1. Some soils are heavier and denser than others, sand and marls being the heaviest, and peaty soils the lightest. In reclaiming peat lands, it is found to be highly beneficial to increase their density by a covering of clay, sand, or limestone gravel.

2. Again, some soils absorb the rains that fall, and retain them in larger quantity and for a longer period than others. Strong clays absorb and retain nearly three times as much water as sandy soils do, while peaty soils absorb a still larger proportion. Hence the more frequent necessity for draining clayey than sandy soils; hence also the reason why, in peaty lands, the drains must be kept carefully open, in order that the access of springs and of other water from beneath, may be as much as possible prevented.

3. When dry weather comes, soils lose water by evaporation with different degrees of rapidity. In this way a siliceous sand will give off the same weight of water in the form of vapour, in one-third of the time necessary to evaporate it from a stiff clay, a peat, or a rich garden mould, when all are equally exposed to the air. Hence the reason why plants are so soon burned up in a sandy soil. Not only do such soils retain less of the rain that falls, but that which is retained is also more speedily dissipated by evaporation. When rains abound, however, or in very moist seasons, these same properties of sandy soils enable them to sustain a luxuriant vegetation, when plants will perish on clay lands from excess of moisture.

4. In drying under the influence of the sun, soils contract and diminish in bulk in proportion to the quantity of clay or of peaty matter they contain. Sand does not at all diminish in bulk in drying, but peat shrinks in one-fifth, and agricultural clay nearly as much. The roots are thus compressed, and air is excluded, especially from the hardened clays, and thus the plant is placed in a condition unfavourable to its growth. Hence the value of proper admixtures of sand and clay. By the latter (the clay), a sufficient quantity of moisture is retained, and for a sufficient length of time; while, by the former, the roots are preserved from compression, and a free access of air is permitted.

5. In the hottest and most drying weather, the soil has seasons of respite from the scorching influence of the sun. During the cooler season of the night, even when no perceptible dew falls, it has the power of again extracting from the air a portion of the moisture it had lost during the day. Perfectly pure sand possesses this power in the least degree; it absorbs little or no moisture from the air. A stiff clay, on the other hand, will in a single night absorb sometimes as much as a 30th part of its own weight, and a dry peat as much as a 12th of its weight; and, generally, the quantity thus drunk in by soils of various qualities, is dependent upon the proportions of clay and vegetable matter they severally contain. We cannot fail to perceive from these facts, how much of the productive capabilities of a soil is dependent upon the proportions in which its different earthy and vegetable constituents are mixed together.

6. The temperature of a soil, or the degree of warmth it is capable of attaining under the influence of the sun’s rays, materially affects the progress of vegetation. Every gardener knows how much bottom heat forces the growth, especially of young plants; and wherever a natural warmth exists in the soil, independent of the sun, as in the neighbourhood of volcanoes, there it exhibits the most exuberant fertility. One main influence of the sun in spring and summer is dependent upon its power of thus warming the soil around the young roots, and thus rendering it propitious to their rapid growth. But the sun does not warm all soils alike: some become much hotter than others, though exposed to the same sunshine. When the temperature of the air in the shade is no higher than 60° to 70°, a dry soil may become so warm as to raise the thermometer to 90° or 100°. Mrs. Ellis states, that among the Pyrenees the rocks actually smoke after rain under the influence of the summer sun, and become so hot, that you cannot sit down upon them. In wet soils the temperature rises more slowly, and never attains the same height as in a dry soil by 10° or 15°. Hence it is strictly correct to say, that wet soils are cold; and it is easy to understand how this coldness is removed by perfect drainage. Dry sands and clays, and blackish garden mould, become warmed to nearly an equal degree under the same sun; brownish red soils are heated somewhat more, and dark-coloured peat the most of all. It is probable, therefore, that the presence of dark-coloured vegetable matter renders the soil more absorbent of heat from the sun, while the colour of the dark-red marls of the new and old red sandstones may, in some degree, aid the other causes of fertility in the soils which they produce.

Granite generally forms hills and sometimes entire ridges of mountains. When it decays, the rains and streams wash out and carry down the fine felspar clay, and leave the (quartz) sand on the sides of the hills. Hence the soil in the bottoms and flats of granite countries consists of a cold, stiff, wet, more or less impervious clay, which often bears only heath, bog, or a poor and unnutritive pasture. The hill sides are either bare or covered with a thin, sandy, and ungrateful soil, of which little can be made by the aid even of skill and industry. Yet the opposite sides of the same mountains often present a remarkable difference in this respect, those which are most beaten by the rains having the light clay most thoroughly washed from their surfaces, and being therefore the most barren.

In reading the above observations, the practical reader can hardly fail to have been struck with the remarkable similarity in physical properties between stiff clay and peaty soils. Both retain much of the water that falls in rain, and both part with it slowly by evaporation. Both contract much in drying; and both absorb moisture readily from the air in the absence of the sun. In this similarity of properties, we see not only why the first steps in improving both kinds of soil must be very nearly the same; but why, also, a mixture either of clay or of vegetable matter will equally impart to a sandy soil many of those aids to, or elements of, fertility—of which they are alike possessed.

SECTION IV.—OF THE CHEMICAL CONSTITUTION
OF SOILS.

Soils perform at least three functions, in reference to vegetation. They serve as a basis in which plants may fix their roots and sustain themselves in their erect position,—they supply inorganic food to vegetables at every period of their growth,—and they are the medium in which many chemical changes take place, that are essential to a right preparation of the various kinds of food which the soil is destined to yield to the growing plant.