Good soil drainage is important to encourage healthy plant growth. Few plants thrive in waterlogged conditions.
Causes of poor drainage
Poor drainage may be indicated by surface puddling after rain. This may be due to a high water table, compacted layers beneath the surface that arise naturally on some soils, or as a result of surface compaction. A particular problem on modern housing developments is that topsoil is often removed prior to building, the subsoil is then compacted by building activity before the soil is replaced.
Testing soil drainage
Poor drainage may be confirmed by pouring water into a 30-60cm (1-2ft) deep hole. If the water remains for hours or even days the soil may benefit from drainage.
Cultivation
With surface compaction double digging and the incorporation of bulky organic matter (about one bucket per square metre) and a 5cm (2in) layer of coarse grit into the top 15-30cm (6-12in) will improve drainage.
Ditches
Where poor drainage is due to other factors it may be necessary to install a drainage system. This may be as simple as digging ditches about 90cm (3ft) deep with sloping sides across the lower end of a slope to take excess surface water. Where open ditches are unsuitable, French ditches can be constructed by filling the ditch with coarse gravel topped with upturned turf and topsoil.
Pipes
More elaborate systems involve the installation of perforated plastic pipe laid in trenches on a firmed 5cm (2in) bed of coarse gravel and covered with 10cm (4in) of gravel.
For larger areas it may be worthwhile hiring a mini-digger or employing a specialist contractor.
Systems are laid below the level of cultivation, usually 40-60cm (16-24in) deep and 3-6m (10-20ft) apart. The trenches must follow a natural slope or a constructed one with a fall of 1:40. A herringbone layout is usually employed with the feeders meeting main lines at an angle of 45 degrees. With perforated plastic pipe the perforations should face downwards to prevent excessive silting.
Excess water
There is no point installing drainage unless there is somewhere for the excess water to go. This may be an existing ditch, authorised outlet or to a soakaway pit, constructed at least 1.8m (6ft) square and filled with rubble.