When to Test Garden Soil for Lead Contamination and What to Do if It’s Present
Learn why and how to check for lead contamination in soil and how to garden safely when it's present.
Even though it was banned nearly 40 years ago, lead is still one of the most dangerous pollutants out there because it does not break down and vanish naturally. Paint manufactured prior to 1978 is one of the biggest causes of lead poisoning. You’ve probably heard horror stories of children eating paint chips and becoming seriously ill. It wasn’t just in paint: decades ago, lead was in gasoline, plumbing pipes and pesticides. And the danger is not just from ingestion, but also inhalation and absorption through the skin. Abatement measures can be taken to make a home’s interior safe, but what about the exterior and surrounding property? There’s really no way of knowing if exterior paint chips or particles are in the soil around your home. If you plan on edible gardening on the site of an older home, it's definitely something to consider. But the good news is, there are ways around it, and you can still garden.
I live in a new home that was built on a site where an older home was torn down. So, when my builder told me that the topsoil around my house — where I planned on putting my garden — was from the foundation of an old house that once stood on the site, I knew immediately that I needed to get the soil tested for lead. What I didn't expect was that the test results, which indicated elevated lead, would set me on a year-long search for straight talk on what to do with my soil.
Online resources and authorities varied when it came to interpretations of risk, as well as advice on how to mitigate the problem. Wanting clear answers, I consulted the directors of two state soil testing labs. They helped, but not completely.
Ultimately, I contacted soil chemistry scientist Michael Essington of the University of Tennessee Institute of Agriculture. Through Essington, I met professor Nicholas Basta of The Ohio State University. Basta is regarded as one of the nation’s top authorities on assessing, cleaning up and reusing soil that's been contaminated with lead and other pollutants. These experts provided the answers I needed to use my garden beds.
Why Test for Lead Contamination
Get your soil tested if you plan on growing edibles beside the foundation of a house that may have been painted prior to the late 1970s. Some houses may have older paint layers hidden under newer siding.
Also test if you are considering gardening at any of the following areas:
- Former industrial sites
- Land near roads in use before leaded gasoline was phased out
- Areas where soil may have been exposed to old lead plumbing pipes
- Old orchard sites that may have been in production when lead arsenate was used as a pesticide
- Old urban areas being considered for new uses
What's the Risk
The risk when elevated levels of lead are present in soil isn’t that the food you grow will contain lead. Instead, it’s that soil containing lead may splash upon produce like lettuce and greens or adhere to root crops and be difficult to fully rinse from their crinkly surfaces. Ingesting some of that soil or breathing in the dust could be hazardous.
Carrots, potatoes and other root crops are difficult to scrub free of soil. When grown in soil with slightly elevated levels of lead, they should always be peeled. In soils with higher lead levels, it’s best not to grow them at all.
Lead is largely not bioavailable to plants. That means plants take up limited amounts of it, including Indian mustard, and sunflowers, which some information mistakenly claims will actually remove lead from soil.
Basta says he and the U.S. Environmental Protection Agency Brownfields Program work with communities to dispel a persistent myth that plants are able to remove lead from soil. While this process, known as phytoremediation, can work with other contaminants, research has repeatedly shown it does not work with lead. The reason is that lead is both stable and insoluble in the soil.
How to Have Your Soil Tested
Many state laboratories test soil samples to make gardening recommendations. When the concern is lead, instead of a general soil test, ask for a screening test, which typically costs around $30.
Make sure the lab tests for the total lead content of your soil. Not all state laboratories will perform the required screening test, so you might have to send a soil sample to a lab in an adjacent state, as I did, or to a private lab.
To find a testing lab, consult your local or district extension agent or contact your state’s land-grant university. A lab can provide instructions on how to collect a sample of your soil. Lead can vary greatly even within small areas, so be sure to follow the recommendations for your site.
How to Interpret Test Results (Hint: a Number is Just a Number)
Screening tests generally provide a specific number for the parts per million (PPM) of lead in a soil sample. Rather than focus on the number you receive, study where it falls in a series of ranges that identify soils with lead as low, medium and high in risk.
Labs and other resources identify these ranges. The challenge is that the PPM numbers used to define risk often vary. In uncontaminated soils, normal lead levels range between 2 and 300 PPM, and average about 35 PPM.
Essington and Basta advise this rule of thumb:
- Low: If the lead present in your soil tests 400 ppm or below, the soil is safe for any use.
- Medium: Lead levels at 400 PPM and above fall into a medium risk range that increases in the amount of risk to an upper threshold of 1,000 PPM.
- High: If your lead level is above 1,000 PPM, use the area for something other than fruit and vegetable gardening.
Three Steps When Soils Are Medium Risk
Since the amount of lead that fruit and vegetable plants are able to take up is limited, focus instead on reducing the risk of accidentally breathing or ingesting the soil. You can do this by incorporating a phosphate fertilizer into the soil. The phosphate will bind to the lead and create crystals that make the lead biologically unavailable to humans.
Compost has the same effect because it is also a rich phosphate source. So proper fertilizer management, whether conventional or organic, will lower the risk of lead exposure to humans.
Mulching the soil is also important. Mulch beds and aisles with organic or inorganic mulch, like plastic, to further lower the risk of inhalation and ingestion. Soil labs advise not to grow lettuce, greens or root vegetables when lead contamination is high. While they can be grown when lead levels fall lower in the moderate risk range, always rinse this produce well, and both scrub and peel root vegetables.
The ideal way to use a site where elevated levels of lead are present — or, in fact, when any contaminants are suspected — is to garden in a raised bed.
Line the bed foundation with landscape cloth to keep roots within the bed. Fill the bed with soil from known and safe sources. Then add as much compost and organic matter as you can to improve soil quality and fertility. Also, add phosphate to the soil as an additional barrier to lead exposure.
When Soils Are High Risk
If you find high values of lead in your soil, keep the soil mulched or covered with grass or other materials. This will prevent wind from blowing the soil into the air. Keep children away from problematic soil and minimize your own exposure, too. Wear garden gloves at all times, but still, wash your hands and skin well after you’ve been landscaping in that area.
For More Information
- U.S. Environmental Protection Agency factsheet on growing gardens in urban soils
- Resources from The Ohio State University on how to investigate, treat, and restore damaged soil
- List of land-grant universities. Extension offices within these universities can direct gardeners to laboratories.