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TRENCHING - Part 1 |
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Some gardeners advocate trenching to a depth of two feet or even more, and others say that simple digging to a depth of eight to ten inches is sufficient. Good and bad results are obtained by both advocates. Those who succeed with shallow preparation are usually working on a loamy soil with good natural drainage, due to an underlying layer of readily permeable gravel. Very light soils or heavy retentive soils need deeper preparation. Trenching of heavy soil without installation of an adequate drainage system will be harmful to any horticultural venture. Trenching of light soil allows the placing of a moisture-retaining layer, or water trap, twelve to twenty-four inches from the surface. This can be of clay or heavy loam. It has become conventional to recommend clay for this layer or even as a small patch under each rose. Heavy loam is better; after all, it contains about sixty per cent of clay.
Those who favour trenching say that rose roots penetrate two feet or more in depth. Those who are content with shallow cultivation are just as confident that the roots spread laterally, making deep preparation a waste of time. Both contentions are perfectly sound. Roots will penetrate only as deep as a soil suits them; trenching increases this depth. A deep soil is a more capacious reservoir for water and plant food than shallow, lightly dug soil. In trenching, the natural drainage of the soil must not be upset, the relative positions of topsoil and subsoil must not be reversed, and large air pockets must be avoided. Trenching is best done in late summer or early autumn, when the soil is fairly dry. Digging clay when it is wet may do more harm than good. If a trench be filled or partly filled with rough lumps of wet clay it may take months or even years for them to break down; meanwhile there are many air pockets. On the other hand, if wet clay be puddled, especially near drainage pipes, it is rendered almost impervious to water and all drainage systems are rendered useless. A garden will be better if the whole area is trenched. This makes for better drainage and obviates the danger of each bed becoming more or less a pit in which water stagnates. Trenching is really an elaboration of thorough digging: the aim is to turn and loosen the surface soil and either drain the subsoil or construct a retentive layer about two feet from the surface. To commence trenching, choose a strip of land three feet wide and the full length of the width of the area to be trenched. Dig from this strip and remove by barrow all the topsoil, leaving the subsoil. If there is enough topsoil take out twelve to eighteen inches of it. In light loams or sandy soils take out two feet of it. This soil should be heaped where the trenching is to finish. Since drainage should have been attended to before the actual trenching is commenced, there is no need actually to remove any of the clay subsoil. If the surface soil is in only a thin layer it is advisable to dig the subsoil to a depth of six to eight inches and at the same time mix into it a little of the surface soil. This will make the clay less dense but drainage will be essential if this is done. The subsoil must remain as subsoil, and should never be mixed with tht upper layers, except perhaps in very small proportions. |
