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Be A Better Gardener: ‘Dr. Dirt’ and the dangers of lead in soil


By Thomas Christopher

For Capital Region Independent Media

Lead levels in the soil in your garden can be higher than you think. Courtesy of Unsplash

We’ve all heard the news about lead in the drinking water in older cities such as Flint, Michigan, and the harm it has caused the local population. Did you know, though, that there’s a real chance you too have been exposed, perhaps in your own backyard?

When I spoke to Clay Robinson recently, a soil scientist and former university professor who also has been very busy educating school students under the moniker of “Dr. Dirt,” he informed me that there is a low level of lead in almost all soils.

The level depends on what he calls the “parent materials,” the rocks and minerals from which the soil was formed. Generally, though, the natural quantity of lead is very low, between 10 to 50 ppm (parts per million), which is harmless to human health.

However, in many cases, human activity has left it significantly, sometimes dangerously, higher. This is especially a concern in older communities. That beautiful, vintage home, for example, was quite likely covered with lead-based paints. Lead wasn’t banned from paint until 1978, and the older the home, the greater the risk. Whereas just 24% of homes built between 1960 and 1970 include some lead-based paint, that figure is 87% for those built before 1940. Over time that paint has flaked off or washed as dust, ending up in the soil around the foundation.

Another common source of lead pollution was auto exhaust during the leaded gas era, which ended only in 1976. These toxic fumes deposited lead in the soil next to roads, which leaves older neighborhoods, especially those near busy highways, especially at risk.

A lesser-known source of lead contamination is lead arsenate, which was a popular insecticide in apple orchards from the late 19th century up through the late 1950s. Homes built on the sites of former orchards are thus also at risk.

A problem with lead, Clay Robinson told me, is that it does not biodegrade or leach out of the soil and can persist for centuries. That’s why lead leaked into household water supplies from the solder used in plumbing systems installed as late as the 1980s also poses a risk. Although the concentration of lead from such water systems is relatively low, when sprinkled over the yard to water the lawn, it can eventually accumulate to significant levels.

Lead-contaminated soil is especially dangerous to children in part because of their size – the dose that might not harm a 200-pound adult is much more concentrated in a 20-pound child. In addition, lead has a particularly harmful impact on their developing brains and nervous systems, can stunt their growth, and cause problems in speech and learning.

Recommendations vary from state to state, but the Environmental Protection Agency has warned parents not to let their children play on bare soils with lead concentrations of 400 ppm or higher. Owners of older homes are well advised to contact local health authorities for help with testing their soil, which is a straightforward and inexpensive process.

One way of coping with lead-contaminated soil is, according to Dr. Robinson, appropriate gardening practices. If your soil proves to be in the dangerous range, 400 ppm or higher, make sure to cover all areas of bare soil, either with a couple of inches of mulch or some perennial ground cover such as turf. Locate children’s play areas and vegetable gardens away from the foundations of the house or the road’s edge, areas where lead levels are likely to be higher. If the lead contamination exceeds 400 ppm where you want to grow vegetables, install raised beds filled with clean soil.

Mixing compost into the soil also has a beneficial effect, as it will dilute the soil, reducing the concentration of lead. Compost also typically contains compounds that bond with any lead found in the soil, making it less available for uptake by plant roots. Fruit harvested from trees or bushes or even tomato vines is unlikely to contain significant amounts of lead. Root crops such as potatoes or radishes are more problematic, as are low-growing leafy vegetables such as lettuces which are likely to come into the kitchen with soil on them.

If soil tests reveal the presence of lead in your soil, contact the local health authorities for their recommendations.

To hear the rest of my conversation with Dr. Clay Robinson, and for more information about gardening on soils contaminated with lead, log onto the Berkshire Botanical Garden’s “Growing Greener” podcast at

Be-a-Better-Gardener is a community service of Berkshire Botanical Garden, located in Stockbridge, Mass. Its mission, to provide knowledge of gardening and the environment through a diverse range of classes and programs, informs and inspires thousands of students and visitors each year. Thomas Christopher is a volunteer at Berkshire Botanical Garden and is the author or co-author of more than a dozen books, including Nature into Art and The Gardens of Wave Hill (Timber Press, 2019). He is the 2021 Garden Club of America’s National Medalist for Literature, a distinction reserved to recognize those who have left a profound and lasting impact on issues that are most important to the GCA. Christopher’s companion broadcast to this column, Growing Greener, streams on, Pacifica Radio and NPR and is available at

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