January 17, 2017
No doubt you’ve noticed – there’s a backyard vegie gardening revolution happening, and – excuse the pun – it’s growing faster than ever. But with most of us living in cities, is our soil up for the job? Urban areas have seen their fair share of industrialisation, building renovations, traffic and pollution over the years, so the risk of heavy metals and toxic contaminants in our soil is more prevalent than ever – meaning your organically grown lettuce may not be as healthy as you think.
Lead is the ‘big one’ – it’s the most prevalent contaminant in Australian urban gardens and, as a result, presents the biggest health risk. Mark Taylor, Professor of Environmental Science at Macquarie University, heads research into lead levels of Sydney’s backyard soils and says that all houses in the inner city likely to have high levels, citing two main sources of contamination: houses painted with lead paint – “when people renovate, they generate dust, which gets in the garden” and areas with greater car use, resulting in tailpipe emissions of leaded petrol. Before it was phased out, “lead petrol issued hundreds of thousands of tonnes of lead into the atmosphere”.
Lead has long-term health effects. High blood lead levels affect the nervous systems of adults, children and animals; and brain development in children, leading to decreased IQ, emotional and behavioural problems. It can also increase the risk of cancer, among other detrimental effects. Children and pets are particularly susceptible, due to their natural tendency to play close to the ground and hand-to-mouth contact activity. Furthermore, children absorb lead more easily than adults, meaning pregnant women should also be particularly careful.
Vegie patches in contaminated backyard soils are a big concern, with root vegetables, leafy greens and herbs the biggest risk. A 2003 study published in the journal Science of the Total Environment found that lead in soil will transfer into the roots, stems and leaves of a plant, while fruits on a plant will have non-detectable levels. However, even for fruits, Taylor says to be wary of soil dust on plants.
“If you’re concerned about reducing your exposure risk, you should build a new raised bed for growing food,” he suggests. It’s a simple way to know what you’re growing your food in, by bringing in new, clean soil. An alternative, albeit more costly, option is to remove the contaminated soil completely and introduce new soil. Taylor sees this as unnecessary, except in the most extreme cases of contamination. “Just make sure the grass is kept nice and long, it’s green and there are no bare patches,” Taylor says. “Mulch your soils and use raised beds
for your vegies.”
The Lead Education and Abatement Design (LEAD) Group advise keeping contaminated soil where you may be growing fruits at a neutral-acidic pH level of around 6.5 and above, explaining that “most of the lead that is present in the soil will become bound to soil particles in a way that prevents it from being incorporated into growing crops”. Adding organic matter also helps lock up lead, making it unavailable for plant uptake. As a last measure, wash any food grown in your backyard thoroughly for dust.
It’s also important to check your backyard chickens. Lead “does not ‘break down’ and leave your system; it accumulates in soft tissues and bones,” says Hannah Moloney, permaculture teacher and founder of Good Life Permaculture. “So chickens, who love to scratch, peck and eat soil, can ingest lead and pass a large portion of it straight into their eggs.” As an alternative to replacing the soil, she suggests covering it and building up your chickens’ home from there. “Cover your contaminated soils with geotextile to seal it off. Then, add a layer of shade cloth as extra reinforcement, to prevent chickens from trying to scratch through the geotextile. Finally, add a deep litter layer of straw, woodchips or another mulch substance for the chickens to scratch, play and live in.”
People need to understand the risks, but know “they can do something sensible about it and still use their garden,” Taylor says. “It’s important that people grow their own food, to have that enjoyment, particularly around kids, of planting things and watching them grow.”
Meanwhile, Taylor suggests that a last major source of contact to look out for is any dust or dirt brought into the house, a great exposure risk according to his studies. “Dust in house comes from a range of sources, including the garden.” Being aware of this and limiting indoor exposure means taking shoes off at the doorway, keeping doormats and floors clean, and regularly washing and brushing pets who come inside from the backyard.
While lead is undoubtedly the main concern for backyard contamination, Taylor also suggests keeping an eye out for old treated pine used for fences or planter beds that contain arsenic. “If you’ve got chromated copper arsenate posts that you’ve used earlier for fencing, that contributes to soil contamination.” He also frequently sees mercury in test results. Avoid contact with both sources of contamination as you would lead.
While all this may sound a little alarmist, Taylor encourages us all to not worry excessively, but to simply take care. “I want people to be alert but not alarmed. Wash your vegies, wash your house, mulch your soil, put lots of organic matter in, which helps bind the metals and reduce the bioavailability, and people can really enjoy their backyard.”
Taylor helps coordinate a free soil metal testing program that you can sign up to in Sydney with VegeSafe if you're concerned.
What is a geotextile?
Geotextiles are tough, permeable fabrics, usually made from polypropylene or polyester. As well as locking away contaminated soil, they’re used for a variety of purposes, such as protecting against erosion, so check you’re using the right type before you begin.
12 Steps to a lead-safe home & garden:
2. Grow edibles in raised beds with new clean soil, lined with a geotextile.
If you can’t build raised beds, stick to fruiting plants and trees.
3. Add plenty of organic matter
Adding organic matter to contaminated soil helps lock away lead into insoluble forms.
4. Keep soil pH above 6.5.
5. Thoroughly wash all fruits, vegetables and herbs grown in the backyard.
6. Keep all soil covered, ensuring no bare patches.
You can do so with a lawn, gravel, thick mulch or hard surfaces.
7. Be wary of all of the above
Take particular care around the ‘dripline’ of the home (the area under the eaves) where contamination is often worst.
8. Ensure that chickens have no access to contaminated soil
Build their pen up with layers of geotextile, shade cloth and mulch.
9. Take shoes off at doorway
and regularly hose down doormats.
10. Keep floors in the home clean
Mop regularly to remove dust, and clean windowsills and other surfaces that collect dust.
11. Clean pets before they come inside.
12. Take care of any old paint flaking from homes, fences or planters by removing safely or painting over.