Clay soil is made up mainly of clay particles. On the positive side, clay soils are rich in nutrients. On the other hand, they can be difficult to dig and poorly drained, becoming waterlogged when wet and then cracking as they dry out during extended dry periods.
If you are unsure about whether your soil is clay or not, there is a simple soil test you can do using a sample of soil from your garden. Click to find out how to do the one-step test for soil type.
As a quick guide, a handful of moist clay soil is easily squeezed to form into a ball or rolled into a sausage shape. The more clay, the better it can be shaped.
In some areas, the garden’s topsoil may be a good garden loam but there may be clay in the subsoil that can impede drainage.
Plants for clay soil
Although clay soils are a gardening challenge, there are plants adapted to grow in clay, including natives from your local area that share your soil conditions. Wattles are a good starting point for clay soils. Clay-tolerant wattles include bower wattle (Acacia cognata) and silver wattle (A. dealbata). Also tolerant of clay soils are bottlebrushes (Callistemon spp.) including ‘Endeavour’, ‘Kings Park Special’ and the compact ‘Little John’.
While most productive plants require good drainage and soil that’s well cultivated to about 30cm depth for good root growth and development, beans and shallow-rooted vegetables such as loose-leaf lettuce can be grown in clay soil.
Ornamental plants for clay soils include asters, daylily, hydrangea and Viburnum tinus. Roses also grow well in improved clay soils.
Improving clay soil
Longer term, clay soils can be modified into clay loams by adding organic material such as compost, rotted manure and mulch, along with gypsum or clay breaker. To do this, work the organic matter and gypsum (calcium sulfate) into the soil prior to planting. Where digging is difficult, laying organic mulch and manure over the soil to break down, assists over the long term.
Adding sand to clay soils can improve drainage, but generally it's hard to add enough sand to make an improvement.
Although gypsum improves most clay soils by changing their structure, there are some types of clay that don't respond to gypsum. Before adding gypsum, check your clay soil with this simple test. Take a lump of soil, put it into a glass jar filled with water and watch what happens (don’t shake the container). If the clay can be improved with gypsum, it diffuses into the water, which will become cloudy. If the clay doesn’t dissolve and the water remains clear, gypsum won’t help, but added organic matter and sand will.
As poor drainage and cultivation are the main concern with clay soils and limit what can be grown, creating raised beds or mounds is a good way to cope with a clay soil or a clay subsoil. Create raised beds or planting mounds at least 30cm high using organic garden mix and compost, to grow plants that need good drainage, particularly productive plants such as citrus and vegetables.
Also starting with small, clay-tolerant plants as seedlings, which are easy to plant, is often better than attempting to plant large potted plants.
When planting, check the drainage by digging a hole, filling it with water and observing how long the water takes to drain away. If the water drains within a few hours, most plants will cope but if there’s still water in the hole the next day, the soil needs further work before planting.