Plants obtain the moisture they require from rainwater held in the upper layers of the soil, and from the water which rises naturally from the subsoil. Any excess is definitely harmful, forcannot grow without air and this is excluded in a waterlogged soil. So long as the soil is well aerated, roots continue to develop, but their growth stops as soon as they reach the level of the water table below which all soil is waterlogged. They can make no use whatsoever of this ‘gravitational’ water, and, if it comes close to the surface, plants will be shallow-rooting and suffer severely in times of drought. In a thoroughly waterlogged soil the plants soon die. Under these conditions, , which removes water to make more available to the plants, is the only cure. The greatest need for occurs with heavy clay soils which hold water firmly and, owing to their impervious nature, prevent any surplus from flowing away. In such soils, where an outlet is available, a series of drains should be laid in herring-bone pattern with 21/2 in- earthenware tiles running into a 4 in. main — the whole system sloping very gently to the outlet. The tiles should be covered with rubble and the trenches filled to the top with soil. Drains in clay soils should be set 2 ft. deep.
Where a steep bank causes seepage on to the garden, a catchment drain made in this way and having the garden side puddled with clay, should be laid along the bank foot. If no outlet is available, sumps should be dug in the wettest parts of the garden and tile drains, made as already described, led into these. A sump is merely a hole, 4 ft. to 6 ft. deep, half-filled with stones, ashes, and brushwood, and to the top with good soil. When draining, a heavy dressing of lime, about 1 lb. per sq. yd., may be mixed up with the soil, in order to reduce it to a fine crumbly condition. The addition ofmould, sand, wood ash, burnt soil, basic slag, strawy horse manure, peat or hop manure will also help to keep the soil open.
In building a rockery it should not be forgotten that alpines are liable to suffer severely from waterlogging. It is therefore advisable to lay a drain during construction, and to include plenty of mortar and rubble in the foundation, except for ericas, dwarfand other lime-hating plants.
Grass cannot grow on waterlogged soil, and a proper drainage system should always be laid down when making a lawn on heavy clay soil. Even with this the surface soil will often become compacted owing to, rolling, trampling and other causes, so that rainwater takes a long time to drain away and moss and bare patches appear. The remedy for this consists in ‘piercing’ by means of a special fork or spiked roller, to break the surface layer and aerate the soil.