The big wet: A DIY guide to improve drainage in your garden
Well-drained soil has both water and air present. It's also warmer, increases nutrient availability to plants and contains more soil organisms. Poor drainage, on the other hand, leads to water-logged soil, which pushes out air and results in stunted plant growth – or even death.
To test your soil drainage, take a spade and dig a hole 30cm deep. Fill it with water and allow the water to drain completely, then fill the hole again. Check the hole in an hour's time.
If less than 5cm of water has drained away, your soil has poor drainage and needs amending.
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Of all the soil-amendment methods, the best is adding organic matter, be it compost, aged manure or anything else that's well rotted. This organic matter binds with soil particles. In the case of clay soil, it forces the small, tightly packed particles apart so that drainage is improved; in sandy soil, it fills the large pore spaces and acts as a sponge so that moisture is retained for longer.
So if you have clay soils, your best bet is to dig in plenty of organic matter, such as compost and aged manure, as well as calcium in the form of gypsum (calcium sulphate). But if your soil has adequate gypsum levels already, use compost alone – too much calcium can interfere with the uptake of other nutrients.
To test the levels of gypsum in your garden, put 2cm of soil in a large jar filled with water, screw on the lid and shake. If the water clears within half an hour, you have adequate amounts. If it takes one to two hours for the water to clear, apply about 50g of gypsum per square metre to your soil to help improve the soil's structure and drainage. Dig it in at least 30cm deep.
If the water is still murky after several hours, add about 200g of gypsum per square metre to your soil.
Repeat this process every four weeks until your test shows soil improvement.
Alternatively, mushroom compost can be used as mulch for clay soils as it's high in calcium sulphate. Avoid using it around acid-loving plants, though.
If your soil is heavy clay, you might consider installing clay or plastic drainage pipes. You can do this by laying pipes beneath the ground and creating a soakaway that will receive water from the drainage pipes. A soakaway is simply a hole dug into the ground, filled with rubble and coarse stones or broken bricks, which allows surface water to percolate back into the ground.
Your soakaway should be located at the lowest point of your garden. Dig it to about one metre square and to a depth of about 120cm. Fill with rubble and stones to about 30cm from the surface. Add 15cm of gravel and then 15cm of soil. Your soakaway will now be at ground level.
When laying your drainage pipes, mark the ground with a thin line of sand where the pipes are to go. Dig up a width of turf and place the turf next to the line, then dig your trench for the pipe. If your ground surface does not slope away, dig your trench so that the top end is higher than the bottom end where it meets the soakaway. You need a slope of 1cm to every 1m of pipe laid. Add gravel, your drainage pipe, more gravel and then the topsoil that you dug out of the trench. Then replace the turf.
If pipes are out of the question for financial or structural reasons, you can simply create trenches, filled with stones and gravel, that lead away from your garden. Alternatively, raised beds are another option.
One more consideration for improving soil drainage is to plant a cover crop. Certain crops can aerate the soil, especially if you plant a cover crop with a deep root system, such as alfalfa, clover, forage radish, turnips and sunflowers. The deep roots can help loosen compacted soil. Before the crops flower they are dug back into the soil to break down, which acts as a green manure, further enriching the soil and improving structure.
- NZ Gardener