According to Carl Rosen at the University of Minnesota, "Background concentrations of lead that occur naturally in surface agricultural soils in the United States average 10 parts per million (ppm) with a range of 7 to 20 ppm (parts per million)." The soil of the Brandeis Garden, not including added compost that is in the raised beds, has a concentration of 92 ppm. This is higher than the background "normal" level, but considering that most of our plants are growing in the compost of the raised beds, it is very low.
Also, fruit (beans, apples, tomatoes, pumpkins...) does not take up lead from the soil. It is more likely to find lead accumulating in the leafy parts of plants (lettuce, herbs...).
Therefore, Rosen continues, "Since plants do not take up large quantities of soil lead, the lead levels in soil considered safe for plants will be much higher than soil lead levels where eating of soil is a concern. Generally, it has been considered safe to use garden produce grown in soils with total lead levels less than 300 ppm."
So, in conclusion, we really don't need to worry about lead levels in our soil! Hooray!
***All quotations taken from "Lead in the Home Garden and Urban Soil Environment" by Carl J. Rosen, University of Minnesota. http://www.extension.umn.edu/distribution/horticulture/DG2543.html
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