A well-drained soil is essential not only for successful plant growth but also where paths and walls are to be built. It is therefore important to consider the drainage of your whole plot, not simply with regard to the growing areas.
Few plants like a lot of water around their roots and in soil which is constantly wet the plants roots will remain near the surface or will start to rot. Wet soils are also cold, which retards plant growth. When drainage is inadequate, not only is air blocked from the plant roots but the general lack of air in the soil means that bacteria cannot live and the bacteria are a vital part of healthy soil.
It is in particular heavy clay soils which suffer from poor natural drainage; sandy or stony soils usually drain quite freely. However, where the topsoil is a stiff loam the subsoil may well be clayey and nonporous, preventing water from draining away completely.
The simplest way of testing your natural drainage is to dig a hole about 600 mm (2 ft) deep and watch what happens after heavy rain – or simply sill it with water. If the drainage is good, the water should disappear in 24 hours; if it is still there after 48 hours the drainage defiantly needs improving. However, before spending money on expensive pipe drainage, bear in mind too efficient a system will impoverish your soil as the plant nutrients will be leached out very easily. Try one of the several natural corrective methods first.
It is important for a gardener to know about the water table. This is the line under the topsoil or subsoil, depending on the depth of these layers, to which water standing in the earth’s crust rises. It is not a horizontal line but conforms roughly to the contours of the ground.
The water table generally rises and falls following wet and dry periods. If it stands about 900 mm (3 ft) below ground level it can be an asset, since water will be available to the deeper plant roots. However extreme fluctuations in the water table are a great danger: if it rises in winter the roots of plants are killed through saturation and if it falls in the summer the plants will suffer from drought. On low-lying ground if there is perpetual standing water (usually in winter), this might mean that the water table has risen above ground level and no drainage system will relive it.
Simple Remedies for Poor Drainage
Any measures taken to improve the texture and fertility of the soil will help to overcome drainage problems. Digging will aerate a clay soil, and mixing in bulky organic matter (such as peat, compost, strawy manure or dead bracken) well below the surface will improve its texture. The addition of inorganic materials such as gritty sand, weathered ashes or gypsum (calcium sulphate) will also make a clay soil more open in texture. As a rough guide, allow a bucketful of organic material and two of inorganic for each square meter or square yard of ground.
Lime will make a clay soil more porous by breaking it up into crumb-like particles; it encourages earthworms, which help to aerate the soil and this improve the drainage. Hydrated lime should be spread on top of the broken surface of the soil, not dug in but simply mixed into the top 75 mm to 100 mm (3 in to 4 in) of topsoil at the rate of about 225 g to a square metre (8 oz to 3 sq ft), preferably just before the winter.
Drainage may be impaired because the lower soil layers have been consolidated into a hard ‘pan’ about 450 mm (18 in) below the surface. This might be caused by continual treading or by heavy construction machinery. Double digging the whole plot may be the most effective remedy.
On new sites in particular it is common for heavy machinery used during the construction to have compacted the ground surface or a particular area of it, making it impossible for water to run through. This may be only a temporary state of affairs, so try to alleviate the situation first by spiking the ground with a fork. Alternatively, use a ‘tyning fork’ which does in more deeply and takes out pieces of earth like an apple corer; then brush fine gravel into the holes.
If you have taken over a new site and some form of drainage is definably needed, this will provide an excellent opportunity to get rid of any builder’s rubble which you have inherited. Broken bricks or lumps of concrete should be used in the bottom of drainage pits or trenches.
Three Drainage Systems
Soakaway Drainage System
A soakaway is the simplest form of drainage for an isolated patch which stays wet after heavy rain. It is also useful to take the overflow and occasional outlet from a small garden pond.
Dig a hole about 1 m (3 ft) square and at least 1 m (3 ft) deep; it should be sufficiently deep to penetrate the impervious subsoil into something more porous below. Fill the hole, first with brick or other large, hard rubble to a depth of about 600 mm (2 ft), then with about 100 mm (4 in) of gravel or ash. Finally fill with excavated topsoil up to ground level. The gravel layer is essential to prevent the coarse rubble becoming blocked with silt and soil leaching through from the soil replaced on top. During very wet weather surplus water will collect in this hole and slowly percolate from it into the lower strata. A soakaway should always be sited well away from the house, as it may weaken the surrounding soil.
Rubble Drainage System
A rubble drain is a short-term drainage run, which may be all that is required on a new site to relive temporary lying water. Dig a trench 300 mm to 540 mm (1 ft to 1 ft 6 in) deep, depending on the depth of cultivated soil (since water collects on the comparatively solid pan of undisturbed ground that lies immediately beneath). Fill the hole at least half full with coarse rubble then with a layer of ash or gravel and finally topsoil.
This channel should obviously run to some form of outlet such as a soakaway – on no account allow your excess water to drain on to your neighbour’s property. The disadvantage of a rubble drain is that it becomes blocked comparatively quickly as soil leaches through from above.
Very bad drainage conditions may be indicated by permanent standing water but check first that this is not just a high water table. If you conclude that the drainage is totally inadequate you will need to lay a system of underground pipes running to a soakaway, placed preferably at the lowest point in your garden.
Earthenware and Plastic Pipes
Earthenware or plastic pipes can be used, laid either in a run or, for greatest efficiency, in a herringbone system. Plastic drains are available in long runs, perforated at regular intervals to take surplus water. They have an advantage over earthenware ones in being flexible; they can be made to curve round a tree root.
The pipes should be laid in trenches with the gradient of the side pipes in the herringbone pattern not greater than one in 250, with the main rib at a smaller gradient to the soakaway. On a steeper slope there is too little time for the water to soak in. The depth and proximity of the pipes in the herringbone system will depend on the soil type and depth of cultivation; the table is included as a guide.
Clay: Spacing: 4 to 7m (12 to 21ft), Depth 600 to 750mm (2 to 2.5ft)
Loam: Spacing: 8 to 12m (24 to 36ft), Depth 750 to 900mm (2.5 to 3ft)
Sand: Spacing: 12 to 22m (36 to 66ft), Depth 900 to 1000mm (3 to 4ft)
The trench to take the pipes should be excavated to the required depth and the base rounded to fit them. Most of the water will enter where the drain pipes meet but some will also percolate through the walls of unglazed clay tile drains or through the holes in plastic ones. Where drain pipes but together, therefore, cover the joint with a piece of slate, broken tile or small piece of tough plastic sheeting to prevent soil leaching through from above.
Cover the pipes with coarse rubble, and then protect this layer with upturned sods of turf if you have had to dig these up. Alternatively, use gravel or sand to prevent clogging. Fill up the trench with topsoil to just above ground level, to allow for sinking.
Construct the soakaway by the same method suggested for draining an isolated wet patch. However, as it is draining a much larger area, it should be deeper.