Lead is a naturally occurring heavy metal. It is however, toxic to plants and humans.
Lead in Soil
Many houses today were once painted with paints that contained lead ‑ unless the paint was removed, that paint will still be there under layers of newer paint. Lead is also used in a number of alloys, flashing, solder and some batteries. Lead is released during combustion of fossil fuels and many manufacturing process produce or release lead. Soil may become contaminated with lead if it is exposed to any of these substances or processes or if water runoff from such substances infiltrates the soil. Mining activity may also lead to lead contamination. Note that soil within a metre of houses and garages is often contaminated with paint chips that contain lead. Lead in soil does not biodegrade nor does it leach easily so even structures that have had all lead paint removed may have lead contaminated soil in this area as a result of previous exposure.
Lead in Humans
Lead is particularly dangerous for children. Even low levels of lead in the blood of a child can irreversibly affect IQ and attention. Lead can cross the placenta so pregnant women who are exposed to lead can pass that lead to their unborn child where it can damage the baby's nervous system. Lead exposure can also cause miscarriage, stillbirths and infertility in both men and women. Soil clings to fingers, toys and other objects that children normally put into their mouths. If that soil is contaminated with lead, children will absorb the lead. Lead contaminated soil should thus be covered to prevent access ‑ mulch, grass and concrete are all effective barriers. If contaminated soil cannot be covered, children's access should be restricted.
Lead is not just dangerous for children however. It can be stored in bones, blood and a variety of tissues (regardless of a person's age at exposure) whereby it can produce a wide range of effects. Large, short term doses of lead can cause, in children and adults, abdominal pain, constipation, fatigue, weakness, headaches, irritability, loss of appetite, memory loss, pain or tingling in the hands and feet, anaemia, kidney and brain damage and even death. Long term exposure can cause similar symptoms along with depression and reduced fertility. Lead is also a probable carcinogen. Adults are less likely to ingest contaminated soil but they may inhale lead or ingest lead contaminated food. Anyone concerned about lead exposure can request blood tests for lead.
Lead in Food and Plants
Food can become contaminated with lead if it is grown in lead contaminated soil. Lead can remain in the soil for up to 2000 years and it is toxic to plants in a variety of ways. It strongly inhibits seed germination, root and plant growth, seedling development, transpiration and chlorophyll production and it negatively affects water and protein content. Lead taken up by plants concentrates mostly in the roots of the plant but its affects are noticeable throughout the plant, especially since it also impedes the uptake of essential nutrients such as magnesium and iron. Plants can act to reduce their uptake of lead and by adding calcium and phosphorus to the soil, you can also reduce the amount of lead taken up by plants.
In addition to direct toxic effects on plants, lead can indirectly affect a variety of plants. At concentrations of 10000‑40000 mg/kg, lead can kill soil bacteria and fungi. This in turn negatively affects the growth of many plants and inhibits organic matter decomposition (which prevents nutrient recycling and thus plant growth). At concentrations between 500 and 1000 mg/kg, lead can bias the types of microorganisms present. This can affect the types of plants that grow best in a particular area.
How Much Lead is Safe?
Depending on where you live, there may be a number of guidelines for soil lead levels that are relevant to you. If you grow edibles, maximum recommended levels for agricultural soil will be relevant. Guidelines for soil in residential areas may also be available and this is particularly useful for those who don't garden but may be exposed to lead in the soil in other ways (anyone can breath in dust and children frequently hands and toys in their mouths). If you garden at a community garden, you might find guidelines for lead in the soil surrounding public infrastructure such as schools and child care centres. Note that some of these guidelines may not specify unsafe levels; they may instead specify levels that should instigate further investigation. As such, ensure you read your guidelines carefully. I can't provide values for every country here (note that many soil testing companies will provide information about ideal or acceptable levels when they give you your results) but the following values are used in Australia and might be useful to you as a guide:
- The maximum recommended soil lead value for agricultural soil is 35 mg of lead per kg of soil.
- If soil lead levels are over 300 mg/kg the Australian environmental protection measure for residential, primary school and child care facilities recommends further investigation.
Environmental protection agencies in California, Canada, Norway and the United States publish guidelines for lead in soil as well: 80 mg/kg, 140 mg/kg, 90 mg/kg and 400 mg/kg respectively.
Dealing with Excess Lead in the Soil
If you have concerns about relatively minor contamination of soil with lead, there are some strategies you can use to reduce the chance of ingesting and inhaling soil. For instance, you can ensure all soil is either covered with plants, or if that's not possible, you could use mulch or even pave the area (what ever works best for the intended use of your soil). This is especially useful for preventing children from eating soil as they often like to play in dirt. You should also ensure that everyone in your family (or all members of your community garden) washes their hands thoroughly after gardening or touching the soil for any other reason.
Also, when it comes to growing edible plants in an area with contaminated soil it is important to note that not all plants absorb lead to the same extent. Leafy greens such as lettuce and silverbeet and root crops such as beetroot and carrots uptake the most lead. Bulbous and tuberous plants such as potatoes, onions and garlic uptake the next highest amount of lead. Both these classes of produce should be grown in raised beds containing clean soil if you have concerns about your ground soil (make sure you test any purchased soil or ask to see an analysis of it before you buy). The beds need to be deep enough to cover the majority of root growth (to ensure significant root mass doesn't penetrate the contaminated soil). Deep rooted green manures should not be used to mulch the soil as these may concentrate lead and then contaminate the clean soil. Corn, cauliflower, asparagus, celery and berries uptake only low levels of lead. Most other fruiting plants such as beans, tomatoes, melons and fruit trees uptake very low levels of lead. If your ground soil doesn't contain too much lead, it may be safe to grow these plants in the ground. If concerns remain about the safety of this produce, tissue tests can be performed on the fruit to determine whether any lead (or unsafe levels of lead) is present in the fruit.
If a soil test reveals very high levels of lead, I would recommend that you seek expert advice on your situation. You may need to replace the soil in your garden in extreme circumstances.
Bioremediation of Soil Contaminated with Lead
Other than being careful not to exposure yourself to lead, you may also be able to deal with lead in soil by removing it. Some plants absorb a lot of lead and it might be possible to use them to assist you. Vetiver grass and Thlaspi caerulescens both accumulate lead and distribute it to their foliage. Such plants could be grown in contaminated soil and the foliage regularly pruned and destroyed off site in an attempt to remove the lead. In the long term, this kind of bioremediation may be useful but it is a slow process and it will not be successful if the land can be re‑contaminated by water leached from uphill contaminated soil or from an active mine or smelter.
- [Australian] National Environment Protection (Assessment of Site Contamination) Measure 1999 Schedule B1 (F2013C00288, measures as amended, taking into account amendments up to National Environment Protection (Assessment of Site Contamination) Amendment Measure 2013 (No. 1)) http://www.comlaw.gov.au/Details/F2013C00288/Html/Volume_2