Although underdraining is the most important means of increasing the depth of the soil, it is not always practicable to lay drains through garden lands. In such cases, recourse is had to very deep preparation of the land, either every year or every two or three years.
[Illustration: VII. Bedding with palms. If a bricked-up pit is made about the porch, pot palms may be plunged in it in spring and pot conifers in winter; and fall bulbs in tin cans (so that the receptacles will not split with frost) may be plunged among the evergreens.]
In small garden areas, this deep preparation will ordinarily be done by trenching with a spade. This operation of trenching consists in breaking up the earth two spades deep. Figure 81 explains the operation. The section at the left shows a single spading, the earth being thrown over to the right, leaving the subsoil exposed the whole width of the bed. The section at the right shows a similar operation, so far as the surface spading is concerned, but the subsoil has also been cut as fast as it has been exposed. This under soil is not thrown out on the surface, and usually it is not inverted; but a spadeful is lifted and then allowed to drop so that it is thoroughly broken and pulverized in the manipulation.
In all lands that have a hard and high subsoil, it is usually essential to practice trenching if the best results are to be secured; this is especially true when deep-rooted plants, as beets, parsnips, and other root-crops, are to be grown; it prepares the soil to hold moisture; and it allows the water of heavy rainfall to pass to greater depths rather than to be held as puddles and in mud on the surface.
In places that can be entered with a team, deep and heavy plowing to the depth of seven to ten inches may be desirable on hard lands, especially if such lands cannot be plowed very often; and the depth of the pulverization is often extended by means of the subsoil plow. This subsoil plow does not turn a furrow, but a second team draws the implement behind the ordinary plow, and the bottom of the furrow is loosened and broken. Figure 82 shows a home-made subsoil plow, and Fig. 83 two types of commercial tools. It must be remembered that it is the hardest lands that need subsoiling and that, therefore, the subsoil plow should be exceedingly strong.