Now dig in the organic matter in the trench bottom to the depth of a spade, mixing it well into the subsoil. If this is heavy clay, some ground limestone can be added to help loosen it. Any modification of pH should be made at this time: you will not have another chance
at the subsoil for a long, long time. One job of trenching should last at least ten years.
If the soil is poorly drained, the time to make corrections is when the trench is open. To lay a drainage tile line, begin digging at the lowest point of the garden and lay the tile away from this trench to a lower spot so water that accumulates in the loosened soil has some place to go.
The next step is to dig a second trench alongside the first. The topsoil from the second trench goes in the bottom of the first, covering the organic matter. Next the lower layer from the second trench is placed on top of the topsoil in the first trench.
This operation, repeated across the garden, works the soil to a depth of three spades, or approximately 18 to 20 inches deep. It places rich soil and organic matter down deep, where the roots of plants can reach them. For a year or two, fertilizer applications to the area should be doubled, to make up for the decreased richness of the subsoil that is now the topsoil layer. Soon, however, as plant residues and roots are worked into this surface soil it improves in richness and tilth. If your subsoil is very poor, improve it before shoveling it into the preceding trench.
NOT ALWAYS ADVISABLE
Obviously, trenching is not always worth the time and effort involved. It would be completely wasted, for example, if only shallow-rooted crops such as petunias and lettuce were to be grown. In rich, deep, black prairie soils such as are found in parts of the Middle West, topsoil may extend down two or three feet, so that the soil dug out of the bottom of the trench would be just as rich as the top.
Certainly, trenching is not an operation for an older gardener or a man with a poor heart. It is hard work, and not to be undertaken lightly. However, rewards are high when the operation is needed and properly done. For example, it is ideal when a hedge is to be planted, in which case only two trenches need be dug.