sand need not be considered any drawback; but it cannot be grown profitably on such soil.

The lime tree and willows (Huntingdon and Bedford) also grow to fair sizes on this soil, but in general they are very branchy, except when they have been artificially pruned. The former is the most suitable tree for the soil, although the willows at from forty to fifty years old are the most profitable, especially when grown at low altitudes. One of the chief drawbacks to growing willows on this sort of soil is their tendency to be blown over. Thorough drainage, so as to allow the roots to get down into the subsoil, is the only remedy. Of course, the tops can be lightened, but that is taking away the vital energies of the trees, and besides, the remedy is only temporary. The poplars do not succeed so well as the willows, and are liable to all their drawbacks, although they are often planted, and sometimes even grow to fair sizes. At low and medium altitudes the Norway spruce can be grown profitably on this kind of peat, especially on the former and in sheltered places. In such situations spruce is one of the most profitable crops that this soil will produce. The Abies Douglasii, also grows in similar situations, at least to a certain age, but of course it is of too recent introduction to form a positive opinion as to the soils on which it will succeed; but in a young state, it promises well.

If the altitude is high and the situation exposed, this sort of soil and subsoil is best suited for growing Scots fir, and they will be found to be as safe and as profitable a crop as the soil can produce. The quality and durability of the Scots fir is much improved by a proportion of sand amongst the peat, especially if the subsoil be granitic. If on a limestone formation, the Scots fir is of a branchy habit, and the timber rougher than when on granite. They grow rapidly up to a certain stage, when they generally branch off into numerous leaders and get flat-headed. The stage at which this appearance occurs depends on the depth of the subsoil, as it is generally allowed to be caused by the roots coming into contact with some substance antagonistic to their development. Under the same circumstances, larch, besides losing their leaders, are affected with blisters and ulcers, and when such is their state, it is considerably reduced in value, and as a profitable crop is a failure. The hardwood stated as suitable for growing on a granite formation is more suitable for growing on a limestone formation under similar circumstances; thus showing that on the granite formations the fir tribes are the most suitable, while on the limestone formation the planter can have a wider choice.

3d, Peat of various depths, but generally shallow, with a moorband pan and hard subsoil, is very common in some parts of Scotland, especially on flat moorland. The soil is invariably wet on the surface, and in its natural state is quite unsuitable for the growth of trees. If thoroughly drained and the pan broken, Scots fir and spruce will grow on it to be a fair average crop. Unless thoroughly drained, the trees either die off young or get twisted into all conceivable shapes, and never attain to anything like timber proportions. The spruce is only suited for growing on low and medium altitudes, but the Scots fir will grow on either. Birch will do the same; but as mentioned before, it is not so profitable, and should only be grown where variety is wanted.

4th, Deep black peat, such as is generally used for fuel, is not suitable for the growth of trees under any circumstances; still, if it can be got thoroughly drained and a portion of the subsoil spread over the surface, a fair crop of spruce may be got to cover the ground; but unless the roots can get through the peat and into the subsoil, this sort of peat will not grow a profitable crop of trees. Some good crops of spruce and Scots fir are grown at low and medium altitudes on peat where the great proportion has been cast off for fuel, and only about one foot in depth left for surface; and where this is practicable, it is the safest way to secure a crop. Under such circumstances, neither the spruce nor the Scots fir grow to a large size, and at from fifty to sixty years old they may be considered at their best as a crop.

5th, Peat on a soft tilly or "boiling" clay bottom is unsuitable for the growth of trees, especially if the "boil" is near the surface. If there is as much surface soil as keep down the soft clay, any of the surface-rooting varieties may be grown, but they never grow to great dimensions, and are liable to be blown over at all stages. If the "boiling" clay can be got under by drainage the case is different, and wood may be grown profitably; but it is of the utmost importance that the drains should be kept at all times clear and in good working order. Spruce and alder are the varieties that will succeed best. The former is the most profitable, except in districts where a demand exists for alder.

6th,. Bog peat, or the natural accumulation of succulent vegetable matter with a soft bottom. This, in its natural state, is the most unsuitable of all for the growth of trees. Still, if it is desirable to grow trees or shrubs, it can be made suitable by thorough drainage, digging over and exposing the peat to atmospheric influence, and adding sand. Treated thus it will grow good spruce, alder, and lime. Taking everything into account, it cannot be said that the undertaking would be profitable; but circumstances might arise where it would be very desirable to have such ground occupied, or partly occupied, by a crop of trees or shrubs and treated in the manner indicated; this has been accomplished at low altitudes.

2. Loam is a soil consisting of clay, sand, and oxide of iron, with more or less of chalk. The qualities are various, from a light sandy loam to a heavy clay loam. If the subsoil and situation are suitable for growing trees, all the different qualities of loam are found to be adopted for the growth of some of the ordinary varieties of forest trees.

1st, Light loam on a sandy or gravelly bottom. This is a likely soil for the growth of trees, and yet there is perhaps no soil so unsuited for the profitable growth of the trees that are generally found growing upon it, especially larch; and it js a matter of surprise that, with so many examples of failure, arch is still persistently planted on such soils. Even when they grow to fair sizes and present a fair exterior, we have found upwards of three-fourths of them to be unsound at the root, and consequently a great portion of the root comparatively useless. This heart-rot is found to begin at an early period; and if larch is to be grown profitably on this sort of subsoil, they must be cut at an early stage for props and spars. During the summer of 1868, on such a soil and subsoil, in a mixed plantation eight years old, most of the larch and poplars died out; while the Scots fir, Laricio, Austrian pine, and plane stood apparently unscathed. The birch and beech were seriously checked, and have not yet recovered from the effects, but have still a languid and sickly hue. The larch were taken out and the blanks made up with Austrian pine, and they have stood the test of 1874. The plantation referred to is at a medium altitude. Scotch fir grown on this subsoil wants the dark green foliage that it has on heavier soils, and does not grow so tall, but the timber is generally pretty clean and of fair quality. The Austrian pine and Laricio do not change so much in the colour of foliage, and grow more freely; but we have not seen any of their timber cut up into scantling. Larch generally grows pretty freely up to thirty or forty years old, when it invariably begins to have a sickly appearance, becomes shorter in the foliage, and sheds its foliage from ten days to a fortnight before trees grown on heavier soils. Beech are generally short in the trunk, with scraggy, spreading tops, and of little value as timber trees. Birch and plane do not lose much of their ordinary habits, only they are of a great deal smaller dimensions than when grown on favourable soils. If the subsoil is damp, although of sand, it is more suitable for the growth of trees. Instances of this are invariably met with in plantations with an undulating surface, where, on the dry subsoil of the heights, the trees of all varieties are a great deal smaller than those in the hollows and low portions of the ground,—the only perceptible differences as to soil and subsoil being that the latter is more moist in the hollows. Of course, shelter must not be lost sight of; but all the difference cannot be attributed to it. This damp subsoil must not be considered as opposed to drainage for plantations, or apologising for stagnant water, as no soil that is surcharged with water is suitable for the growth of trees. On the other hand, where the water percolates freely through the soil and subsoil, it is not as a rule injurious to the growth of trees, but rather favourable. Under such circumstances, larch is the most profitable tree that can be grown, provided the altitude and situation are such that it will not be liable to be damaged by spring frosts. In inland districts the larch suffers most from frosts on southern exposures, and at altitudes ranging from 300 to 450 feet. In such situations, the crop should consist of two-thirds larch and one-third of Scotch fir, plane, oak, or beech, for all of which the soil and subsoil referred to is suitable, at altitudes not exceeding 1000 feet; above that altitude, the Scotch fir alone is most suitable.

2d, Light loam, with a hard clay or gravelly and clay subsoil, on a granitic formation, and at low and medium altitudes, is well suited for growing silver-fir, lime, plane, and beech. Scotch fir and larch are also often grown—in fact, commonly grown; but on this soil the Scotch fir does not produce the same quantity of timber as the silver fir; and the larch only succeeds for about fifty years, when it is very frequently affected with heart-rot. On such a subsoil as this, very fine silver firs, plane, and beech have been grown at medium altitudes. Compared with the spruce, Scotch fir, larch, oak, and elm growing in the same plantation, the silver firs were much healthier, and contained at least one-third more timber than the other varieties; while the plane had grown as rapidly as the beech. The same sort of soil and subsoil, over limestone or whinstone formation, is best suited for the growth of oak, plane, elm, ash, larch, and silver fir, provided the ground is naturally dry, or has been previously drained. Under such circumstances the oak continues to grow rapidly, until the hard subsoil becomes too hard for the roots to penetrate, when its top loses the leading shoots, and becomes more bushy, and its growth as to height is generally at an end. The wood is, however, tough at all periods of its growth; but in general it is from sixty to seventy years old before much heart-wood is formed. On heavy loams, over limestone, we have seen large oaks with only one inch of sap-wood; while on light loams we have often seen the very reverse. Plane trees are more surface rooters; consequently they are not so much affected with the quality of the subsoil, provided the soil is deep enough; but on a light, shallow loam, they are invariably of small dimensions, unless they are supported and derive nourishment from the subsoil. Elm and ash grow freely, especially in glens or hollows; but neither of them attain the same size as they do on heavier soils. The ash is, however, generally of good quality. Larch is not so much affected with heart-rot on the limestone as it is on the granite ; but it grows with more taper, and is shorter, and is very frequently affected with blisters. The silver fir grows freely on booth, and the difference is not so perceptible.

3d, Medium loam, of good depth and open subsoil. This variety of soil is well adapted for oak, elms, and planes, as also most of the newer conifera3, at low and medium altitudes. On this soil the plane grows to great size, and being generally sound, commands a high price. In fact, for some years back, good plane trees of large size have been about the best selling of our home wood. On deep loams they are proportionately larger in size; but we have not been able to detect much difference in the quality of the timber. Elms also prefer a deep loam; in act, the deeper the soil the larger is the elm, both Scotch and English; but both require a considerable amount of shelter to produce good timber. In exposed situations both varieties are frequently affected with ring-shake, which is sometimes aggravated by heart-rot. When such is the case, the value of the timber is considerably deteriorated. This sort of soil is also well suited for growing larch, limes, chestnuts, and in fact almost all the ordinary varieties of forest trees ; but larch, and the kinds first mentioned, are those that are considered to be the most profitable, except at high altitudes, when the Scotch fir must be preferred.

4th, Medium loam, with a clayey subsoil, in glens or sheltered places, is the most suitable for ash. In such situations, it is the most profitable tree that can be grown, as, besides producing a heavy crop if allowed to grow to maturity, it yields a fair revenue during the whole period of its growth, young ash being in great request at all stages. When grown on the granite formation, the wood is more brittle and shorter in the grain than when it is grown on limestone ; but a clayey bottom is preferable to either. As an ornamental tree, the ash grows well at high altitudes and in exposed places; but under such circumstances, the trunks are invariably short and rough, and consequently the wood is not of so much value as when grown on suitable soils and under favourable conditions. This sort of soil is also suitable for oak, plane, Spanish chestnut, black Italian poplars, beech, elm, silver birch, and Scotch fir; and, at low altitudes, spruce and willows. In fact, this is one of the soils in which there can scarcely be a mistake made in selecting suitable trees, as, under ordinary circumstances, it will grow to fair dimensions all the common varieties. Of course, there is a difference as to the value of crop; but there is no doubt that the ash, plane, black Italian poplar, larch, silver and Scotch fir are the most remunerative. Poplars and larch being the most rapid growers, attain to a large size on this sort of soil when about fifty or sixty years old.

5th, Heavy loam or clay. This is most common at low altitudes. Oak, ash, elm, and plane are the varieties most commonly met with on this soil, and they all grow to fair sizes and are of good quality, especially the oak and ash, many fine specimens of which are growing on clay soils, and often under very adverse circumstances as to situation. However, as a profitable tree for the growth of which this soil is suitable, the black Italian poplar is the best. It generally contains as many cubic feet of timber at 50 years old as the oak does at 100 years, and the price of poplar per cubic foot is sometimes as high as the oak, but in general it averages about one-fourth less. Owing to the branches of the poplar being wide-spreading and brittle, they are often damaged with the wind when grown in exposed places; but being free and rapid growers, they soon recover their appearance. When grown among other trees the poplar soon overtops the whole, and it is therefore more suitable for growing in masses than in mixed plantations. To plant this sort of soil successfully, good-sized well-rooted plants must be used, as small plants are always stiff to grow on clay soils; but when once they are fairly established, they grow rapidly.

6th, Sand. Until recently sand was considered to be unsuitable for the growth of anything but bent, but the success that has attended the planting of the Culben Sands, in Morayshire, has proved that sand is capable of growing a fair crop of Scots fir. The Pinaster also grows well on the Continent on sandy soils, and there is no reason why it should not be considered as suitable in this country. The successful growing of Scots fir on sand has been attended with a good deal of expense, but the results have been very encouraging, and such as ought to induce the proprietors of such lands to turn them to some better account than lying waste, exposed, and carried away by almost every wind that blows.

Conclusion.—Such is a description of the soils and subsoils suited for the various species of forest trees, coniferous and hardwood. The conclusions arrived at are the result of careful observation of the different varieties of forest trees under all circumstances. More species could easily have been enumerated, but our study in this report and our object in daily practice is to recommend the varieties from which the largest amount of revenue can be obtained, coupled with the varieties for the growth of which the soil and subsoil is most suitable. We feel that had the nature of the different soils and subsoils been more scientifically described and designated, this report would have been more interesting and readable to the savans of the Highland and Agricultural Society; but we believe that the soils and subsoils, along with the altitudes and situations as described, will be understood by the greater part of those who are interested and engaged in the planting of forest trees, or the management of plantations and wood lands.

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