It is important to evaluate the level of lead in the soil you plan to grow in by conducting a soil test. I recommend Logan Labs, as you can get a full panel with trace minerals from one place. Logan does send the sample out for lead testing – they don’t do it in-house because most growers concerned with trace minerals are really caring for their soil, and cared-for soil doesn’t present a big lead hazard… let me explain:

The only ways to get lead contamination from soil is to eat the soil itself or to breathe in dust from soil that is not covered or kept moist. If you are treating your soil right, you won’t have either of these problems, until you go to harvest your roots. Harvesting roots represents a very real hazard. There are lots of bits of soil left on the roots, and those can get into the tincture.

How does soil get contaminated with lead?

Lead exists naturally in soil at levels of 10-50 parts per million (ppm) [some sources say the average is 10 ppm, with the natural range from 7-20; under 50 is considered ‘safe’ but anything over 20 is usually the result of lead contamination]. Areas near smelters, tailings from metal ore mines, fossil fuel-fired electrical power plants, or cement factories often have elevated soil lead levels. Lead-arsenate sprays used in the 1910s-50s in nut and fruit orchards may have left concentrations of lead and arsenic in soils at those old orchard sites. Lead can also get into the soil from paints and leaded gasoline (from car emissions). Lead concentrations were highest in paints before 1960, and leaded gasoline was used until the mid-1980s in Massachusetts.

Lead accumulates in the top 1-2 inches of soil unless soil has been disturbed or tilled. Once deposited in the soil, it is very slow to move and can persist for a long time. Lead is held tightly on surfaces of very fine organic matter particles, which tend to stick to skin and clothing (as dust). It is also found in airborne soil dust. Almost all of the lead in soil is in solid form and does not dissolve readily in water.

WHAT IS THE RISK?

Plants do not absorb significant amounts of lead. Lead does not accumulate in edible parts of vegetable and fruit crops. External lead on unwashed food is a greater concern. If you grow and harvest vegetables and have high-lead soil, you can wash vegetables thoroughly in 1% vinegar solution. Vegetables that can trap dust like broccoli, lettuce, kale, kohlrabi, radishes, cauliflower, Swiss chard pose a higher risk. These should also be rinsed. Root crops are also more of a concern, if left unwashed, as more surface area is in contact with soil. A .5% soap solution may also be used to lift the small particles from food surfaces.

The risk of lead poisoning goes up with direct exposure to soil with high lead levels. If soil is uncovered, dusty, remaining on edibles, or otherwise breathed in or eaten, the risk of lead poisoning goes up. Riskiest spots:

• Play areas (bare soil, kicked up)
• Property adjacent to heavily traveled streets and roadways, especially older high-traffic streets (when cars that traveled them were using leaded gasoline).
• Planting borders or beds adjacent to a house or other structure painted before 1960.

Testing for lead:

The best way to determine if soil is a high risk for lead is to test it. I recommend Logan Labs, as you can get a full panel with trace minerals from one place.

Reading a soil test:

• < 50 ppm = little or no lead contamination, not special precautions needed.
• 50-400 = some lead present from human activities, grow any vegetable crops and choose gardening practices that limit dust or soil consumption.
• 400-1200 ppm = do not grow leafy vegetables or root crops and choose gardening practices that limit dust or soil consumption.
• > 1200 ppm = not recommended for vegetable gardening. Mulch and plant perennial shrubs, groundcover, or grass. Use clean soil in raised beds or containers for vegetable gardening.
• If lead levels are over 5,000 ppm, paving or lead abatement is required by the USEPA.

Some sources say that any level less than 300 is considered safe to grow food plants, if children are not present (who tend to take plants from the soil, brush them off, and eat them), and even at soil levels above 300 ppm, the lead risk is from contaminated soil or dust on the plants rather than uptake of lead by the plant. Washing is important if you know you have higher lead levels.

Decreasing lead hazard in the garden:

• Reduce dust exposure by covering bare soil with mulch or mulch plants like groundcovers or cover crops.
• Apply soil amendments to bind lead with organic matter, as recommended by soil testing and analysis. You can decrease the bioavailability and therefore toxicity of lead in soils by several soil management practices:

1. Maintain near-neutral soil pH. Lead is made relatively unavailable to plants when the soil pH above 6.5.
2. Add phosphorus when soil tests indicate a need. Phosphorus reacts with lead to form insoluble compounds, making lead unavailable. It can be supplied with manure, compost, or as rock dust.
3. Add organic matter, which enhances the formation of organic compounds that bind lead, making it less able to form dust. In soils with high lead levels, adding one-third by volume organic matter will significantly reduce lead availability. Composted leaves, garden compost, and well rotted manure are good sources of organic matter.

• Intensive gardening that uses tilling and leaves the soil bare is a practice that requires the most risk and the most care, because dust and mixing are part of the process. To reduce lead hazard, you can resort to these practices (or turn to permaculture, stop tilling, and plant living mulch):

1. Use barriers like geotextile or solid plastic to separate contaminated and uncontaminated layers of soil.
2. Use raised beds filled with uncontaminated soil.*
3. Grow plants in containers using uncontaminated soil.
4. Replace contaminated soil with uncontaminated soil.

*Basically, you don’t need to build raised beds if your only concern is lead. If you also have compacted soil, a bad back, and problems with water-logged soil, raised beds can be an awesome and fast solution. Raised beds pose expense and effort, and if you don’t really need them… well. I’m for the most part against bringing in imported soil, even if it is contaminated. There are ways to clean, sequester, and grow so that the risk is minimized. If the problem is taken in holistically, solutions are self-serving and easy to implement.

References:

Angima, S.D. and Sullivan, Evaluating and Reducing Lead Hazard in Gardens and Landscapes. D.M, EC 1616-E March 2008.

Rosen, Carl J. Lead in the Home Garden and Urban Soil Environment. Extension Research Soil Scientist Department of Soil, Water and Climate