Clients often ask, ‘Gav, where shall I put my raised garden beds?’ My usual response is to say, ‘Why did you buy them? Is there a problem with your soil?’. Folks often install raised garden beds when they don’t really need to. There’s a bit of a mindset that says if you want a vegetable garden you need raised beds because ‘that’s what you’re meant to do, right?’ Well, not necessarily is my answer.
Unless your soil is rocky, stony, contaminated (more about that later), or your gardening on a balcony or roof, then you’re usually better off using the soil that you’ve got. Rather than spending hundreds of dollars on raised beds and bags of soil, using the soil in your yard is a better investment in most cases. Even poor draining soils can be remedied by mounding and gypsum applications (for sodic clay soils). Back yard soils are one of those things we don’t even think about, yet they are a precious resource and if managed well will allow you to successfully garden season after season.
In our Greater Brisbane region native soils are quite variable, ranging from beautifully fertile red loams to stony and leached out ‘schists’. Suburbs like Sunnybank, Nudgee, Graceville and Aspley have generally excellent garden soils and it comes as no surprise that many of our city’s market gardens and orchards of yesteryear were located in these places. On the other hand the hills and ridges of the inner city – such as Red Hill, Paddington, Highgate Hill, Herston and Kangaroo Point – have ancient, highly weathered thin top soils and in many places the top soil has eroded away.
I love how our region’s fertile soils and abundance were celebrated by past generations in local place names like Redlands, Richlands and Orange Grove Road. If you’re wanting to buy property with good fertile soils then understanding this history is a good first step. Looking at soil maps of Brisbane is also a worthwhile exercise.
Soil testing will indicate if your soil is fit for purpose. For my clients I test for pH (acidity levels), drainage, colour, smell and texture to get a basic feel. Observing the vegetation on the property and surrounds also adds to the picture – are the trees stunted or magnificent, are there any signs of nutrient deficiencies in the leaves, are the plants generally in good or poor condition? If clay is present I also test for salts (sodicity).
For folks serious about creating a high yielding vegetable garden then a more detailed analysis from a registered soil laboratory is warranted. A report will be produced that indicates nutrient deficiencies and how to correct them. A laboratory that undertakes Albrect soil testing and analysis such as Southern Cross University’s EAL service is recommended.
Potential sources of soil toxicity are many. Old house paint, roof flashing and sheeting, old plasterboard, backyard incinerators (from a long time ago), garage workshops are all potential sources in Brisbane. Yards adjacent to busy roads may also be a sink for lead pollution – a legacy from the days of high lead content in petrol. In our region the main contaminants of concern for gardeners include lead, zinc, and asbestos. Understanding the history of a site can tell us a lot about possible contamination issues.
The VegeSafe service run out of Macquarie University is a free program ($20 donation ask for) that will analyze your soil for eight heavy metals. This a fantastic citizen science project that is building a detailed map of soil contamination across the nation. I undertake the soil sampling for clients which are posted to the lab and they usually provide results within three weeks.
Whilst contamination may seem a daunting problem, in my experience I have rarely encountered any issues in backyards. Research has shown that low to moderate heavy metal contamination can be buffered by good organic gardening practices such as applying humus and liming to raise pH. Friends and colleagues in Brisbane have occasionally reported high to extreme lead levels adjacent to older timber houses where lead paints and roof flashings were used. In those rare cases, the affected soil is either completely removed and replaced with a good quality landscaping soil, or raised garden beds are used.
Do you need a raised garden bed?
If soil testing and other investigations show that your soil is fit for purpose then ‘gardening in-the-ground’ is a value-for-money way to go. Of course, raised beds are good idea if you need to sit or stand whilst gardening. One of the downsides of raised beds is finding access to good quality soil. As of writing my research indicates that all of the garden soil products on the Brisbane market are essentially sands mixed with organic additives, such as pine bark, and minerals. Over a year or two the additives either leach from the soil or are taken up by plants. So, fairly quickly you’ll be left with just sand. Additions of compost, animal manure every season will restore fertility and water holding capacity in the short term. Clays, such as bentonite and kaolin, added in appropriate amounts can be useful.
Wicking beds are useful if watering will be infrequent (for example at a community garden) and an irrigation system is not possible. There’s a plethora of products and online how to guides on this topic. I’ll be writing more about this soon.