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July 29th, 2022
Lead and other heavy metals are often found in urban soils. It can be frustrating and scary to learn that your soils have high levels of lead. I wanted to share how I go about assessing backyards to see if they have lead, how to evaluate how much is too much, and what to do about it if you have lead, so you can still garden safely and confidently!
Leila Darwish in her book Earth Repair: A Grassroots Guide to Healing Toxic and Damaged Landscapes, speaks to a deeper set of questions we can ask ourselves: "From the contaminated soil in backyards, community gardens, and vacant lots to brownfields, big oil spills, and nuclear accidents, how can we transform toxic and damaged landscapes into thriving, nourishing and fertile places once more? How can we respond to environmental disasters in radical, accessible, and community-empowering ways?" Wrestling with these broader questions is important work. It is deeply linked to environmental justice as minority groups, people of color, and low-income communities are disproportionately impacted by environmental pollutants and degraded soils. These are daunting issues of our time. Yet we can work to address them, beginning right where we are, learning the principles of building life-giving soils and dealing with toxic or degraded lands starting in your own backyard or a community site near you.
One of the major sources of lead in soils is from old leaded paint. Lead was also in leaded gasoline as well as old pesticides like lead arsenate used in old orchards (yes, it does kill things… but... lead and arsenic? What were we thinking?). In working with hundreds of urban gardens in and around the Boston area, I helped test a lot of soils. The good news is that there are pockets of low or no lead even in cities. The bad news is that you also never quite know where it will pop up (old burned trash heap or dump that is now buried, old gasoline site, etc…). Because of this, I strongly recommend taking a soil test. Check your state or local testing laboratories to see if they will give you a lead estimate. I’ve linked a few I know of in the resource section below that test for lead fairly inexpensively ($12-25/test). See the resource section for how-to take a home soil test.
Second, be smart about how you test. If you have an old house, fence, or shed that you are pretty sure has had lead paint, plan to test the soil within 2-4 feet next to that structure separately from the rest of your yard. Often I have seen a halo or “lead zone” around houses, but the rest of the yard comes up very low. If this is the case, it gives you options!
It is normal to have some lead in soils. It is only a problem when the levels get way higher than what occurs naturally in soils. If you have “high” or “medium-high” lead levels (according to your soil test!), please be especially careful if you have small children or are pregnant. The options I list below are great for mitigating lead in soils with low to low-medium lead levels in soils. If you have high or very high lead levels, these techniques can still be helpful to bind up and immobilize lead in the soil, but growing in those soils still should be avoided. Instead, grow in containers or raised beds with a barrier to prevent roots from coming in contact with contaminated soils on site.
Actions:
Maintain healthy levels of organic matter in your soils. Organic matter (what makes soils dark, sweet-smelling, and rich in color) can hold onto lead and make it less available to your plants, especially when your basic plant nutrients are readily available. Compost, mulches, composted manure, humates, and cover crops are all good ways to increase the organic matter in your soils.
Maintain pH at 6.5-7. Lead is less available in soils with a higher pH (more basic or alkaline). Acid soils (low pH) enable more lead to be available. An ideal veggie garden has a pH between 6-7pH, just slightly acidic, but not too much! A soil test is also a good way to get to know your pH level. Adding lime, compost, and correcting nutrient deficiencies are all ways to adjust the pH to be more basic if you tend to be much more acidic.
Balance soil nutrients (esp. Ca, Mg, K). Soil nutrients like calcium (Ca), Magnesium (Mg), and Potassium (K). Plants prefer these nutrients over lead but will uptake lead when basic soil nutrients are limited or not available. Look at the “base saturation” levels on a soil test (if available) and shoot for the following ratio 68:12:4 of Ca:Mg: K. If that sounds too complicated... don't worry! Most soil tests offer simple recommendations if these nutrients are deficient! Balancing major nutrients can also help ultimately balance the pH to minimize lead uptake. Pay particular attention to adequate levels of calcium in the soil as plants deficient in calcium are more likely to take up lead. A great way to add calcium is to add high calcium lime or gypsum to compost or directly to the soil in the spring or fall.
Cover the soil surface and minimize dust. Lead uptake is much more likely to happen from eating dust on the surface of your crops than from eating the crops themselves. Avoid dust on your crops by keeping the soil surface covered with protective mulches. You might also choose to trellis your crops to keep them upright, grow crops that hold themselves upright (raspberries vs. strawberries), and/or be extra mindful of washing produce from the garden.
Creative mulch options
Prioritize fruiting crops. Fruiting crops such as melons, tomatoes, raspberries, peppers, and apples, for example, are much less likely to uptake lead than leaf or root crops. Consider prioritizing them in your garden if lead is present but not at high levels.
Other creative solutions: These creative solutions can help mitigate challenges with lead in the soil if you want to remediate an area that you are not able to use over time. They will not completely remove lead from the soil, so continue to use the above strategies to minimize lead problems in a garden.
Use Biochar or Humates to help tie up the lead in the soil (similar to increasing organic matter content), and make it less likely to be taken up by plants, especially if soil nutrients are balanced.
Plant Hyperaccumulators. Some plants such as sunflowers and ragweed hyper-accumulate lead. They remove lead VERY slowly from the soil, and to be successful, they must be removed at the end of the growing season.
Mycoremediation: Some mushroom species can remove heavy metals from the soil, but careful monitoring must be done to remove contaminated mushrooms as they appear. "Spent substrate" from commercial local mushroom production – aka the leftover/compostable material from commercial gourmet mushroom production - is full of enzymes that have the capacity to break down hydrocarbons and literally rip apart certain organic pollutants. It also is a slow-release fertilizer so can benefit plants even in a healthy, non-polluted system.
Biological products to break down pollutants – There are now many biological products where beneficial soil biology is being used to break down pollutants and create healthy underground ecosystems that hold and make heavy metals less available to plants. “OP8” is one example of a product developed by Advancing EcoAgriculture that specifically targets the breakdown of pollutants like hydrocarbons and other toxins from polluted water and soils. More active and healthy soil biology in the soil also can create conditions where lead is not able to move!
Grow soil. Be an earth repairer, if you can, wherever you are. Be extra cautious with contaminated soils and safe as you do this important work! Don't burn out, but gradually, steadily work towards your goal... there are an unbelievable number of allies, human and non-human, who will help you do this work.
"Let the beauty we love be what we do. There are hundreds of ways to kneel and kiss the ground."
~ Rumi
Resources:
Soil Testing Labs that test for lead:
How-to take a home soil sample (5 min video from UMaine): https://www.youtube.com/watch?v=2FAAYz-ihnI
University of Massachusetts Amherst Soil Testing Lab does a quick lead scan in their "Routine Soil Analysis": https://ag.umass.edu/soil-plant-nutrient-testing-laboratory/fact-sheets/sampling-instructions-for-routine-soil-analysis
University of Maine Soil Testing Lab "Standard Soil Analysis screens for lead: https://umaine.edu/soiltestinglab/home/faq/
And if you want to dive deeper into repair, recovery, and remediation, check out these resources by some truly amazing land repairers of our time. It is not easy work, and we must be smart and safe while doing it, but we have a responsibility and call to heal and work with the lands we have so deeply injured:
Earth Repair: A Grassroots Guide to Healing Toxic and Damaged Landscapes by Leila Darwish, https://earthrepair.ca/
Mycoremmediation Handbook: A Grassroots Guide to Growing Mushrooms and Cleaning Up Toxic Waste with Fungi by Alex Dorr
Organic Mushroom Farming and Mycoremediation: Simple to Advanced and Experimental Techniques for Indoor and Outdoor Cultivation by Tradd Cotter.
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