Tenacity of the soils. Pure clay, 100. Quantity of oxygen absorbed by 77·165 grains f the moist soil in 30 days, from 15 cubic inches of atmospheric air. Expressed in cubic inches.
Siliceous sand 0 0·24
Calcareous sand 0 0·84
Light clay 57·3 1·39
Stiff clay 68·8 1·65
Heavy clay 83·3 2·04
Pure clay 100·0 2·29
Carbonate of lime 5·0 1·62
Humus 8·7 3·04
Gypsum 7·3 0·40
Garden soil 7·6 2·60
Soil from Hoffwyl 33·0 2·43
Soil from Jura 22·0 2·25

The experiments detailed in the preceding table speak in a great measure for themselves, and scarcely require detailed comment. It may be remarked, however, that the columns illustrating the relations of the soil to water are probably more important than the others. The superiority of a retentive over an open soil is sufficiently familiar in practice, and though this is no doubt partly due to the former absorbing and retaining more completely the ammonia and other valuable constituents of the manures applied to it, it is also dependent to an equal if not greater extent upon the power it possesses of retaining moisture. A reference to the table makes it apparent that this power is presented under three different heads, which are certainly related to one another, but are not identical. In the second column of the table is given the quantity of water absorbed by the soil, determined by placing a given weight of the perfectly dry soil in a funnel, the neck of which is partially stopped with a small piece of sponge or wool, pouring water upon it, and weighing it after the water has ceased to drop from it. This may be considered as representing the quantity of water retained by these different soils when thoroughly saturated by long continued rains. The column immediately succeeding gives the quantity of that water which escapes by evaporation from the same soil after exposure for four hours to dry air at the temperature of 66°. The fifth, sixth, seventh, and eighth columns indicate the quantity of moisture absorbed, when the soil, previously artificially dried, is exposed to moist air for different periods. These characters are dependent principally, though not entirely, on the porosity of the soil. The last may also be in some measure due to the presence of particular salts, such as common salt, which has a great affinity for moisture, but is chiefly occasioned by their peculiar structure. It is to be remarked that clay and humus are two of the most highly hygrometric substances known, and it is peculiarly interesting to observe, that by a beneficent provision of nature, they also form a principal part of all fertile soils. The quantity of water imbibed by the soil is important to its fertility, in so far as it prevents it becoming rapidly dry after having been moistened by the rains. It is valuable also in another point of view, because if the soil be incapable of absorbing much water, it becomes saturated by a moderate fall of rain, and when a larger quantity falls, the excess of necessity percolates through the soil, and carries off with it a certain quantity of the soluble salts. Important as this property is, however, it must not be possessed in too high a degree, but must permit the evaporation of the water retained with a certain degree of rapidity. Soils which do not admit of this taking place are the cause of much inconvenience and injury in practice. By becoming thoroughly saturated with moisture during winter, they remain for a long time in a wet and unworkable condition, in consequence of which they cannot be prepared and sown until late in the season, and though chemically unexceptionable, they are always disadvantageous, and in some seasons greatly disappoint the hopes of the farmer.

The extent to which the imbibition and evaporation of water takes place is very variable, but they are obviously related to one another, the soils which absorb it least abundantly parting with it again with the greatest, facility; for it appears that siliceous sand absorbs only one-fourth of its weight of water, and again gives off in the course of four hours four-fifths of that it had taken up, while humus, which imbibes nearly twice its weight, retains nine-tenths of that quantity after four hours' exposure. Long-continued and slow evaporation of the water absorbed by a soil is injurious in another way, for it makes the soil "cold"—a term of practical origin, but which very correctly expresses the peculiarity in question. It is due to the fact, that when water evaporates it absorbs a very large quantity of heat, which prevents the soil acquiring a sufficiently high temperature from the sun's rays. The soils which have absorbed a large quantity of moisture shrink more or less in the process of drying, and form cracks, which often break the delicate fibres of the roots of the plants, and cause considerable injury: the extent of this shrinking is given in the fourth column.

The relation of the soils to heat divides itself into two considerations: the amount of heat absorbed by the soil, and the degree in which it is retained. Of these the latter only is illustrated in the table. The former is dependent on so many special considerations, that the results cannot be tabulated in a satisfactory manner. It is independent of the chemical nature of the soil, but varies to a great extent according to its colour, the angle of incidence of the sun's rays, and its state of moisture. It is, however, an important character, and has been found by Girardin to exercise a considerable influence on the rapidity with which the crop ripens. He found in a particular year that, on the 25th of August, 26 varieties of potatoes were ripe on a very dark-coloured sandy vegetable mould, 20 on an ordinary sandy soil, 19 on a loamy soil, and only 16 on a nearly white calcareous soil.

The tenacity of the soil is very variable, and indicates the great differences in the amount of power which must be expended in working them. According to Schübler, a soil whose tenacity does not exceed 10, is easily tilled, but when it reaches 40 it becomes very difficult and heavy to work.

On examining the table it becomes manifest, that as far as its mechanical properties are concerned, humus is a substance of the very highest importance, for it confers on the soil, in a high degree, the power of absorbing and retaining water, diminishes its tenacity and permits its being more easily worked, adds to its hygrometric power and property of absorbing oxygen from the air, and finally, from its dark colour, causes the more rapid absorption of heat from the sun's rays. It will be thus understood, that though it does not directly supply food to the plant, it ministers indirectly in a most important manner to its well-being, and that to so great an extent that it must be considered an indispensable constituent of a fertile soil. But it is important to observe that it must not be present in too large a quantity, for an excess does away with all the good effects of a smaller supply, and produces soils notorious for their infertility.

Such are the important physical properties of the soil, and it is greatly to be desired that they should be more extensively examined. The great labour which this involves has, however, hitherto prevented its being done, and will, in all probability, render it impossible except in a limited number of cases. Some of these characters are, however, of minor importance, and for ordinary purposes it might be sufficient to determine the specific gravity of the soil in the dry and moist state, the power of imbibing and retaining water, its hygrometric power, its tenacity, and its colour. With these data we should be in a condition to draw probable conclusions regarding the others; for the higher the specific gravity in the dry state, the greater is the power of the soil to retain heat, and the darker its colour the more readily does it absorb it. The greater its tenacity the more difficult is it to work, and the greater difficulty will the roots of the young plant find in pushing their way through it. The greater the power of imbibing water, the more it shrinks in drying; and the more slowly the water evaporates, the colder is the soil produced. The hygrometric power is so important a character that Davy and other chemists have even believed it possible to make it the measure of the fertility of a soil; but though this may be true within certain limits, it must not be too broadly assumed, the results of recent experiments by no means confirming the opinion in its integrity, but indicating only some relation between the two.

The Subsoil.—The term soil is strictly confined to that portion of the surface turned over by the plough working at ordinary depth; which, as a general rule, may be taken at 10 inches. The portion immediately subjacent is called the subsoil, and it has considerable agricultural importance, and requires a short notice. In many instances, soil and subsoil are separated by a purely imaginary line, and no striking difference can be observed either in their chemical or physical characters. In such cases it has been the practice with some persons not to limit the term soil to the upper portion, but to apply it to the whole depth, however great it may be, which agrees in characters with the upper part, and only to call that subsoil which manifestly differs from it. This principle is perhaps theoretically the more correct, but great practical advantages are derived from limiting the name of soil to the depth actually worked in common agricultural operations. The subsoil is always analogous in its general characters to a soil, but it may be either identical with that which overlies it or not. Of the former, striking illustrations are seen in the wheat subsoils, the analyses of which have been already given. In the latter case great differences may exist, and a heavy clay is often found lying on an open and porous sand, or on peat, and vice versa. Even where the characters of the subsoil appear the same as those of the soil, appreciable chemical differences are generally observed, especially in the quantity of organic matter, which is increased in the soil by the decay of plants growing upon it and by the manure added. In general, then, all that we have said regarding the characters of soils both chemically and physically, will apply to the subsoils, except that, owing to the difficulty with which the air reaches the latter, some minor peculiarities are observed. The most important is the effect of the decay of vegetable matter, without access of air, which is attended by the reduction of the peroxide of iron to the state of protoxide, and not unfrequently by the production of sulphuret of iron, compounds which are extremely prejudicial to vegetation, and occasionally give rise to some difficulties when the subsoil is brought to the surface, as we shall afterwards have to notice.

The physical characters of the subsoil are often of much importance to the soil itself. As, for instance, where a light soil lies on a clay subsoil, in which case its value is much higher than if it reposed on an open or sandy subsoil. And in many similar modes an important influence is exerted; but these belong more strictly to the practical department of agriculture, and need not be mentioned here.