A soil that is both moisture-retentive and well-draining is achievable, but it is helpful to know how soil holds water -- especially if you need to improve the drainage of a waterlogged area or lawn.
How Soil Holds Water
A soils ability to hold on to water as it flows through varies from soil to soil. Some water is held in pores, the spaces between soil particles, but the main source of water for growing plants is capillary water, which clings to the surface of each soil particle.
Dry soils, predominantly of sand and gravel, have large pores that allow them to drain quickly. Because they are made up of fewer, larger particles, they hold less capillary water and dry out rapidly.
Wet soils, usually with a high clay content, are made up of lots of very small particles, giving them the potential to hold plenty of water. The pores in between are much smaller so water is held for longer, and the soil stays wet for prolonged periods.
reasons for poor drainage
Poor drainage, where surplus water cannot drain away, can be caused by:
Surface compaction, or 'capping', which occurs most commonly on silty soils where little organic matter is present, or where not much vegetation covers the soil. If soil worked to a fine tilth becomes wet (through rain or irrigation), and then dries quickly, a crust or cap perhaps 1 cm {1/2 in) thick may form.
Deep compaction, which is most commonly seen in the gardens of new houses where subsoil and topsoil have become mixed, and the soil mass has been pressed by heavy machinery. Deep cultivation through double digging is the only effective solution.
Hard pans, which are compacted layers of soil, usually within 30 cm (12 in) of the surface. These prevent water from draining well, producing waterlogged soil above. The cause can be cultivating wet soil or repeated walking over an area.
Heavy clay, which with its minute soil particles may hold onto large volumes of water even where drains are present.
TESTING DRAINAGE TIP: Dig a hole to about 50 cm (20 in) deep and 30-45 cm (12-18 in) in diameter. This will part-fill with water after two to three hours of rain but should empty within 48 hours as the water drains away. If it does not, you will have to take steps to correct your drainage.
Solutions to Poor Drainage
Techniques for improving drainage will depend on the lie of the land, soil depth and soil type (heavy clay soils have the worst drainage), and how much moisture the intended plants will tolerate. Always avoid walking on soil that is waterlogged and never attempt to cultivate it, as this makes the problem worse.
Installing a drainage system under lawns or cultivated areas may be the only effective solution to severe waterlogging. Although initially disruptive, it should work well for up to 50 years. Even digging a shallow trench, filling it a third deep with a layer of aggregate then replacing the soil, will provide a solution for about 10 years.
Adding aggregate: mixing in fine grit, horticultural sand or boiler ash helps to open up heavy soil. On very wet soils, cut narrow slits, about 8 cm (3 in) wide and 10 cm (4 in) deep, and fill them with aggregate.
Varying the depth of cultivation over different areas avoids creating a hard pan. Dig heavy soils in the autumn before they become too wet, and leave as rough lumps over winter so that the wind and frost can dry them out and help break the soil down to a finer tilth.
Spiking lawns using a hollow-Lined aerator removes cores of soil about 1 cm (1/2 in) in diameter and up to 15 cm (6 in) long. This breaks through the compacted layer, allowing air in and moisture to evaporate. The holes can be filled by brushing in an open, sandy compost, which acts like a wick, drawing moisture up towards the surface.
Growing plants on mounds or ridges keeps most of the roots above the saturated soil. Raising the soil level in one part will also help it dry out more quickly. Plants that prefer free-draining soil (like strawberries, lavenders and most herbs) will benefit from being grown 'high' even if the soil is not particularly wet.
Raised beds are ideal for heavy clay soils: water drains from the raised bed, allowing it to dry out ready for sowing or planting, and collects in the trenches where it evaporates. For raised beds, dig a series of trenches 20-30cm (8-12in) deep, placing the soil in between them to create beds approximately 1.2m (4ft) wide.
Installing a land drain means that the upper soil layers, at least, will be drier. A sloping narrow trench is dug from the top of the garden, across the land towards the lowest point. Porous clay or perforated plastic pipes are placed within a porous layer (ash or gravel) before the trench is refilled with soil. (Clay pipes are about 40cm/16in long and plastic drains are bought in flexible coils up to 30 m / 100 ft long.)
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