This section is from the book "Handbook Of Hardy Trees, Shrubs, And Herbaceous Plants", by W. Botting Hemsley. Also available from Amazon: Handbook of hardy trees, shrubs, and herbaceous plants.
So much depends upon the nature of the soil affected by different plants, that we should like to devote much more space than we have at our disposal to the consideration of this question. Under the most favourable climatal conditions many plants have no chance of flourishing unless they are supplied with, or placed in, a suitable soil. It is true that a vast majority of plants will succeed in any ordinary, free, tolerably rich soil; but, on the other hand, there are others that prefer a light dry soil, a rich heavy moist soil, or a peaty soil, and so on.
According to the composition of the soils, the greater or less will be their power of absorbing and radiating heat, and retaining or discharging moisture. In other words, a soil may be either a good or bad conductor of heat; and evaporation and downward drainage of water will be either slow or rapid, in proportion to its density and depth. It is obvious, therefore, that the extremes of these conditions - though both are suitable for certain classes of vegetation - are alike uncongenial to the majority of ornamental plants in cultivation. From observations in Scotland,1 over a period of nine years, the temperature at three inches below the surface has fallen to 26°.5 in loose sandy soils, and at a depth of twelve inches the freezing point has only once been registered. In clayey soils, on the other hand, the lowest temperature at three inches was 28°, whilst at twelve inches it frequently fell to the freezing point, and once even 32° was recorded at a depth of twenty-two inches. Hence it follows that a moderately light porous, sandy soil, being a feeble conductor of heat, and readily parting with its moisture by evaporation and drainage, is eminently adapted for tender shrubs, bulbous plants, etc. A heavy adhesive soil retains an excess of moisture for many plants in wet seasons, and during a period of drought contracts and hardens as evaporation goes on, and opening in broad deep fissures permits the escape of moisture to a still greater depth than a loose soil.
1 Buchan, ' Handbook of Meteorology.'
Ordinary alluvial soil contains, according to its quality, more or less of the materials consumed by growing plants, such as alkaline oxides (lime, aluminium, potash, &c), almost always in combination, as neutral salts, with carbonic acid (carbonates), silicic acid (silicates), and sulphuric acid (sulphates). Phosphate of lime and carbonates of iron, copper and other metals, and soda occur, besides many others, in small quantities, little influencing the cultural operations. Besides the foregoing inorganic constituents, the soil embodies more or less decayed remains of vegetables and animals that have lived upon or in it. This detritus, which has received the name of humus, and which imparts a dark colour to the soil, is more or less rich in phosphate of lime, as well as nitrogenous and carbonaceous substances, and their presence in abundance indicates a fertile soil.
Soils are usually divided into three primary groups, namely, argillaceous, calcareous, and silicious, according to the predominance of clay, lime, or silica - the latter in the form of sand, resulting from the disintegration of granite rocks or sandstone. These substances are rarely found in a state of absolute purity, being usually mixed in some proportion. But the preponderance of one over the others is sufficient to give the name to a soil, and indicate its particular qualities.
Pure, or nearly pure clay is a compact, heavy earth, soft to the touch, and impervious to water. It dries with difficulty, but will acquire excessive hardness from long exposure to the sun. In this state of purity it defies all attempts at cultivation, and even with from 12 to 15 per cent, of silicious and calcareous matter it is equally intractable. A soil into whose composition clay enters to the extent of 40 per cent, would be termed argillaceous, but in this proportion it would be suitable for the cultivation of many things. Argillaceous soils are naturally more tenacious and difficult to work in proportion to the quantity of clay they contain. The soils belonging to this group are commonly designated heavy pr stiff soils.
Lime or calcareous matter results from the attrition of marble and limestone rocks (carbonates of lime). In a state of purity it is white, and receives the name of chalk. In this state it constitutes a poor soil, absorbs little heat from the sun's rays, dissolves under the action of rain, blisters and flakes from frost, laying bare the roots of plants growing upon it. Earth containing from 40 to 60 per cent., the remainder being composed of equal parts of argillaceous and silicious matter, is termed calcareous, and is generally very favourable for cultivation. It thus constitutes a light soil, easily worked; but a larger proportion of lime renders it very adhesive when moist.
Silicious, or sandy soil is diametrically opposite in its physical qualities to argillaceous soils. It is rough or gritty to the touch, light, friable and loose, permitting the passage of water with great facility, drying with the slightest sunshine, and rapidly accumulating heat. Pure silicious sand, such as results from the disintegration of sand-rocks, is in its nature quite unproductive; but if mixed with a certain proportion of humus, and especially if it contain from 15 to 20 per cent, of argillaceous and calcareous matter, it is suitable for the support of many plants. A warm, poor, well-drained soil of this description is eminently favourable for the growth of tender subjects. Fine silicious sandy soil, with sufficient humus or vegetable mould to appear brown or nearly black, plays an important part in horticulture under the name of peat-earth. On account of its lightness it is admirably adapted for raising small seeds, but frequent waterings are necessary in consequence of the facility with which it dries up. It is, moreover, the only soil suitable for the culture of plants that grow naturally in peaty or boggy places, such as Heaths, Rhododendrons, Azaleas, and the Ericaceae generally, and a few members of other families, collectively known as American plants, though in point of fact many of them are not of American origin.
Combinations of the three principal elements above described vary exceedingly in their relative proportions, and also by the addition of other matters which modify their physical properties. Hence the term mixed soils, as applied to the soils designated, according to their composition, argillo-arena-ceous, in which lime is almost wholly wanting; argillo-cal-careous, consisting mainly of clay and lime; calcareo-silicious, nearly destitute of clay. Ferruginous soil receives its name from the presence of a large quantity of carbonate of iron, giving it a reddish tinge. But these secondary distinctions are of little importance in practical gardening, as the soil, on account of its limited area, is almost always so greatly modified by improvements and the addition of manures.
Besides this primary division of soils according to their constituent parts, there is another distinction to be considered, namely, in regard to the nature of the upper layer, or surface soil, and of the stratum immediately below, termed the sub-soil, or subjacent rock, as the case may be. The surface of arable soil ranges from an inch or two to several feet in thickness, and is equally variable in the class of vegetation it will support. Greater importance perhaps attaches to the nature of the subsoil than is usually accorded to it, for on this depends the necessity or otherwise of artificial drainage, and the choice of trees, shrubs, and herbs that will permanently flourish in certain situations. Its thickness, as well as its mineral-ogical composition, is, of course, indeterminate. It may consist of solid rock, or beds of gravel, sand, etc, or it may also be composed of soil suitable for cultivation, and will thus serve to enrich the surface layer when it lias been more or less exhausted by the crops taken from it. In the case of a heavy, impervious clayey subsoil, artificial drainage is beneficial, or, as in some instances, indispensably necessary to ensure success to the cultivator.
It comes within our province to say but little respecting the improvement or enriching of ground by the addition of natural and artificial manures. To effect this object it is obvious that the nature of the soil to be manured should be considered; for some manures or moulds that would improve a heavy loam or clay would deteriorate the quality of a light sandy loam, and vice versa. Another thing to take into consideration is the use to which the ground is to be put. Luxuriance in the growth of some things is undesirable, as for instance in small shrubberies, and where tender subjects are planted. And then different subjects delight in different soils. A lawn rarely requires any stimulating manures, as they induce the growth of coarse herbage. We have seen a good piece of grass spoiled by the injudicious application of liquid manure. Except for hot-beds and the purpose of mulching the surface of the soil around newly-planted shrubs and trees to diminish the amount of evaporation in dry hot weather, little use is made of what is termed green dung in the pleasure-garden. Thoroughly rotten leaves (leaf-mould), fibrous turf, and farm-yard manure, wood-ashes, soot, lime, sand, etc, are employed according to the composition of the soil and its deficiency in certain desirable constituents. Inorganic, or purely artificial mineral manures, are seldom needed save where the crops are taken off the ground, in which case it becomes necessary to replace some substances taken from the soil. The principal thing to bear in mind in manuring is the fitness of the manure used for the soil it is wished to improve.
Vegetation is soon scorched up where the soil is sandy and shallow; but where there is a good depth of sandy loam, the quantity of water is more uniform throughout the year; neither an excess in winter, nor relatively so great a deficiency daring a dry summer. Much may be done by deep and thorough tillage and frequent breaking of the surface to render heavy soils more productive, because the water will thereby be drained from the surface when there is an excess, and attracted towards it when there is a deficiency. The greater the amount of evaporation the lower the temperature; thus natural or artificial drainage affects the soil in two ways by relieving it of its superabundant moisture. It is important to bear in mind that a light soil, although exposed to greater extremes of temperature at and near its surface, maintains a higher temperature below a certain depth, and also that there is less evaporation from its surface. These two conditions materially modify the effects of frost, and are of as much importance to the practical horticulturist as the strictly climatal peculiarities of a district.