Lead in Urban Gardens

Urban gardeners are at risk of lead poisoning due to lead contamination in soil. I recently had my soil tested, and the lab estimated that my soil contains lead at the rate of about about 550 ppm (parts-per-million). To make this concrete, a garden plot on my land that is just 10 feet long by 10 feet wide may contain over 3 pounds of lead. (My calculation is based on 110 lbs per cubic foot as the average density of soil, and considers only the top six inches of soil, which constitute the bulk of the soil tilled for gardening. Note also that ppm is indeed a ratio of masses.)

How much lead is too much? As long as children are not playing in the soil, various sources recommend that gardening is "safe" with lead concentrations under 300 ppm. The soil testing service that I used puts 550 ppm on the low end of its "medium" risk category, and gives some recommendations for how to garden despite the lead. The recommendations fall into three categories:

1. Reduce/dilute/mask lead in the soil, for example by maintaining a high pH above 6.5, adding clean soil, or adding organic matter. My soil pH is 6.7, and I'm adding lots of organic matter.
   
2. Avoid eating dirt. Wash veggies, peel root crops, and keep soil from blowing into your mouth by mulching and/or keeping the garden well-watered. My garden is mulched heavily.
   
3. Choose plants that uptake less lead.
   

Regarding point #3, recommendations become more nuanced: As a general rule, plants uptake so little lead that the main risk is in eating small amounts of dirt on produce that is not well-washed. This study, for example, affirms that very little lead gets into plants and that the biggest risk factors for lead uptake are having a low pH and a lack of organic matter in the soil. However, we must be careful not to over-generalize these positive results. Not all plants refuse to uptake lead. For example, sunflowers and morning glories are known to uptake so much lead that one proposed way of cleaning lead out of soil is to grow these plants in the soil and dispose of the contaminated plants in a landfill.

In conclusion, I find that most vegetables may be safely grown in my soil after taking the precautions suggested above. However, certain kinds of plants might not be safe. I would love to find a more complete analysis of the rates of lead uptake for different kinds of plants (even in the case of sunflowers, I still don't know if the lead makes it into the sunflower seeds or just stays in the roots and leaves).