Heavy Metals in Soil

by DIG-IT

by Dr. Joseph Heckman

Every gardener knows the joy of working with soil on their own plot of land. Most soils are clean and wholesome in their natural state and pleasant to touch and work with by hand. Healthy soils carry out vital functions such as providing a home for living organisms, recycling nutrients, filtering water, and producing nutritious food.

Unfortunately, just as air and water resources can become polluted so can soils. Pollution with toxic (heavy) metals, such as lead, arsenic, or cadmium, is a serious concern because once these elements enter the soil they can persist for a long time. Gardening with these soils is still possible, but precautions are needed to protect family members.

Soils with elevated levels of heavy metals can have negative effects on human health. Major pathways of exposure include plant uptake, inhalation of soil dust, or direct ingestion. The developing brain and IQ of children are especially vulnerable to toxic metals, especially lead. The mouthing behavior of children and playing on the floor increases their risk of exposure to contaminated soil.

Heavy metals can move into the food chain. Plant parts vary in their tendency to uptake and concentrate them. Leafy greens like lettuce or Swiss chard and root vegetables like carrot or potato generally uptake heavy metals more than fruit structures like a tomato.

Causes of Contamination
Knowing how soils become contaminated with heavy metals is important so that gardeners can prevent its pollution. Knowing the history of land areas can help predict which soils may already be contaminated.

Some common causes of lead contamination of soils in the past were exhaust from vehicles using leaded gasoline, lead-arsenic pesticides used on farmland, and lead bearing paint chips near buildings. Thus, soils near busy highways, old orchard sites, and near the walls of older painted buildings are more likely to be contaminated with lead now. A soil test for lead should be performed to determine if it is contaminated with heavy metals.

Causes of heavy metal pollution of soils today include release of metals from pressure treated lumber, certain fertilizers, and some soil amendments. Although pressure treated lumber products are being phased out, many existing structures continue to release heavy metals such as chromium and arsenic into soils.

Fertilizer Materials
While most commercial fertilizers are not a significant source of heavy metals, there are a few products of concern. Nitrogen and potassium fertilizer materials are generally free of significant toxic content; however, phosphate fertilizer materials often contain cadmium, the amount present depending on the source of the rock phosphate ore.

Micronutrient fertilizers are sometimes derived from industrial byproducts that may be contaminated with cadmium, lead, arsenic, or other toxic elements. Both organic and chemical fertilizers are at risk. Generally, the most serious cause of contamination results from attempts by industry to recycle industrial byproducts into the fertilizer market. A recent survey of zinc fertilizers for sale in New Jersey found one product of concern because it contained 83 ppm (parts per million) cadmium.

No Federal Standards
While there are standards to regulate the presence of heavy metals in sewage biosolids products applied to farmland, no federal standards for heavy metals in fertilizers exist. Regulation of fertilizers is the responsibility of the individual states. In the state of Washington, all commercial fertilizer products must be tested for nine heavy metals and the results are posted on the web. This web site can be used by gardeners in other states to look up heavy metal concentrations in fertilizers.

Some amount of heavy metals will be present in most fertilizer products and soil amendments, whether man made or derived from natural sources. Gardeners should select products with the lowest concentrations.

In the home gardening market, a commercial product of concern is a brand named “Ironite," which contains as much as 3600 ppm arsenic and 2900 ppm lead. The high levels of heavy metals and their solubility suggests that this product should be classified as hazardous waste. In August 2005, the New Jersey Department of Agriculture issued a “stop sale" on Ironite 1-0-0, but other Ironite products remain on the market. Ironite 1-0-0 is still for sale in many garden centers across the United States.

How To Protect Yourself
All gardeners should have a soil fertility test performed to determine the nutrient needs of their soil. Knowing what nutrients are already at a sufficient fertility level allows them to avoid applying unnecessary fertilizer. About 75% of garden soils in the Mid-Atlantic region have above optimum fertility levels for phosphorus, which may contain cadmium. Thus, avoiding unnecessary phosphorus fertilizer application to your garden may prevent the addition of more cadmium.

On soils already contaminated with heavy metals there are several precautions recommended to minimize human exposure. If the soil is highly contaminated with lead, it should not be used for growing food crops but may be used for ornamentals. If moderately contaminated, it is not advisable to grow leafy vegetables and root crops, while it may be acceptable to grow fruit crops. All produce should be carefully washed to remove dust or soil.

Wear gloves when working contaminated soils and remove gloves and shoes at the doorway to avoid tracking soils indoors where children play on the floor. Cover children's outdoor play areas with a dense turf sod to prevent contact with contaminated soil. Cover high foot traffic areas, such as under swing sets where it is not possible to maintain a dense turf, with a thick layer of wood chips. If the contaminated soil is to be removed and replaced with clean soil, have the new soil tested for heavy metals in advance.

While it is hoped that this article helps to create awareness of how to protect public health from the presence of toxic heavy metals, it is not intended to take the joy away from working with soils in the garden.

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published May 29, 2006