Learn about safe levels of lead in soil, testing soil for lead, lead remediation, and how to take a few simple precautions to take when gardening in urban soil.
Using raised beds with clean soil can go a long way toward reducing the impact of lead in your garden soil.
Growers are taking urban soil by storm. Guerrilla gardeners blanket abandoned properties with wildflower seeds, and guerrilla grafters coax fruit from ornamental pear and crab apple trees. Urban farmers crop vacant lots and high-rise rooftops. Meanwhile, neighbors band together to create gardens on barren ground, cultivating community as they garden together. Individually, gardeners line their foundations with beds of greens and beans, their patios and balconies with tomatoes and peppers in pots.
Green, growing plants and vibrant flowers drastically improve upon broken concrete, crumbling brick, rusted chain-link, and shattered glass. However, there are different challenges to gardening in the city, and one of the biggest issues for urban gardeners is lead in soils.
Lead is a dull, grayish-white heavy metal, which is soft, workable, and incredibly useful both in its pure, metallic form and in many different compounds. Ancient Rome’s water flowed through lead pipes — the word plumber comes from plumbum, lead’s Latin name. Fashionable Elizabethan ladies and gentlemen wore cosmetics of red and white lead. Until relatively recently, medicines, paints, and gasoline all contained lead compounds. Stained glass windows, car batteries, and fishing weights all depend on lead.
As useful as lead may be, the stuff is also a health hazard. More than a half million children ages 1 to 5 in the United States are living with high enough levels of lead to damage their health, according to the Centers for Disease Control and Prevention (CDC). Lead poisoning affects children in many ways, including irritability, appetite loss, weight loss, muscle pain, vomiting and constipation, memory loss and learning difficulties, developmental delays, and even death in severe cases. Although adults can be affected by lead, children, especially those under 6, are at the highest risk of serious lead poisoning. Minor symptoms of lead poisoning will go away over time if the lead source is removed, but severe damage is permanent. There is no known safe threshold of lead in the blood for children; any lead at all in the body is a concern.
What does this have to do with urban gardening? Lead contaminates many city soils. All soil contains a small amount of lead, measured in parts per million (ppm). Normal soils can contain 10 to 50 ppm of lead. Many governments and researchers have established guidelines for lead in soil. Some give a threshold as low as 100 ppm, and some set it at 300 or 400 ppm or more; overall, acceptable levels have continuously fallen since 1970. But lead levels in urban soil may exceed 200 ppm and can be as high as 10,000 ppm, particularly where lead-based paint has been in use.
Soil lead comes from many sources. Many buildings bear deteriorating coats of lead-based paints. Leaded gas exhaust lingers in soils near gas stations and busy streets. Discarded car batteries and other automotive wastes contaminate areas surrounding garages, junkyards, and manufacturing facilities. Lead also leaks into soil from stained glass windows, solder in discarded electronics, and even some aging brass fixtures.
The most common way people get lead poisoning is not from eating contaminated crops (as vegetables don’t uptake much lead) but rather from eating or breathing dust or soil containing lead, or from absorbing lead through their skin, especially through cuts or breaks. Children under 6 are at the highest risk because they often play in or eat dirt, and their bodies absorb more lead than adults’ bodies do. One estimate suggests a child can absorb at least 40 percent of the lead they take in. Therefore, safe gardening around suspected high-lead soil involves avoiding breathing in or ingesting dust and dirt and being careful when working with the soil itself.
That’s not to say all urban soils contain that much lead, only that your soil could be contaminated. While testing soil for lead is possible, that proposition can be complicated. Inexpensive lead test kits are available, but most test only for the presence of lead and its compounds. They don’t tell you how much lead is there, or the forms of lead you have.
To determine how much lead is in your soil and in what form, simply send a series of samples to an accredited testing laboratory through your local extension office. While laboratory testing can be expensive, usually at least $60 (some local organizations might offer testing for a lower price or free of charge), it can be money well spent.
Lead remediation is not usually practical or economical. Soil cleanup involves professionals stripping all of the topsoil and hauling it away to a landfill. The area then must be backfilled with soil brought in from a clean site. The process can be extremely expensive and is often underwritten by the United States Environmental Protection Agency or your state’s agency. Check for available help in your area if you have high-lead soil and wish to mitigate it.
Because plants don’t absorb much lead, your vegetables will be safe to eat. Even so, the CDC and other groups recommend taking different precautions for three groups of vegetable crops — fruits, leaves, and roots — according to their lead risk levels.
Fruiting bodies, such as tomatoes, peppers, and raspberries, are the least likely to contain lead; the risk is virtually nonexistent. Leafy crops tend to contain higher concentrations, but most of the risk of ingesting lead is from the soil on them. Wash your lettuce and cabbages carefully and discard the older outer leaves. Because root crops form in the soil, they carry the greatest risk of holding lead in their skins and in soil embedded in the skin. Wash your potatoes and carrots carefully with water containing vinegar (in a 1 percent solution) or edible soap, and discard the skins when you prepare your vegetables for cooking.
Besides these crop precautions, you can garden safely if you change the way you work the soil. For starters, choose your garden site with care. Avoid planting at the base of a building that may have been painted before 1978 (the year lead-based paints were banned in the United States). Stay clear of street edges, areas near highways, or places where car batteries, car parts, or wrecked cars have been stockpiled.
Garden on your ground, not in it. Build raised beds and fill them with clean soil and compost. You don’t need to build bottoms on them, although you can if you want to. Keep your planting soil covered with a mulch of dried grass clippings, shredded leaves, or straw at all times. Similarly, cover your walkways with wood chips or grow grass on them. Keeping your soil covered, both in your growing beds and around them, minimizes the risk of water from a hose or rainwater splashing soil on plants or on yourself.
If you know that your garden plot contains soil with high concentrations of lead, take appropriate precautions to avoid ingesting it while working. Leave your dirty clothes at the back door if you have crawlers or toddlers around the house. If you’re coated with soil dust, resist the urge to hug your little one until after you shower.
If mitigation is out of the question, then lock up the lead by amending the soil, using the following tips.
Keep your soil pH neutral or slightly alkaline (between 6.5 to 7.0). Lead in acidic soils is far more bioavailable than in neutral to alkaline soils. The easiest way to raise the pH is to add lime according to the recommendation of your soil test.
Amend your soil with lots of compost. High levels of organic material help bind up toxic metals. There’s no such thing as too much compost, either. Many potting mixes, for example, contain from a third to a half organic matter.
Supplement your soil with rock phosphate at the rate of 11 pounds per 100 square feet or with fish bone meal, which also contains phosphate. Phosphate binds to organic lead to form non-bioavailable pyromorphite. The lead will still remain in your soil, and will still register in lead tests, but your body won’t absorb it.
Plants don’t absorb much lead in soil, so your fruits and vegetables will be safe to eat. Most of the risk of consuming lead from what you grow actually comes from ingesting soil or dust on produce. These tips can lower your risk of taking in lead:
• Plant more fruiting crops, such as tomatoes, squash, peppers, okra, cucumbers, peas, or beans. These plants accumulate the least amount of lead.
• Wash greens and leafy parts of plants thoroughly, preferably outside in the garden first. Remove and discard the outer leaves.
• Root crops take up the most lead (though still a negligible amount). Wash root vegetables with a water-vinegar solution; remove the skin before eating.
Andrew Weidman lives, gardens, and writes in Lebanon, Pennsylvania. A former Master Gardener, he enjoys helping fellow gardeners solve problems by discussing and researching solutions for them.
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