Soil Contamination in Home Gardens

While growing your own food can be incredibly rewarding, it’s important to be mindful of the soil quality in your garden. Soil contamination is an issue in some gardens, and it can pose significant risks to both the environment and human health. Contamination can occur in any garden but gardens set in older houses or in urban areas with an industrial past are more likely to be affected. The good news is that soil contamination needn’t mean the end to your gardening journey; there are easy steps that you can take to garden and grow food safely.

How Does Soil Become Contaminated?

Urban soils can be contaminated with a variety of heavy metals transported by air, soil or water. In urban areas past industrial activities, urbanisation, the use of lead paint and transportation infrastructure have all left their mark on garden soil. Common contaminants include lead, arsenic, cadmium, chromium and zinc.

Lead contamination in particular is a significant concern, especially in areas with older buildings where lead-based paints may have been used. Lead can also be present in soil near roads where pollutants from seven decades of leaded petrol use may have become persistent in the environment. According to the EPA’s GardenSafe initiative, soil in urban areas can also be contaminated by other pollutants from industrial activity, construction, or even past use of pesticides and fertilisers.

Why is Soil Contamination Harmful?

Contaminants are a concern because they can impact human health, biodiversity and the wider environment. For example, lead is recognised as a dangerous neurotoxin that can cause developmental delays in children and increase the risk of dementia in older people. Even small amounts of lead in our environment can be harmful to humans.

Do Contaminants Enter the Food You Grow?

Whether plants absorb heavy metals and other contaminants, and where those elements travel within plants, depends on the species. For example, leafy vegetables like spinach, lettuce and kale are particularly susceptible to absorbing lead and other toxins from the soil, while fruit trees may not be as susceptible. Trials at Delprat Garden in Newcastle have found that beans, watermelon, pumpkins and carrots accumulate toxins in the edible parts of the plant (fruit or roots). Sunflowers and mustard plants, however, accumulate lead and zinc in their roots but not in their leaves or flowers. Fig and tomato fruits in contaminated areas have been tested and found to contain no toxins.

The dangers are not just limited to direct ingestion of contaminated produce. When soil is disturbed through digging or weeding, pollutants can become airborne in dust and settle on other surfaces or be tracked into houses on the soles of shoes. Gardeners working the soil with bare hands can also encounter contaminants. Similarly, toxins can be ingested by children and pets who play in the garden or soil.

Is Your Soil Safe? Get a Test!

The first and most important step in relation to soil contamination is to know what’s in your soil. A professional soil test will reveal but can be costly. Fortunately, home gardeners in Australia can join citizen science efforts to map soil contamination and may be able to get a soil test done for free or the cost of a small donation:

1. In Victoria, the EPA citizen science unit offers free soil tests through its GardenSafe program. Gardeners send up to three soil samples to GardenSafe and receive a report with information on soil nutrients and soil type as well as whether contaminants are present.
2. Elsewhere in Australia gardeners can make a small donation ($20 is suggested) and receive a similar report from VegeSafe, which is associated with Macquarie University. (Note that this service is sometimes suspended due to a backlog of samples.)

Once you know what’s in your soil, you can make informed decisions about what to plant, what actions to take and whether remediation is necessary.

Gardening with Contaminated Soil

Discovering that your soil contains dangerous levels of heavy metals may be disheartening but it doesn’t need to mean an end to your gardening adventures. There are several simple steps that gardeners can take to reduce the impact of contaminated soil and grow healthy food:

1. Raised garden beds: Raised garden beds are one of the most effective ways to avoid direct contact with contaminated soil. Fill your raised bed with purchased topsoil and you can then grow any type of edible crop including leafy greens.
2. Mulch: Covering your soil with mulch limits soil disturbance and prevents soil sticking to your shoes and dust from spreading contaminants through the air.
3. Don’t plant edibles under eaves: If you live in an older house that may have been painted with lead paint, avoid planting edibles under the dripline of eaves or directly beneath windowsills. It’s fine to plant ornamentals in these places.
4. Provide safe play areas for children: install a sandpit or other play space where children can dig and play without coming into contact with your soil.
5. Give chooks deep litter: make sure chickens can’t scratch in contaminated soil by installing a deep litter system that separates them and their food from the soil.
6. Wear gloves and wash hands: wear gloves while gardening so your skin doesn’t come into direct contact with contaminated soil, and wash your hands after gardening.
7. Wash veggies after harvesting: this will remove soil that may be attached to your crop before you consume it.
8. Don’t wear shoes indoors: Contaminated soil (as well as dog poo and other nasties) can be carried indoors in the soles of shoes.

Phytoremediation: Nature’s Solution to Soil Contamination

One promising solution to soil contamination is phytoremediation, which is the use of plants to clean up polluted soils. Phytoremediation is seen as a better option than removing soil or capping it with a layer of clay. While these methods entail contaminants remaining in soil, phytoremediation has the ability to remove heavy metals from soils permanently.

Certain plant species have the ability to absorb contaminants from the soil. The toxins then accumulate in the plants’ tissues. As noted above, plants like sunflowers and mustard greens have been shown to absorb heavy metals such as lead, arsenic and cadmium. The plants can then be removed and composted in a dedicated compost heap. The question of how to dispose of the resultant compost, which has a smaller volume of plant matter but still contains heavy metals, is still being worked on. Delprat Garden in Newcastle is one site where research into phytoremediation is being done. You can see the results of their research here.

Urban gardening can be a fulfilling and sustainable way to grow your own food, but it’s essential to be aware of the risks associated with soil contamination. By testing your soil, choosing appropriate crops, and using some simple strategies, you can mitigate the dangers and create a healthier, more productive garden.

References:

1. Gardensafe https://www.epa.vic.gov.au/for-community/get-involved/citizen-science-program/gardensafe/garden-safely-with-epa
2. Vegesafe: https://www.360dustanalysis.com/pages/vegesafe-about
3. Bulleen Art & Garden: Soil Contamination https://www.baag.com.au/soil-contamination/
4. Gardening Australia, “Soil Safety” https://www.abc.net.au/gardening/how-to/soil-safety/103472326
5. Gardening Australia, “Plants to the Rescue” https://www.abc.net.au/gardening/how-to/plants-to-the-rescue/102076168
6. Taylor M and Filippelli G, ‘Wearing shoes in the house is just plain gross. The verdict from scientists who study indoor contaminants‘ The Conversation 2022.
7. Delprat Garden: https://delpratgarden.com.au/garden-performance