Is your garden soil contaminated? What you should know

by Connie Oswald Stofko

Is the soil in your garden contaminated? If so, is it okay for you or your kids to be working in the soil? Will the vegetables you grow there be safe to eat?

The answers to these questions are complicated, and we don’t have all the information we would like.

That’s according to Hannah Shayler, extension associate at the Cornell Waste Management Institute. She is working on Healthy Soils, Healthy Communities, a research and education partnership with urban gardeners. Shaylor was one of the speakers at a Buffalo Garden Symposium held in September by the Gender Institute at the University at Buffalo.

While much of the work of Healthy Soils, Healthy Communities is done with community gardens in urban areas, it is applicable to home gardens in urban, suburban and rural areas as well, she said.

People may be concerned about contaminants in their soil because they’ve had a community garden plot for years with soil that has never been tested, or they may be starting a new community garden, or there may be children working in the garden. Their first question is: Should I test my soil?

The answer isn’t simple, Shayler said. You need to do some research to find out what possible sources of contamination exist now and existed in the past.

Is your community garden near a toxic waste site or near an industrial site that may be using harmful chemicals? How was the site of your garden used in the past? Was there a house on that spot that contained lead paint? Were decks or swing sets on the property built from pressure treated wood? Even former farmland and orchards can contain contaminants– arsenic and lead were once used as pesticides.

Soil contamination may be more likely if the site has had any of the following: lead paint, high vehicle traffic, use of fertilizers or pesticides, industrial or commercial activity, treated lumber, petroleum spills, automobile or machine repair, junk vehicles, furniture refinishing, fires, landfills or garbage dumps. Find out more in Sources and Types of Contaminants from Cornell.

There’s no single soil test that will pick up all contaminants, so you need to determine what contaminant or contaminants are likely to be in your soil, then test specifically for those. In addition to metals such as lead, researchers are also concerned about polycyclic aromatic hydrocarbons (PAH), which are byproducts of burning and can cause many health problems, including cancer.

Testing can be expensive, Shaylor noted. Testing might cost $50 per sample, and you need to take samples from various spots in the garden.

For more information on testing, see Cornell’s Guide to Soil Testing and Interpreting Results. It’s very important that you send your samples to a certified lab, Shaylor said. Here is a list of laboratories certified by the New York State Department of Health Environmental Laboratory Approval Program.

Once you have your results, what do the results mean? That’s tricky, Shaylor said. There is no clear definition of  “safe” because there is no single standard that defines acceptable levels of contaminants in garden soils. Regulatory agencies are working on that, and in the meantime, gardeners can find some guidance in Cornell’s Guide to Soil Testing and Interpreting Results.

Contaminated soil can affect us in several ways. We might touch the soil and bring our hands to our mouth, ingesting the soil. We might breathe in the dust.

The food we grow in contaminated soil could become contaminated. However, Shaylor noted that contaminated dust on leafy vegetables or root vegetables is more of a concern than contaminants taken up and into the plant.

The steps you take in your garden depend on the results of your testing and how you want to use your garden. There are some general steps gardeners can take, including:

• Top dress the garden area with clean materials such as uncontaminated soil, compost, manure or peat moss, or incorporate these materials into your soil. If you have a square yard of soil and add a square yard of compost, you’ve cut the contamination in half.  Because organic matter in compost breaks down over time, incorporate new material often.
• Use raised beds.  A layer of landscape fabric will prevent plant roots from entering the contaminated soil below the bed. Avoid using railroad ties, telephone poles and pressure-treated lumber to build your beds because they contain chemicals that can migrate into the soil. You should use uncontaminated soil and compost. However, Shaylor noted that there aren’t any regulations governing commercial soil products and what you buy may not have been tested for contaminants.
• Mulch walkways and other areas to reduce dust and soil splashback onto crops, or maintain healthy grass or other ground cover.
• Create a barrier to separate underlying soil from children’s play area surfaces. Consider laying down landscape fabric and put clean play materials such as sand or wood chips on top.
• Adjust soil pH to near neutral. At lower pH levels (acidic soil), lead is in a more available form for plants and it’s easier for plants to take up the contaminant.
• Wash vegetables thoroughly and peel root vegetables.
• Consider growing food plants that are less likely to contain contaminants when eaten. It’s much easier to rinse dust off a tomato than it is to wash dust out of all the crooks and crannies of a leaf of kale. A list of food plants that are more suitable for growing in contaminated soil is contained in the Best Practices fact sheet.

For more information, check out another presentation by Shaylor on this topic here.