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Toxic Heavy Metals in Farm Soil
Publish Date:Summer 2010
VolNo:Vol. V No. 3
In 2008, Dr. Dan Sullivan and I published an OSU publication about heavy metals in garden and landscape soils due to many questions of concern about this topic. Here I will summarize the main points in this publication as it relates to farming. A heavy metal can be defined as a chemical element with a specific gravity that is at least five times that of water (considered one (1) at 39 oF for water). Specific gravity is a measure of density of a given amount of a solid substance when it is compared to an equal amount of water. Examples of heavy metals that fall into this category include arsenic, cadmium, iron, lead, chromium, copper, zinc, nickel, and mercury.
Not all heavy metals are toxic to humans. In small quantities, metals such as iron, copper, manganese, and zinc are essential for good health. Heavy metals such as lead are also good industrial ingredients e.g. used in car batteries. However, these heavy metals become toxic when they do not get metabolized by the body and end up accumulating in the soft tissues. Ingestion is the most common route of exposure to heavy metals. In plants, uptake of heavy metals depends on the plant species and bio-availability of the metal in the soils. Since most of the ingestion of heavy metals occurs from consumption of plants, then addressing how plants acquire heavy metals can aid in controlling heavy metal toxicity.
If you happen to ingest heavy metals, that alone is not enough to cause toxicity. In laboratory animals, absorption of toxic metals may occur as a result of chronic deficiencies of calcium and magnesium in the body and in other cases, excess levels of aluminum mobilizes calcium and heavy metals to move from bones to the central neural tissue. Of the many heavy metals of concern, lead and arsenic have been found to be higher than federally set levels in most soils studied (note that all heavy metals exist naturally in the soils largely in complex forms with other minerals – see Table 1, showing average abundance of total heavy metals in the earth’s crust and in typical soils).
Arsenic is the most common cause of acute heavy metal poisoning in adults (but the source is not from soils). Arsenic is released into the environment by the smelting process of copper, zinc, and lead and from manufacture of chemicals and glass. Lead on the other hand is the leading cause of heavy metals poisoning with major source coming from soils. Excess levels of lead in soils greater than 400 ppm result from prior use of lead paint around houses, lead-arsenate sprays for pest control during 1910-1950s, use of leaded gasoline (up to 1996 in Oregon), locations close to former smelters & tailings from metal ore mines, and proximity to fossil fuel-fired electrical plants. Therefore the culprit to look for when looking at heavy metals in soils is lead toxicity.
A local study on lead contaminated soils was carried out in Multnomah County in 2001 around homes built before 1930. They found that in bare soil play areas lead concentrations were often above the EPA limit of 400 ppm. The main reason why lead is found in close proximity to the loading point is because lead is held tightly on surfaces of very fine clay and organic matter particles and therefore accumulates in the top 1-2 inches of soil unless disturbed by excavation and tillage. Therefore if you think you might have lead contamination in your farm or home, the best procedure to follow is to collect soil samples and have them analyzed for lead content. OSU publications EM 8677 and EC 628 provide laboratories that can do heavy metal soil testing and how to sample soil for home gardens and small acreages for soil testing, respectively. If you are testing for farming purposes, always take soil samples to tillable depth depending on how deep your current equipment disturbs the soil.
Once you receive you soil test back, use Table 2 to interpret what you need to do for your specific soil in question. Soil tests showing less than 50 ppm lead, generally show no lead contamination while those showing greater than 1,200 ppm lead, are not recommend for any gardening practices, rather they should be mulched heavily and planted to perennial plants that do not need harvesting of food for human consumption. Other abatements in soils testing high in lead are to use container or raised bed gardening with clean soils and installing a barrier (e.g. geotextile fiber) between good soils and contaminated soil below.
If your soil tests for lead higher than 50 ppm, you might need to use some soil amendments to reduce lead toxicity. These include:
1. Maintaining a neutral soil pH above 6.5. Lead up take by plants is reduced when pH is above 6.5.
2. Add phosphorus when soil tests indicate a need. Phosphorus reacts with lead to form insoluble compounds, therefore reducing toxicity.
3. Add organic matter, (OM) which in turn binds lead and makes it less soluble in soil water. When testing OM, soil pH soil should be maintained above 6.5 to reduce uptake by plants.
4. What about lead in water? If you still have leaded water pipes, you should test your
water for lead content. It is recommended to replace these pipes or keep water off edible plants. What about lead in fertilizers? Most fertilizer and soil amendment products do not significantly increase health risks. Fertilizer manufacturers are required to test products for lead, and tell Oregon Department of Agriculture or Washington Department of Agriculture. Check online for Oregon http://oregon.gov/ODA/PEST/fertilizer.shtml and online for Washington, http://agr.wa.gov/PestFert/Fertilizers/Metals.htm. Compost makers and distributors are not considered fertilizers and therefore are not required to provide lead analysis data to regulatory agencies. However, many composters determine lead levels in their products and will supply the analytical information to consumers upon request. Check out the Winter 2010 issue of the Small Farm Newsletter for arsenic related toxicity from treated lumber.
It is important to note that plants do not absorb or accumulate substantial amounts of lead. Lead does not readily accumulate in the fruiting part of vegetables and fruit crops (e.g. corn, beans, squash, tomatoes, strawberries, and apples). Since lead is tightly bound to clay particles, higher concentrations of lead will therefore be on surfaces of leafy vegetables from lead laden dust (e.g. brassicas), and on surfaces of root crops (e.g. carrots and potatoes) if soils are contaminated. Actually, there is more concern about lead contamination from external lead on unwashed produce than from actual uptake by plants. This raises the need for everyone to always wash their produce before eating/cooking and places a big responsibility on growers to always wash their leafy vegetables before marketing them since lead laden dust can blow from distant places. Remember that soil contaminated with lead looks and smells like normal soil. Lead does not biodegrade since it has a half-life of about 53,000 years.