There are two distinct methods by which these ends may be, in some measure, attained,—by the use of _mechanical_, and by the application of _chemical_, means. Mechanical operations produce changes _chiefly_ in the physical properties of the soil,—chemical means alter its elementary constitution. Ploughing, draining, mixing, &c. belong to the former class of operations; manuring and irrigation belong to the latter. It will be proper to consider these methods separately. SECTION I.—OF MECHANICAL METHODS OF IMPROVING THE SOIL. 1. _Draining._—The first step to be taken, in order to increase the fertility of nearly all the improveable lands of Great Britain, is to drain them. So long as they remain wet, they will continue to be cold. The heat of the sun’s rays, which is intended by nature to warm the soil, will be expended in evaporating the water from its surface; and thus the plants will never receive that genial warmth about their roots which so much favours their rapid growth. Where too much water is present in the soil also, that food of the plant which the soil supplies is so much diluted, that either a much greater quantity of fluid must be taken in by the roots,—much more work done,—or the plant will be scantily nourished. The presence of so much water in the stem and leaf keeps down _their_ temperature likewise, when the sunshine appears; an increased evaporation takes place from their surfaces, a lower natural heat, in consequence, prevails in the interior of the plant, and the chemical changes on which its growth depends proceed with less rapidity. By the removal of the water, the physical properties of the soil also are in a remarkable degree improved. Dry pipe-clay can be easily reduced to a fine powder, but it naturally, and of its own accord, runs together when water is poured upon it. So it is with clays in the field. The soil expands, becomes close and adhesive, and excludes the air from the roots of the growing plant,—the access of which air appears to be almost an essential element in the healthy growth of the most important vegetable productions.