Garden Word of the Day
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Lead Contaminated Soil
Eating lead-based paint is a bad idea. You don’t want it in your garden soil, either. But how do you know if it is there and what can you do about it if it is?
Lead is a soft, heavy metal that has been used to make paint, pipes, bullets, batteries, pewter, leaded glass, and in gasoline. Lead is still used to make high voltage power cables, lead-acid batteries, solder, and wicks for cheap tea lights.
Damage caused by lead
Lead is a neurotoxin that accumulates in bones and soft tissues, causing brain, kidney, liver, reproductive system, digestive system, and nervous system damage. It also reduces intellect and is believed by some to be associated with increased rates of crime and violence. Many historians attribute the Fall of Rome to the fact that their pipes many of their food containers were lined with lead.
Most countries have banned the use of lead in products that might cause exposure, but not all. Countries such as China, India, and Indonesia still use lead in many products which is why it is important to verify that planting containers, coffee cups, and other food-related items are safe to use. Red and yellow ceramics are the most likely to contain lead.
Where does lead come from?
Lead was added to gasoline as an anti-knock agent in 1921. By the 1970’s, over 75% of the U.S. population had elevated lead levels in their blood. That number dropped to just over 2% twenty years later, after lead was removed from gasoline. All those fumes, spewing forth for over 50 years, contained lead. That lead settled on roads, yards, gardens, and fields. Rain and irrigation water leached some of that lead into rivers, lakes, and oceans.
Lead can also find its way into your garden soil by sanding, chipping, or sandblasting lead-based paint from older buildings, or when old lead pipes, roof flashing, or lead-batteries are allowed to sit on the ground and break down.
How much lead is in your soil?
Lead occurs naturally in the soil. While there is no safe level of exposure, natural concentrations range from 10 to 30 parts per million (ppm). Areas where leaded gasoline was still in use in 2014 were found to have lead levels of 100 to 1,000ppm. Homes painted with lead-based paint that were located near high traffic roads could have had lead levels as high as 3,000ppm.
Until you get your soil tested, there is no way of knowing how much lead is there. I use the UMass Extension Soil Testing Lab. My soil test lists anything below 22ppm as acceptable. My results were 2.1ppm in 2015 and 2.0 in 2019. Lab-based soil tests are inexpensive and they provide valuable information both for your plants’ health and your family’s health.
How to manage lead contaminated soil
If your soil is contaminated, your biggest health risk is breathing in dust that contains lead. One of the easiest ways to reduce the risk of inhaling lead dust is to grow cover crops or mulch over the area. You can also cover the contaminated area with 4” to 6” of clean soil, to reduce the risk of dust.
You can also use certain amendments that bind to the lead, making it less likely to be absorbed by plants or released into the air via dust. Lead will bind to organic matter, such as aged compost, but this treatment needs to be repeated as the compost breaks down. Depending on your soil’s phosphorus levels, the addition of more phosphorus may improve the binding action. Too much phosphorus is bad for plants, so check your soil test results before using this method.
As soil pH increases, becoming more alkaline, plants absorb more lead. Maintaining a soil pH of 6.5 to 7.5 is ideal, both for plant health and to reduce lead absorption.
Can I grow edible plants in lead contaminated soil?
Plants can grow in soil with lead levels as high as 500ppm. Lead moves very slowly through plants, staying mostly in the roots. According to the University of California Department of Agriculture and Natural Resources, “Fruits such as tomatoes, peppers, melons, okra, apples, and oranges and seeds such as corn, peas, and beans generally have the lowest lead concentrations and are the safest portions of the respective plants to eat [when] grown in lead-contaminated soils.”
Crops that should never be grown in lead contaminated soils include leafy greens, such as chard and collards, and root vegetables, such as beets, carrots, potatoes, and turnips. These crops are better grown in raised beds with clean potting soil.
Also, if you know your soil contains high levels of lead, be sure to wash all produce thoroughly to remove any lead dust that may be present.
Finally, pencil leads have never been made from lead. They are made with graphite.
Now you know.
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