1128. The regulation of moisture is the next point demanding attention; for when the soil is pulverised, it is more easily dried by the penetration of the air; and a due supply of water is essential to plants, not only because it furnishes an important portion of their food itself, but because it serves as a medium by which they imbibe other food. In hot weather artificial supplies of water become necessary ; as when the temperature is increased, the evaporation becomes greater. Hence the origin of watering by surface or subterraneous irrigation, manual supplies to the root, showering over the leaves, steaming the surrounding atmosphere, &c. This is only to imitate the dews and showers, streams and floods of nature; and it is to be regretted that the imitation is in most countries attended with so much labour, and requires so much nicety in the arrangement of the means, and judgment in the application of the water, that it is but very partially applied by man in every part of the world, excepting, perhaps, a small district of Italy. But moisture may be excessive; as when plants have too much hydrogen, which they obtain from the decomposition of water, they begin to decay. Thus from certain soils at certain seasons, and from curtain productions at particular periods of their progress, it may be necessary to carry off a great part of the natural moisture, rather than to let it sink into the earth, or to draw it off where it has sunk in and injuriously accumulated, or to prevent its falling on the crop at all; and hence the origin of surface-drainage by ridges, and of under-draining by covered conduits, or gutters ; and of awnings and other covers to keep off the rain or dews from ripe fruits, seeds, or rare flowers.
