Clay soil tends to become waterlogged and compacted, making it difficult for fledgling plants to establish a good root system. With good soil management practices and a bit of work, you can improve heavy clay to create soil that’s a pleasure to garden with. Here’s how.
- Cultivate the soil. Loosen the topsoil to about a spade’s depth and break up the clods to enable water, air and roots to penetrate more easily. Don’t cultivate the clay too much, as over time this can turn it to dust, making it prone to erosion and compaction, and harmful to worms and other beneficial organisms. Never dig clay when it’s wet.
- Add sand and gravel. When these larger particles are mixed with the tiny clay particles, it separates the clay, which aids drainage and aeration. This is not really feasible on a large scale, but can work a treat in special areas, such as a vegie patch, or when you’re trying to establish individual plants.
- Add calcium. Clay soils that are sodic (high in sodium) tend to have poor structure. Gypsum (calcium sulfate) is known as the ‘clay breaker’ because of its ability to replace sodium with calcium. Adding a handful of gypsum per square metre of clay soil helps to improve the structure of the top layer of the soil.
- Add organic matter. Any organic material that you throw onto, or dig into, a clay soil is beneficial. Whether it’s rotted animal manure, compost, fresh vegie scraps dug in, or mulch laid on the surface, these all boost biological activity by feeding soil life, and they improve soil structure by assisting the formation of aggregates.
- Grow green manures. Before green manure crops set seed and reach maturity, chop up the plants and leave them on the soil’s surface to break down and feed the soil. If you leave the roots in place to break down in the soil, they create channels that become pathways for the movement of air, water, nutrients and soil organisms. Green manures include bulky, leafy, fast-growing plants such as oats, millet, buckwheat, alfalfa, peas and mustard.
You’ll find more details about improving a heavy clay soil in the November 2020 issue of ABC Gardening Australia magazine, out now.