Managing contaminated soil
Builder’s rubble, cement wash-off and heavy metals can all spoil your quest for a thriving garden. JOSH BYRNE explores your options for identifying and testing problem soils, and limiting your exposure
Successful gardening starts with understanding your soil. Texture, depth, fertility and pH all have an influence on the types of plants that will happily grow in your garden, and these factors can vary greatly from site to site. Another consideration is the history of the land and whether there is a risk of contaminants, such as debris and residue from building activities, or more serious concerns arising from the presence of heavy metals and residual chemicals.
Soil contamination is an unfortunate reality in many urban environments, as well as some rural areas. It doesn’t mean you shouldn’t garden on these sites, but it is important to understand the issues and the risks. Here are some of the common problems and ways to identify or test for them. Once you know ‘what lies beneath’, you can either remediate the soil, or choose to garden in raised beds.
rubble & residue
In garden areas, building materials or their residue are a nuisance at best and a hindrance to successful plant growth at worst. Leftover brick and rubble from construction or demolition are common on building sites and, once covered with soil, readily forgotten. Even small amounts can form barriers to root development if beds are confined and root space is tight.
On new builds, it’s worth inspecting the site after the builder has finished to check for leftover material prior to garden beds being set out and new soil brought in.
Cement residue from concrete and mortar is another problem. Cement is highly alkaline and, when present in soil, it raises the pH to the point where plant growth can be severely stunted. One of the main ways this material ends up in soil is when tradies wash down their tools and wheelbarrows. If the location where this is happening is over a future garden area, then the soil will be affected and plant growth compromised. This is particularly problematic on small sites, where wash-down space is limited and cement residue will be concentrated.
To avoid this problem, barrows and tools should be scraped clean and any surplus cement-based products set aside to dry before being binned. If on-site washing down is necessary, it should be done over an area that will be paved, rather than above a future garden bed. If you have beds that may have been contaminated with cement-like material, a test will confirm if high pH is a problem. A reading of 9 or above is serious, and the soil should be dug out and replaced.
metals & chemicals
The presence of heavy metals, such as lead and cadmium, as well as pesticiderelated chemicals and hydrocarbons, is common in urban environments as the result of previous industrial land use and the use of products that contained these substances. For example, there were high concentrations of lead in most paint until the 1970s and it can often be found in soil alongside older buildings, where the paint has deteriorated.
Persistent chemicals, such as those in organochlorine pesticides that are used to control termites and ants around homes and sheds, remain in the soil for a long time. Hydrocarbons from oil can also end up in gardens from road run-off, minute dust particles or illegal dumping.
These substances are unlikely to be at concentrations where they will present problems for your plants, but they aren’t good for you. It’s important to understand the history of your site, especially in older suburbs or areas that may have a legacy of industrial activity and chemical use.
While some plants can take up and accumulate contaminants, the main risk of exposure comes from physical contact and ingestion of contaminated soil. This can come from working in the garden, or eating root crops and ground-dwelling
vegies with soil residue on them, or from kids ingesting soil through hand-to-mouth contact. Poultry also shouldn’t be exposed to areas of concern, as some contaminants can bioaccumulate (become concentrated) in chooks and be passed on through eggs.
If you’re concerned about the possibility of contamination in your soil, you should consider getting the soil tested to see if the levels are within safe limits. There are a number of analytical laboratories around the country offering this service (see box, top right). They will provide instruction on how to collect samples from your garden and how to submit them for testing.