The substratum on which the surface soil rests, is, both for building and gardening, of more importance than the soil itself. No surface soil whatever, on a wet or springy subsoil, or even on a soil not springy, but clayey and retentive, can be rendered fit for either garden or field cultivation without considerable expense. On the other hand, any soil on a subsoil not absolutely retentive, and not too open, may, at a moderate cost, be highly improved. Rock, of almost every kind, forms a favourable subsoil, because it is generally neither too retentive nor too porous: it is proverbially the best foundation for building on; and if the surface soil is of sufficient depth, it is fit for every purpose of cultivation.

There are some descriptions of decomposing sandstone rock, impregnated with oxide of iron, which form unfavourable subsoils for the culture of fruit trees, and produce canker in them; but such subsoils can be freed from the bad effects of the iron to a sufficient depth for the culture of herbaceous vegetables, by frequent dressings with quicklime. Dry gravel forms a good subsoil; but, unless care be taken not to found the walls of a house too deep, and to have a perfect exterior drainage, the surface water will, in rainy seasons, find its way into the floor of the sunk story, if there be one. A gravelly subsoil is not the best for a garden; because, in the summer season, it absorbs too rapidly the water of the surface soil. On marshy or peaty soils or subsoils, no house whatever should be built; but such surfaces and subsoils may be rendered peculiarly suitable for certain descriptions of garden culture, by judicious draining, or by a command of water combined with the power of laying the whole dry to a certain depth at pleasure.

In the case of irregular rocky surfaces such marshy or peaty spots are not unfrequent; and, when the house can be placed on a rocky knoll, and the garden formed in an adjoining hollow, the concurring circumstances may be considered favourable.