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Could your ground be contaminated with lead?

Background lead in soil is 10 to 50 ppm; above 400 ppm, the EPA says not to grow vegetables in it. How to test, and how deep clean soil must be in a raised bed above.

The short answer

Test before you grow food. Typical background lead in soil is 10 to 50 parts per million, according to the University of Maryland Extension. Above 400 ppm, the EPA recommends not growing vegetables in the soil itself. A raised bed filled with clean soil can still work above contaminated ground, with enough depth and the right crops.

Get a soil test

The University of Maryland Extension says to have the lab test the soil for lead if you plan to grow food in a raised bed.

What the levels mean

Lead in soil and what the guidance says
Soil leadGuidanceSource
10 to 50 ppmTypical background levelUniversity of Maryland Extension
Above 400 ppmAvoid growing vegetables in the soilEPA, as cited by the University of Maryland Extension
401 to 1,000 ppmLeafy greens and root crops only in containers or in raised beds of clean topsoil and compost at least 8 inches deepUniversity of Maryland Extension
Above 400 ppmOnly shallow-rooted plants in raised beds with at least 16 inches of clean soil on top of garden fabric, or containersEPA

The two bodies set the depth differently for the same range: Maryland’s 8 inches applies to 401 to 1,000 ppm; the EPA’s 16 inches applies to anything above 400 ppm. Where they differ, the deeper figure is the cautious one.

Building over contaminated ground

Fill with clean, purchased soil, and ask the supplier for its own test results. NC State Extension describes sealing the bottom of the bed with an impermeable barrier so roots do not reach the soil below, with drainage holes drilled near the bottom. The EPA’s guidance puts garden fabric under the clean soil. The bottom-of-the-bed guide compares the materials, and the soil calculator gives the clean soil you need for 16 inches.

References