The soil must have adequate drainage otherwise air may be excluded and the more beneficial micro-organisms may be destroyed. Soils which have poor drainage are often sour and acid. It will be necessary to improve this acidity by applications of hydrated lime. Wet soils are cold ones, and this means that plant growth is severely retarded. The situation is even more critical in the northern, colder parts of the country. Waterlogged soils cause roots to rot and a combination of all these problems can produce complete failures in some gardens.
Soils which are well-drained have sufficient natural coarse, gritty material or sand and many soils have a high proportion of small stones also. A high humus or organic content will also ensure good drainage. It is usually the clay soils which are the most difficult with regard to drainage, although a
hard 'pan' or layer beneath the surface of some soils can also present a problem. Such a pan is usually produced by mechanical cultivation which, in some instances, can consolidate the lower soil layers. Setting the plough or cultivators to cultivate to the same depth season after season will also produce this hard, unbroken layer. Varying the cultivating depth occasionally usually overcomes this difficulty.
Clay soils are composed of finer particles and these tend to pack so tightly together that they soon form a solid mass through which excess water cannot pass easily. Improving the drainage here consists in opening up these fine particles. This can be done by liming the soil. The particles of soil cling together in large granules after this treatment. If sharp, gritty material such as coarse sand or well-weathered cinders is worked in, the clay particles will be separated and made more open. Bulky materials such as peat, composted vegetable waste and strawy manure are invaluable as soil conditioners. Gypsum is another preparation which has proved excellent for the breaking up of heavy, waterlogged clay soils.
Where cultural methods are not sufficient to provide a marked improvement in difficult conditions, it will be necessary to improve drainage by a system of drains or drainage trenches. The most efficient method is to use field or pipe drains. These are expensive, especially if drainage on a large scale is necessary. The pipes are sold in several sizes; those 5-7cm (2-3in) in diameter are the best for the amateur.
Trenches are dug out to receive these pipes, at least 40cm (15in) deep. All trenches should slope in one direction and this slope need not exceed 1 in 40. The trenches should be arranged in a herringbone fashion and should lead to one main trench which runs from the highest point in the garden to the lowest. The side or intermediate trenches should meet this main trench at an approximate angle of 45°.
The pipes should be laid, for preference, on a 5cm (2in) layer of coarse gravel or cinders. Each pipe should be kept about 1 cm (1/2in) away from its neighbor and the junction covered with a piece of slate, broken tile or a small piece of tough plastic sheeting. More gravel or cinders should be carefully placed around and on top of the pipes as work proceeds. The gaps between the pipes are essential to allow excess water to enter them and drain away inside the pipes. Frequent checks should be made with a little water from a watering can or hose pipe to see that water flows steadily along the pipes.
The main pipe line must be taken to a suitable outlet such as a ditch or soak-away. The latter can be constructed by digging out a large hole as deeply as possible and filling it in with stones,
clinker, gravel or ashes. This hole must be at least 60-90cm (2-3ft) square and deep. Under no circumstances must drainage water be allowed to flow on to neighbors' property. Where a stream or ditch is available for the emptying of drainage water, the local City Waste & Water Department should be consulted to make quite sure whether it is permissible to discharge the water in this way.
An efficient drainage system can be provided if trenches are lined with rubble and coarse cinders. A similar system of trenches should be taken out and the bottom half filled with rubble. This layer of rubble should be then covered with about 15cm (6in) of coarse cinders. The trench is then filled up with soil. Surplus water will run through the coarser base material and finally into the large drainage sump at the lowest part of the garden.
It is possible to use a third system, although this is not quite so satisfactory as the others. This method employs brushwood which is laid in bundles at the bottom of the trench systems. The brushwood is then covered with soil to the surface of the surrounding ground. The unsatisfactory part of this method is that the brushwood gradually rots away and loses its efficiency. It will be necessary to renew the system every few years, and unless the layout is small, this will involve a great deal of time and labor.
Where a new site is taken over, it is a very good idea to examine it thoroughly to see whether or not the soil requires attention to drainage. If it does it will provide an excellent opportunity to gather 41 the usual kinds of rubble which can be found on a new or neglected site. Builders often leave behind them a surprising amount of broken bricks, old paint tins, and lumps of concrete.
All this type of waste should be placed in convenient piles in the garden and used in the bottom of drainage trenches. If insufficient is available from the garden, it is quite likely that the local builder will be only too glad to supply some from his building sites.
During the planning of a drainage system for a waterlogged or poorly drained garden, it is a good plan to look ahead and visualize the positions for structures such as sheds, greenhouses or home extensions. All these buildings shed water and it will certainly aggravate the situation if this water is allowed to flow into the garden.
The position of a convenient drainage trench should be marked with a stake so that, later on, excess water from a gutter or down-spout can be directed into this drainage trench. The emptying of a fish pond is also facilitated if the water is directed on to an area of ground which is drained in this way, or if an outlet pipe is built into one of these drainage trenches.