By John Weldon, President, Yarraville Marrow Growers Association
If you live in the west of Melbourne, the highly reactive clay subsoil upon which your house is built has likely made itself known to you via its ability to move stumps and slabs, crack walls and make doors impossible to shut, or open.
In the garden too, these hard-as-a-rock in summer, porridge-like-in-winter clay-based soils can cause big issues. These soils are sodic, which means they are heavy in sodium salts. This affects their structure. Such soil may be chock full of nutrients, but it can be too dense for roots to penetrate, it holds water, drowning some plants, and it can be hard to work with.
So what do you do?
Well, you could move to the east of town where the soil is loamy, friable and stable, but if you’re determined to stay on the right side of the bridge and you want a garden, you’ll need to improve your soil. Here’s how to do just that.
Gypsum is the first word that comes to most people’s minds when they talk about breaking down clay soil. A kilo or two of gypsum (from your friendly neighbourhood garden centre) per square metre, sprinkled and watered in, will help break up the soil. The calcium in the gypsum replaces the sodium and by the magic of science opens up the soil structure, all without altering your soil’s pH level. Just be mindful, when adding gypsum to a lawn. You don’t want to bury the grass – better to add your gypsum gradually over a period of time watering it in as you go.
Adding a few generous handfuls of gypsum to holes dug for larger plants, shrubs and trees will make it easier for their roots to spread outwards. Dig the gypsum into the hole as best you can and, again, water in well.
The downside of gypsum is that it’s not a quick fix. It’ll take six months or so to have any real effect. If you want to break clay more quickly, especially when planting trees, try a liquid soil breaker (back to the garden centre you go). This will work faster than gypsum, but will still need a month or two to get going.
Organic matter – compost, manure, leaf mould – dug into your clay soil, will add more nutrients and also open up the structure, making it much more workable. Backyard chickens, or rather the well-rotted, poop-rich bedding and the straw from their runs, is a great source of organics.
Perhaps the quickest solution is to build raised beds. Cover the ground in a thick carpet of cardboard, pile good garden soil and organic matter on top and avoid your clay soil altogether.
There’s no quick fix to creating workable soil, nor is it a one-time fix, you’ll likely need to add gypsum yearly and to keep digging in that organic matter when you can, but all soil types need ongoing maintenance to keep them productive and healthy. Putting in the work to unlock the potential of your clay soil will be worth it. Unlocking your front door when the stumps have moved again is another matter.
*Thanks to Agriculture Victoria for their advice on soil structure in Melbourne’s west.
