Use it or lose it
Take a peek at Josh Byrne’s garden for tips on how to maximise water
Clay soil has many positive qualities. It’s good at holding moisture, and when it allows plant roots to grow deeply, it provides excellent anchorage and a nice stable temperature. It’s often pretty fertile, too, as it has the ability to store a range of essential nutrients for plant growth.
However, as the smallest member of the soil-particle gang (which includes silt and sand), it’s a sociable little thing with a strong desire to bond with its fellow clay mates. Because of that, clay soils tend to become waterlogged and compacted, which makes it difficult for fledgling plants to establish a good root system.
If you’re dealing with clay soil that has the density of a rubber tyre, don’t despair. ‘The Patch’ at the Royal Tasmanian Botanical Gardens in Hobart is built on clay, and is proof that 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 are five ways to treat clay soil.
1 Cultivate the soil
This is the most rudimentary but accessible way to get started. All you need is a digging implement, such as a spade (and perhaps a crowbar), and some sweat. The idea is to simply loosen the topsoil to about a spade’s depth and break up the clods, to enable water, air and roots to penetrate more easily. Never dig clay when it’s wet – it’s bad for your back, and bad for your soil, as it leads to further compaction.
While cultivation can make compacted clay soil more workable for you, too much cultivation over time can turn it from clods to dust, making it prone to erosion and compaction. It’s also harmful to worms and other beneficial organisms living in the soil. Contrary to what my big sister told me, chopping a worm in half doesn’t make two!
2 Add sand and gravel
When I was younger, I remember gardeners adding coarse sand and fine gravel to help improve clay soils. When you mix these larger particles with the tiny clay particles, it separates the clay, which aids drainage and aeration. You need a lot of materials to do this well, and when coupled with the labour involved, it’s not really feasible on a large scale if, say, you want to do your entire garden. However, it can work a treat in a special area, such as a vegie patch, or when you’re trying to establish individual plants.
3 Add calcium
Clay soils that are sodic (high in sodium) tend to have poor structure, while clays that are rich in calcium often have good structure, allowing air, water and plant roots to penetrate more easily. Calcium improves the soil’s structure by enhancing aggregation, helping to join organic matter to clay particles. So instead of forming a fine dust or dense clods, the clay particles combine to create multiple small groupings, or aggregates, with lovely air spaces between them.
Gypsum (calcium sulfate) is a common soil additive, and 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. However, bear in mind that gypsum is a short-term solution, and is best combined with other methods of soil improvement for long-lasting results.
4 Add organic matter
Any organic material that you throw onto, or dig into, a clay soil is going to be beneficial. Whether it’s rotted animal manure, glorious 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.
Mulch is essentially nature’s downy quilt. Maintaining a surface layer of organic mulch, such as straw or bark, defends the soil against the harmful effects of the sun and wind, and slows water to give your clay soil more time to absorb moisture, thus preventing erosion.
Well-rotted compost and manures can also be laid on the surface, instead of dug in. Indeed, with a hard-to-dig clay, this may be the only option. Worms will find their way there and eventually work the organic matter into the soil for you. Like the old saying goes, work smarter, not harder.
5 Grow green manures
Green manures are crops grown purely to assist the soil by increasing organic matter and improving fertility, and they’re great for improving a clay soil. Good examples include bulky, leafy, fast-growing plants such as oats, millet, buckwheat, alfalfa, peas and mustard. Before the crops set seed and reach maturity, you chop up the plants and leave them on the 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.