Garden Drainage and irrigation
Healthy plant growth and development depend upon the roots having enough moisture and air, as well as food, warmth and light. In a poorly-drained, waterlogged, the spaces between the soil particles are filled with water, excluding air. The lack of oxygen affects plant roots both directly, because they cannot grow without it, and indirectly, by creating soil conditions which favour disease organisms, such as club root and root rot, and the build-up of substances injurious to crops.
Poorly drained soils are slow to warm up in spring and remain cool in summer, so that growth of crops is slow. Wet conditions also prevent the ‘fixing’ of nitrogen gas from the air in the nodules on the roots ofand beans. Normally, in well-drained soil, bacteria in these nodules convert the nitrogen into nitrates, which are major plant nutrients, and these are taken up by the crops. Just like plants, these bacteria need oxygen to live and do their work, and this is not available in waterlogged soil. Countless other beneficial organisms in the soil, too, will die if they do not receive enough oxygen.
Manures and fertilizers may become ‘locked up’ in waterlogged soils so that they are unavailable to your crops; this clearly involves a waste of both money and time.
Badly-drained soils are also often acid, which makes them unsuitable for growing crops. Finally, such soils are often wet and sticky, and, particularly in the case of clay soils, very difficult to cultivate.
The advantages of good drainage are: earlier growth of crops; heavier yields; an extended growing season; better use of manures and fertilizers; and a more easily worked soil.
Soils differ considerably in their ability to retain moisture and in the rate at which water moves through them. Well-drained sandy loams are ideal for early crops, as they are quicker to warm up than the moisture-retentive heavy loams and clay soils. Do remember, however, that well-drained soils dry out more quickly in summer, so that crops will suffer unless you water them frequently and regularly.
Natural drainage depends on two main factors: the composition of the soil and the nature of the subsoil.
Light, sandy, gritty, ‘open’ soils are free draining, while heavy silt and clay soils are not. Subsoils of clay or rock will prevent the downward movement of water regardless of the topsoil type.
Testing your soil
Dig out two or three trial holes, about 60 cm (2’) deep in different parts of the garden during winter, fill them with water, and cover them with a flat sheet of iron or other impervious material to exclude any rain. If there is no water left after 24 hours, drainage is satisfactory, but if water still remains after 48 hours or more, then drainage of some form is needed. In order to avoid false readings, the cover over the hole should be replaced after each inspection.
The presence of wild plants such as rushes, sedges and marsh marigolds is also an indication of wet land.
There are two basic drainage methods: surface drainage and underground drainage.
The former serves to trap surface water and prevent excessive run-off and soil erosion before the water has had a chance to percolate into the ground.
Surface run-off of water can be a considerable problem on steep slopes, and a system of rubble or brushwood-filled trenches across the slope may remedy this. You should take similar steps to prevent rainwater from running onto your soil from a large expanse of paving or other impervious material.
The purpose of these systems is to dispose of underground water and lower the water level in the soil, thus increasing the depth of soil available for crops and cropping.
There are a number of variations, but suitable methods range from a simple ‘soakaway’ on small plots having no suitable outlet, to a network of near horizontal underground pipes or channels draining into a main drain for larger sites. On low-lying sites surrounded by higher ground, it may be necessary to grow plants onas well as using underground drains.
Remember, however, that drainage should be attempted only if it is absolutely necessary; often digging and adding humus (and lime on clay soils) will cure drainage problems, and will involve far less time and expense.
If you do have to drain, you may need to obtain specialist advice on the number, size and type of drains to use, the depth at which to lay them, and the degree of fall. Soil type will govern much of this; other factors include your choice of crops, and the local climate.
Making a start
The basic tools required for draining are: a spade and fork; a wheelbarrow; a measuring stick; a spirit level; a straight edge and some wooden pegs.
The best time to make the excavation is during the drier months of the year. If you are not accustomed to heavy work of this nature, do not attempt more than an hour or so at a stretch.
Before you start to dig, consider how you are going to dispose of the excavated material, which is usually best removed from the site, unless you have some holes to fill.