Whether your garden is located in the heart of Manhattan or the bucolic pastures of Kentucky, you should be aware of the possible risks posed by growing food in contaminated soil. Of course, not all sites are equally at risk, so you should first investigate the history of your gardening site. For example, Cornell University’s Dilmun Hill Student Organic Farm has soils contaminated with lead and arsenic, not surprising given the site’s history as an orchard. Before the 1960s, American orchards routinely managed pests with natural chemicals containing stable and persistent heavy metals, including lead arsenate. Other common avenues through which soil contaminants might enter gardening sites include peeling off of lead paint (banned in 1978); proximity to a current or historic industrial site, landfill, or frequently travelled road (lead from older gasoline, petroleum runoff); and contact with pressure treated wood containing chromate copper arsenate (CCA treated wood for residential use was banned in 2003). Children are often even more susceptible to the harmful effects of soil contamination than adults, as they often play in the dirt and can ingest or inhale contaminated soil.
If your site’s history or soil test results give you reason to be concerned about soil contamination, the Cornell Waste Management Institute offers excellent resources to help you make a prudent, informed decision about soil testing and interpreting results. Their fact sheets are particularly valuable in helping you assess your risks. Unfortunately, there are no universally agreed upon safety thresholds for contaminant levels in soil; different agencies and nations vary in leniency.
The CWMI’s Healthy Soils, Healthy Communities project is carrying out research at Dilmun Hill Student Organic Farm as well as in urban community gardens, measuring the amount of heavy metals present in tomato, lettuce, carrot, and bean tissue grown in soils with low, moderate, and high levels of contamination. They chose these four vegetables to test the common hypothesis that fruits, like tomatoes and beans, may be safer to eat than root or leaf crops grown in contaminated soil. In their experimental plots, each vegetable receives three different management treatments – compost, mulch, and bare soil – to determine whether soil amendments can bind or dilute heavy metals. Interestingly, in their test plots vegetables can be seen thriving in even the most severely contaminated soils. Lead and arsenic are not toxic to plants like they are to humans, so you should not make the dangerous mistake of using vegetable health as a bio-indicator of soil contamination levels! If you garden at an urban community garden and are interested in learning more about the Healthy Soils, Healthy Communities Project, please contact Extension Associate Hannah Shayler at firstname.lastname@example.org.
There are many techniques available to minimize exposure to contaminated soils, requiring varying amounts of time, money, and expertise to implement. One of the easiest and most common approaches is to create a raised bed. If properly designed, a raised bed can keep plant roots from coming into contact with contaminated soil. The farm managers at Cornell’s Dilmun Hill are experimenting with geotextile fabrics as well as cardboard barriers above the native soil, over which they liberally apply a mixture of compost and topsoil. The bed perimeter is then mulched to reduce weeds as well as the risk of gardeners inhaling dust from the contaminated soil adjacent to the bed.
Researchers are measuring the amount of heavy metals present in both the soil and plant tissue for each barrier method, helping gardeners better understand which approaches most effectively minimize the risks associated with contaminated soil.
While it is sobering to realize that even the most gorgeous vegetables grown in a rural setting can be dangerous to our health, you can determine if a garden site falls under any of the common risk factors. If it does, there are a wealth of resources and techniques available to help you test for contaminants and respond appropriately. Ultimately, the best way to ensure that the produce you are eating is grown in healthy soil is to grow it yourself!