Is your garden soil toxic?

Urban farming is hailed as one of the best ways to boost health, strengthen food security and shrink the giant carbon footprint of commercial agriculture.

But cities are polluted, and pollutants can get deposited in soil, that of leafy gardens and thriving vegetable patches too.

Lead, for example, the most common such contaminant, makes its way into soil and onto plants from petrol emissions, old building rubble and industrial processes. Once in the soil, it’s not easily removed, and can persist even if cities clean up their act with measures like introducing lead-free fuel.

In South Africa, where the concept of community and school food gardens is being actively promoted, environmental health scientists are concerned that this might lead to toxin exposures from eating the produce -- especially in historically poor areas located near mine tailings or factories.

What’s the risk?

The health benefits of a vegetable patch, in most cases, will outweigh exposure to soil contaminants -- so don’t stop growing and eating your own food.

Short of a major toxic spill or high occupational exposures, most toxins we encounter in the environment, taken singly, don’t pose a major health risk – at least not that we know of at this point.

 But studies on toxin exposure tend to look at the effects of single chemicals, which, at low levels, are hard to link to significant health effects. We don’t just encounter one or two chemicals from one or two identifiable sources in our lifetimes however: we encounter multiple chemicals from multiple sources, and scientists don’t yet know what these may be doing to us in the long term.

So while we’re waiting for more reliable information, we should surely aim to ease the “chemical load” on our bodies whenever possible -- including using the following methods to ensure "clean eating" from city gardens.

How to avoid soil contamination 

For real peace of mind, you can get your soil professionally tested. If you find your soil is contaminated, or you don’t want the bother and expense of professional testing, here’s how to protect yourself:
  • Grow food plants in clean potting soil, available from nurseries, and use other parts of your garden for ornamental plants. It’s not enough (although it helps) to sprinkle a layer of clean soil over the ground; you need to first put down a layer of barrier fabric (also available from nurseries) to prevent the roots growing down into potentially contaminated soil beneath.
  • Build raised beds with clean soil to grow food crops in more contaminated areas.
  • Avoid growing edible produce directly adjacent to buildings, where lead levels are likely to be highest. Planting vegetables on road verges is probably not the best idea.
  • Choose your crops wisely. Different parts of plants absorb and retain contaminants differently. Toxic metals are largely excluded from entering the reproductive parts (fruits, seeds), remaining instead in the vegetative parts (leaves, stems and roots). These are pretty safe options: tomatoes, eggplant, peppers, squash, corn, cucumber, melons, peas and beans (shelled), tree fruits like apples and lemons, and berries (well washed). Rather only grow the following where you’re sure the soil isn’t contaminated: green leafy vegetables like lettuce, spinach and cabbage; broccoli and cauliflower; root crops like carrots and potatoes. Roots are also more likely to be contaminated because they are in direct contact with the soil.
  • Wash your produce well to remove the soil. Add a little vinegar to the rinsing water (1:100 parts vinegar to water).
  • Peel root vegetables and remove outer leaves from vegetables like lettuce.
Kids especially vulnerable

As well as avoiding toxins from edible plants, it’s sensible to protect yourself, and any children in the home especially, from exposure to urban soil and dust:
  • Teach kids not to put soil or dirty hands in their mouths, and to wash their hands before eating. Children are more vulnerable to pollutant exposures generally, because they receive larger doses relative to their body size, and because their organ systems are still developing. They’re also closer to the ground and put their hands in their mouths more.
  • Cover areas of exposed soil with clean soil or compost, ground cover or paving.
  • Use gloves when gardening, and remove gloves and shoes when you come indoors.
Kootbodien, T et al. (2013) Heavy metal contamination in a school vegetable garden in Johannesburg. South African Medical Journal
Shayler, H et al. (2009) Soil contaminants and best practices for healthy gardens. Cornell Waste Management Institute, Department of Crop and Soil Sciences.

Image of carrots: Shutterstock
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