Clay soil seems like a gardener’s curse at first – in Winter the soil in your lawn and garden beds turns into squelchy mud that sticks to your spade and boots, and in Summer it can barely be penetrated with a spade because it’s so hard. Poor drainage can lead to problems with root growth and plants can end up rootbound in their planting holes. Take heart though, the tiny particles in clay soil mean that it is nutrient dense, plus its water-retaining properties keep roots cool when it’s dry. Below are a few things you can do to make clay soils more fertile.
Gypsum (calcium sulphate) is the main component in plaster and blackboard chalk, and it also improves the structure of clay soils because it causes the tiny clay particles to clump together, creating more aeration. Add gypsum to holes when planting or dig it into soil at 0.5–1kg per square metre. Dolomite lime can also be added to clay soil but be aware that it will make your soil more alkaline (which is a good thing for vegetable gardens but not good for acid-loving plants).
Avoid planting into clay soils when wet, eg in Winter, as you risk compacting the soil even further. If you must walk on the soil use planks so you limit the area you’re walking on. Potatoes can be used to help break up clay soils - improve your soil and grow food at the same time!
Regularly mulch the top layer of soil with organic matter, such as compost, leaf litter, sheep pellets, pumice, scoria or bark. Fork it into the top layer of soil or pile it on top and let worms create humus for you as they break it down.
Choose plants that are known to cope well with clay soils such as roses, viburnums, mānuka and kawakawa. When planting trees or shrubs, plant them 'proud' on a raised mound of around 25cm to improve drainage.
Water flow can also become an issue with clay soils. If water can’t drain away your plants become susceptible to root diseases like phytophthora.
An easy way to check your drainage is to dig a hole 30cm in diameter and 30cm deep. Fill it with water and leave it to drain. Once drained, fill it again. If the water hasn’t drained away in 12 hours, you have drainage issues and your plants are probably going to struggle.
As well as improving the organic structure of your soil, you can remedy drainage by digging a trench that directs the water away from your garden bed and channels it to a hole in the ground, called a soakway, which is filled with gravel and stones. The soakway should be at the lowest point of your garden and always more than 5m from your house. Or install perforated plastic pipes or terracotta pipes which lead to a soakway or drain. Improve drainage in wet lawns by pushing the tines of a garden fork into the lawn to aerate it.