Lead occurs naturally in soils, typically at concentrations that range from 10 to 50 ppm (parts of lead per million parts of soil which is equivalent to mg/kg milligrams of lead per kilogram of soil). Lead is not a nutrient that is essential for plant growth. Because of the widespread use of leaded paint before the mid-1970’s and leaded gasoline before the mid-1980’s, as well as contamination from various industrial sources, urban soils often have lead concentrations much higher than the normal background levels. These concentrations frequently range from 10 ppm to as high as 10,000 ppm at the base of a home painted with lead-based paint. Lead does not biodegrade, or disappear over time, but remains in the soil for thousands of years.
Soil lead is held tightly on the surfaces of very fine clay and organic matter particles. Therefore, when lead is added to the soil surface, it tends to accumulate in the upper 1 to 2 inches of soil unless the soil has been disturbed by activities such as excavation for building or tillage for landscaping and gardening. Added lead also will become most concentrated in very fine soil particles, which tend to stick to skin and clothing and form airborne soil dust.
Not all of the lead in the soil is available to plants (or to the human body, should the soil be eaten). The availability of soil lead depends on how tightly it is held by soil particles and on its solubility (how much of it will dissolve in water). At low soil pH (pH <5.0, acidic conditions) lead is held less tightly and is more soluble. At near neutral or higher soil pH (pH >6.5, neutral to basic conditions) soil lead is held more strongly, and its solubility is very low. Lead is held very tightly by soil organic matter, so as organic matter increases, lead availability decreases. Some lead added to soil may combine with other soil elements to form lead-containing elements. One such mineral that has extremely low solubility is lead phosphate (pyromorphite). Formation of this element is favored by high soil pH and high levels of lead and phosphate, conditions that would occur with the application of ground agricultural limestone and large amounts of phosphate fertilizer to a lead-contaminated soil. The most serious source of exposure to soil lead is through direct ingestion (eating) of contaminated soil or dust. In general, plants do not absorb or accumulate lead. However, it is possible for some lead to be taken up into the plant if the soil tests high in lead. Studies have shown that lead does not readily accumulate in the fruiting parts of vegetables and fruit crops (ie corn, beans, squash, tomatoes, etc). High concentrations are more likely to be found in leafy vegetables and on the surface of root crops.
There is more concern about lead contamination from external lead on unwashed produce than from actual uptake by the plant itself. If you garden is close to a busy street or highways, remove the outer leaves of leafy crops, peel all root crops, and thoroughly was the remaining produce in water containing vinegar (1%) or soap (.5%).
Most laboratories will measure the total sorbed lead (using EPA methodology). This is not a true measure of the amount of lead that may be soluble to the growing plants, however because of limited testing using other extractants there is little to no data that will have correlated useful results. Laboratory test results normally will report soil lead concentrations in terms of µg/g (micrograms per gram), mg/kg or ppm (parts per million). These are all equivalent units of measure. The table below indicated the degree of lead contamination indicated by various soil lead concentrations.
|Degree of lead contamination indicated by various soil lead concentrations|
|Soil Lead Level (Total Sorbed Lead Test)
ppm or mg/kg
|Level of Lead Contamination|
|<150||None to very low|
Lead in Residential Soils: Sources, Tests and Reducing Exposure. Pennsylvania State University. 1999
Lead in the Home Garden and Urban Soil Environment. University of Minnesota Extension. Bulletin FO-02543. 2002