“The whole earth’s population depends on the top six inches of soil.”
- Jules Dervaes
If anyone knows how vital soil is, it’s farmer Jules Dervaes. With his family of three adult children, Dervaes grows around one tonne of produce annually. Nothing special you say? When you consider that the family harvest that tonne from one tenth of an acre in downtown Pasadena, California, I sit up and take notice.
That top six inches of soil, the 25cm or so that we call topsoil, is perhaps the most abused resource on earth. We depend on it for our sustenance, but treat it like dirt. In Australia we have some of the oldest soils on earth, but do we treasure them like a precious heirloom? Hardly. We let our soils blow away in the wind and wash down the creek when it floods. Worse still we build suburban housing estates on top of our best quality soils, stripping the precious topsoil layer in the process.
The irony is that the Darling Downs is rare in possessing some of the best quality agricultural soils on earth. According to the Australian Soil Classification, they roughly fit into two categories: On the various upland plateaus such as Toowoomba, Highfields, and Hampton the soils are red volcanic clay loams known as ferrosols; in the valleys and floodplains, the soils are generally cracking black clays known as vertosols.
In practical terms, ferrosols are slightly acid, have a high iron content and are fertile. Sometimes ferrosol topsoil can be two or more metres deep, which means that these soils tend to drain freely. The name vertosol, on the other hand, is derived from the latin “to turn”. This describes the tendency of vertosol soil to shrink and swell, and anyone who’s worked these soils will know too well the way they crack when dry, and become sticky when wet. Vertosols can be up to one metre deep, they are usually slightly alkaline to neutral, and have the redeeming feature of being quite fertile.
The problems with black vertosol soil are obvious: alkalinity and drainage. Alkalinity can be corrected to some extent by adding sulphur, but it’s a relatively hard task. Stickiness is a trickier beast altogether. Due to a clay content greater than 35%, vertosol soils tend to suck up, and prevent moisture from draining away.
Ferrsol soils are generally much easier to work. The problem with them tends to be cultural, having little to do with the actual soil itself. Most ferrosols are found smack bang where developers like to establish housing estates. Modern builders like to construct houses using a timber frame on a concrete slab, which necessitates a “pad” of level ground. To obtain this pad, earthworks are undertaken, usually in the form of a cut and fill. Can you see a problem taking shape?
More often than not, the cut made during earthworks exposes subsoil, and what usually happens is that the homeowner is handed the keys to a shiny new house surrounded by a compacted, infertile, hard-pan that’s useless for growing anything but kikuyu. If you’re facing a situation like this, don’t despair. With a combination of time and toil, the situation can be rectified. Have a yarn to the builder. Ask if it’s possible that any removed topsoil be retained for use on garden beds. If the answer is no, you might have to tough it out and work with what’s left after the tradies pack up and leave.
Your options are to build raised garden beds or to improve the left over subsoil. The former requires that lots of soil be imported to the site, but if raised beds aren’t an option, you’ll need to work with the subsoil. Ideally you should spend the first couple of years adding gypsum and organic matter before any plants go in. Most people’s patience doesn’t stretch that far though, so a faster alternative is to add gypsum, loosen the soil with a rotary hoe, break up any underlying hard pan with a garden fork, and mix in lots of well rotted manure or compost. Mulch heavily with sugarcane.
After a few months, you’ll be ready for some plants. In subsequent years, keep adding organic matter, gypsum if necessary, perhaps some lime, and mulch. Eventually, you’ll end up with healthy soil that can support a broad range of plant species, but it will require time. Have no doubt that soil can be built. Like Rome though, it isn’t built in a day. Try seeing the soil building process as an opportunity to practice being patient. A chance to step off the treadmill and do things the old fashioned way. For the old ways are often the best ways.
First published in The Chronicle 26th September 2009.
