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How to Protect Your Urban Farm from Soil Contaminants

A large concern with your urban farming site is the contamination from past use of your site. Old industrial cities, in particular, may have soils contaminated with a variety of chemicals.

Contaminated soil is an especially important issue if you’re growing edible crops or have young children or grandchildren who may ingest the soil. Many of the chemicals enter the body through ingestion, but some risk of exposure through breathing in chemical dust is possible as well.

Common contaminants to be aware of

The chemical lists look scary, but don’t let them deter you from growing food and flowers in the city. The reality is that even though most urban soils have some detectable contaminants, the levels in most cases may not be high enough to warrant action on your part.

Source Previous Site Usage Specific Contaminants
Paint (before 1978) Old residual buildings; mining, leather, tanning, landfill operations; aircraft component making Lead
High-traffic areas Next to heavily trafficked highways and roadways; near roadways built before leaded fuel was phased out Lead, zinc, polycyclic aromatic hydrocarbons (PAH)
Treated lumber Lumber treatment facilities Arsenic, copper, chromium
Burning waste Landfill operations PAHs, dioxins
Contaminated manure Agriculture Copper, zinc (from copper and zinc salts added to animal feed)
Coal ash Coal-fired power plants; landfills Molybdenum, sulfur
Sewage sludge Sewage treatment plants; agriculture Cadmium, copper, zinc, lead, persistent bioaccumulative toxins (PBTs)
Petroleum spills Gas stations; residential/commercial/industrial uses (anywhere an above-ground or underground storage tank has been located) PAHs, benzene, toluene, xylene, ethyl benzene
Pesticides Widespread pesticide use, such as in orchards; pesticide formulation, packaging, and shipping Lead, arsenic, mercury chlordane, and chlorinated pesticides
Commercial/industrial site usage Factories manufacturing building supplies, chemicals, and other potentially hazardous materials PAHs, petroleum products, solvents, lead, other heavy metals (such as arsenic, chromium, cadmium, mercury, zinc)
Dry cleaners Service businesses using chemicals for clothes cleaning Stoddard solvents, tetrachloroethene
Metal-finishing operations Factories using hazardous chemicals during painting and finishing work Metals and cyanides
*Adapted from the EPA

To determine whether the toxin levels are high enough to warrant action, you simply have to hire a professional to do a soil test to check for heavy metals or other pollutants. You can work with the cooperative extension service in your state or a private soil lab.

These tests can get pricey if you’re testing for a number of possible contaminants, so narrowing the list to the most likely culprits is a good idea. Doing a soil test on healthy soil every three to four years is fine.

You may be able to find city, state, or federal programs to help offset the cost of these tests. Check online or with your local health department.

The soil test results give you the soil’s contaminant levels and the safe standard for each contaminant. You can use this information to determine what action (if any) that you take.

Tips to avoid contamination in your soon-to-be garden

Creating a little green patch in the city can be tough when you’re worried about contamination from chemicals and other toxins. However, don’t let it stop you from trying. Here are some tips to avoid contamination:

  • Locate your garden away from building foundations. Lead-based paint chips are most likely found close to buildings where painting occurred. This advice is especially important if the building is old enough to have had lead-based paint used on it. (Lead was banned as a paint additive in 1978.)

  • Build raised beds, lay a sheet of landscape fabric on the bottom, and bring in fresh soil and compost to fill them. However, don’t use chemically treated lumber in your raised bed construction; otherwise you risk introducing new toxins to your soil.

  • Build a fence or plant a hedge as a barrier to block dust from potential sources of contamination, such as highways or railroad tracks. After all, it’s not only old chemicals and pollutants that are a concern. Present-day vehicle and industrial exhaust can also drift into your yard and contaminate the soil.

  • Mulch thickly (roughly 4 inches) around your plants to minimize their contact with the soil.

  • Teach young children not to eat dirt or unwashed vegetables. Most contaminants get into the body through ingestion. As a result, all produce should be cleaned thoroughly before storing or eating. And, of course, mud pies are a no-no.

  • Wear gloves in the garden and wash your hands thoroughly after working in the garden. Even though most contaminants are introduced through the mouth, some folks can also have skin reactions to chemicals in the soil. So gloves are important. Also, if you don’t wear gloves and wash after gardening, you risk putting the chemicals into your mouth when eating, coughing, or otherwise touching your face.

  • If you’re growing vegetables, grow fruiting crops, such as tomatoes, peppers, beans, and okra, rather than root crops, leafy vegetables, or herbs. Fruiting crops are held above the soil and are less likely to have contaminants on them compared to these other crops.

    If you do grow leafy vegetables, be sure to remove the outer and bottom leaves before eating. These parts of the plant are in closest contact with the soil and have the most potential to be contaminated. And if you grow root veggies, peel them to remove the skins where contaminants may reside.

  • Add organic matter to the soil through compost or cover crops. Organic matter makes metals less mobile in the soil and lessens the amount taken up by the plants.

  • Maintain a pH of 6.5 or more. The higher pH makes metals less mobile in the soil and lessens the amount taken up by the plants.

  • Replace contaminated soil. You can physically dig out contaminated soil and send it to a toxic waste site. Then bring in new soil that you know isn’t contaminated.

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