Lead Safe Gardening: Practical Fun Facts
Yes, this article title was chosen with tongue in cheek. (How fun can lead really be?) However, given my haziness around managing an invisible, scentless soil contaminant like lead, finding concrete information about its behavior under different soil conditions has been pretty exciting. Even, well, fun. Let's jump in!
Introductory Tidbits & Bioavailability
Soilborne lead clings to the surfaces of very fine clay and organic matter particles. For this reason, lead added to the surface of the soil tends to accumulate in the upper 1 to 2 inches rather than migrating downward. Of course, if the soil has been disturbed by tilling or excavation, this trend no longer applies.
The fine particles that attract lead tend to stick to skin and clothing as airborne dust (Stehauwer). Those working in areas with lead-contaminated soils should be mindful of this by bathing and removing clothing immediately upon returning home.
Not all soilborne lead is bioavailable to plants--or the human body. The bioavailability of lead rides on its solubility, and how tightly it is held by soil particles. Lead is held very tightly by soil organic matter. As organic matter in the soil increases, lead availability decreases (Stehauwer). This is one more affirmation that home composting and mulching is worth the effort! Note that if you are trying to extract lead from the soil as in scenario #1 below, adding compost may interfere.
Interestingly, the presence of lead may decrease the rate of decomposition of organic matter in the soil. There has been documentation of reduced microbial activity in soils contaminated with heavy metals like lead (Smith). According to a journal article by researcher Wiliam H. Smith, "Accumulation of heavy metals in the litter and upper soil horizons of natural forest ecosystems has resulted in the hypothesis that litter decomposition and nutrient cycling may be reduced in soils with excessive heavy metal input. Mechanisms proposed to account for reduced decomposition include; contamination of organic matter with persistent heavy metals, interference with soil enzymes and direct microbial toxicity" (Smith).
At a soil pH below 5, lead is held less tightly and is more soluble. This is also true for other cationic metals, including cadmium, mercury, copper, thallium, zinc, antimony, and barium (Darwish). At a pH greater than 6.5--neutral to basic conditions--soil lead is bound more strongly, and its solubility is very low. Both pH scenarios may be useful for soil remediation and lead-safe gardening depending on the desired outcome.
Two Approaches to Lead-Safe Gardening
In some scenarios, folks may wish to extract the lead from a contaminated area to grow an in-ground garden. However, it often makes more sense to render lead less bioavailable and create a barrier on top of existing soil. The new garden would be planted in introduced soil above it.
In scenario #1, we want to increase the bioavailability of lead so that bioremediating plants can absorb and remove it. In scenario #2, we want to decrease the bioavailability of the lead so the environment is safer for children and wildlife.
In either scenario, performing an initial soil test will be a helpful starting point. The chart below offers a starting point for interpreting soil testing data:
Chart source: Stehauwer, viewed Nov 2019.
This Penn State Extension article offers great tips for how to interact with soil that tests in each of these categories.
It's important to note that the acceptable limit of lead in parks and residential soils in the US is much higher than the limits in Canada and Europe. While anything under 400ppm is acceptable in the US. Canada draws the line at 120ppm. For sites where food is grown, the acceptable limit in Canada is 70ppm. Sweden's acceptable limit is 80ppm; Italy's is 100ppm. This stark difference is due to the role of corporate pressure in the making of US environmental laws (Darwish, 113).
Approach #1: Removing Lead from the Soil
Grassroots removal of lead from the soil often involves these steps:
Test the soil for pH and baseline levels of lead and other contaminants.
Add soil amendments to decrease soil pH, making lead more bioavailable to plants. This can include acidic materials such as sulfur, pine needles, sawdust, peat moss, coffee grounds, and leaf mold, in addition to bagged soil amendments (Darwish, 86).
Cultivate plants that excel at extracting lead from the soil
Remove contaminated plant material from the site and "safely" dispose of it
Repeat the entire process, starting with soil testing to assess the success of the previous round.
See Leila Darwish's Earth Repair for more details about this process and more resources to aid your efforts.
The following list offers examples of plants that have been found to extract lead from the soil. *Note: at least one of these plants is considered to be an invasive species. Please do your research and choose species that can be responsibly introduced into your ecosystem.
Sunflowers (Helianthus annus)
Brown mustard (Brassica juncea)
Alpine pennycress (Thlaspi caerulescens)
Geranium (Pelargonium spp.)
Wild buckwheat (Eriogonum spp.)
Willow (Salix spp.)
Quaking aspen (Populus tremula)
Alyssum wulfenianum
Honey locust (Gleditsia triacanthos)
Vetiver grass (Vetiveria zizanioides)
Bald cypress (Taxodium distichum)
Corn (Zea mays)
Tomato (Solanum lycopersicum)
Alfalfa (Medicago sativa)
Sheep fescue (Festuca ovina)
Water hyacinth (Eichornia crassipes)
Wheat (Triticum aestivum)
Scenario #1 requires a commitment--both of time and money, particularly for soil testing. It gets even more complicated when a community is dealing with multiple contaminants, which is common in urban, industrial, and conventional farming communities. For example, while cationic metals like lead become more bioavailable at a low pH, the opposite is true for anionic heavy metals like arsenic and chromium. Unfortunately, lead and arsenic are often found together, as are zinc and cadmium (Darwish, 86). Some remediation projects have taken the time to pull out cationic heavy metals using pH-lowering amendments, and then extracting anionic heavy metals by raising the soil pH with lime. Since each phase will require numerous seasons of plant growth and soil testing, this takes a lot of time--and a lot of soil-testing money to ensure that your desired outcome has occurred.
For folks who are limited on either time or money, it may be preferable to simply immobilize the lead in the soil (if that is a primary contaminant of concern), build raised beds on top, and call it a day. Which brings us to scenario #2--immobilizing soilborne lead.
Approach #2: Immobilizing Soilborne Lead
Besides time and money-saving, there are other advantages to gardening on top of--rather than directly in--soils that have experienced contamination. One is that we don't have to arrive at soil conditions and pH that are favorable for our vegetables or herbs. We can add soil amendments that make known contaminants less dangerous without concern for harming our crops with undesirable soil conditions, such as a high pH.
This chart illustrates how pH can affect plants' nutrient absorption, in general:
Scenario #2: Immobilizing Soilborne Lead
If we create alkaline soil conditions at a site by adding lime, planting native plants that thrive in alkaline soil conditions would be a responsible next step. Here are a few examples:
Clematis virginiana (virgin's bower)
Black-eyed Susans (Rudkeckia spp.)
Columbine (Aquilegia spp.)
Goldenrod (Solidago spp.)
Meadow rue (Thalictrum aquilegifolium)
Viburnum shrubs (Beaulieu).
Besides increasing the pH, adding phosphorus is another method that can render lead less bioavailable. Soilborne lead may combine with other elements to form lead-containing minerals. One example is lead's pairing with phosphorus to form lead phosphate or pyromorphite. Lead phosphate has an "extremely low solubility," making it an effective way to immobilize soil-borne lead (Stehauwer).
Formation of lead phosphate is supported by high soil pH (which already makes lead less bioavailable) and high levels of lead and phosphate. Applying large amounts of phosphate fertilizer would be the quickest route (Stehauwer). Organic fertilizers, bat guano, fish bones, and planting buckwheat all add phosphorus to the soil as well, but they are generally less potent, and doing the math to figure out how much to apply is a bit less straightforward.
Since phosphorus makes arsenic more bioavailable to plants, this may not be a desirable route if high levels of arsenic are also present in the soil (Darwish, 113). Some experts recommend using iron-rich compost to immobilize both lead and arsenic in the soil.
This is a great time to talk to a soil scientist about how much of these amendments to apply. After getting soil testing results from Cornell Cooperative Extension, a Cornell soil scientist was happy to answer my questions over the phone.
It should be noted that soil testing is less useful when the desired outcome is the immobilization of lead. Testing won't indicate what percentage of the lead that's present is bioavailable or not (111).
Once you've amended existing soil at a contaminated site to promote immobilization of lead, you're ready to create a barrier on top of the soil (which still contains lead) so you can start your garden on top. Be sure to check out Worcester Roots' Lead-Safe Yard Manual for instructions on where to go from here!
Wrapping Up
This article is intended to offer an orientation--not to offer enough detail to safely get you started with soil remediation. However, I hope you feel empowered knowing which soil conditions can render lead less absorbable by plants and people--making it less dangerous.
Never miss an article: Join the Sweet Flag Herbs newsletter to receive an email when a new A Nourishing Harvest article is posted.
Sources:
Beaulieu, David. "Alkaline Soil and Plants That Don't Mind Alkalinity." Updated Dec 8, 2019. www.thespruce.com/soil-and-plants-that-dont-mind-alkalinity-2131000
Darwish, Leila. Earth Repair: A Grassroots Guide to Healing Toxic and Damaged Landscapes. 2013.
Smith, William H. "Lead Contamination of the Roadside Ecosystem." Journal of the Air Pollution Control Association, 26:8, 753-766, DOI: 10.1080/00022470.1976.10470310. 1976. www.tandfonline.com/doi/pdf/10.1080/00022470.1976.10470310
Stehauwer, Richard. "Lead in Residential Soils: Sources, Testing, and Reducing Exposure." Penn State Extension. Viewed Nov 2019. https://extension.psu.edu/lead-in-residential-soils-sources-testing-and-reducing-exposure
