For example, could the presence of cars, pets, or garbage, etc. make the food in your garden unhealthy to eat?
Unless you’re near a factory or a highway, food grown in a home garden should be fine, as long as it’s washed properly. Lead would be the biggest potential problem, and it could be in the soil around foundation plantings from lead-based paint that peeled off the house or was peeled off and allowed to fall on the ground when the house was painted. Proximity to a highway, especially a highway that was in place before lead was banned in gasoline in the 70’s, could also pose a lead hazard for garden soil. If you’re concerned about it, a soil test will indicate lead levels.
Lead is only problematic in acidic soils, where the pH is less than 6.5, so adjusting pH can help if high lead levels are present. Even if toxins like lead are present in soil, plants are fairly selective about what they absorb from soil, and in soils with a lot of organic matter, lead is usually chelated, or bound up, in organic compounds and not taken up by plants. Ferns are one exception. They absorb large amounts of lead, and are actually being used for bioremediation of lead-contaminated brownfields. But unless you’re eating fiddleheads from around your foundation, most urban-grown vegetables are fine, as long as they’re washed thoroughly.
As BF said above, lead appears to be the biggest problem with urban farms, and that is from lead showing up in the soil rather than drifting in on air pollution. Soil researchers suggest having soil tested for problems and taking steps to help mitigate the risk of contamination, including putting down mulch on affected areas to prevent dust from rising from the dirt. You can also haul in clean dirt from elsewhere or construct raised beds with “clean dirt” to keep your vegetables away from contaminated soil.
Any land surrounding a very old home – which is the case in many older American cities – is likely to have a high lead content. Still, this should not dissuade a gardener from planting in it. Trucking dirt in from elsewhere might be wasteful (and contribute the same pollutants we’re trying to fight) or could bring invasive diseases with it. There are certain plants, however, that you are safe planting.
If you have found your soil to be rich in lead, then stay away from root vegetables and leafy greens – these things can take up lead and might not be safe to ingest. Plants with above-ground fruit, however, are normally OK. In another vein, many plants can help remove lead from soil. Corn and sunflowers have been shown to do so; and “Brassica juncea and Brassica carinata, two members of the mustard familyl… appeared to be the best at removing large quantities of chromium, lead, copper, and nickel.”
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