Get The Lead Out: How To Test Your Soil For Contaminants

Because if there are toxins in your soil, there will be toxins in your food.

If you’re just getting into urban gardening, you’re undoubtedly excited to break out the fancy tools and get your hands dirty. You’ve selected those heirloom seeds with care, plotted out the beds, and stocked up on organic fertilizer. Now all that’s left to do is plot your urban oasis, right?

Not so fast. It’s important for city-dweller’s to be mindful of the contaminants that often lace urban soil—a problem that has plagued urban gardening initiatives for decades. Centuries of mining, manufacturing, and the use and accumulation of manmade toxins (from pesticides, paints, batteries, sludge, and more) have led to higher-than-normal concentrations of heavy metals like lead, cadmium and arsenic in most urban soils.

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The excess accumulation of heavy metals in most urban soil is dangerous to humans and other animals, largely because it is so chronic; these metals don’t leave the bloodstream of the plant or animal that ingests them, and they will remain prevalent in whichever food chain they pollute. When plants attempt to grow in contaminated soil, they inevitably take up these hazardous compounds through their roots, which ultimately cycle back into the human population through consumption. (Meaning: if the plants you eat are contaminated, you will be, too.)

Contamination will affect everything in a garden, from plants to the wildlife that comes to feast upon them. People who live in or near homes built before 1978 (when lead-based paint was taken off the market) or near roadways (a source for a streaky stream of harmful particles) are at an even greater risk.

Bioaccumulation of toxins is prevalent in greens like spinach or collards, but is for the most part absent in fruits (e.g. your tomatoes are safe). Nevertheless, gardeners with high concentrations of lead or other compounds in their soil should reconsider planting until they’ve tested (and treated) the soil (more on that below.)

Some plants are capable of stabilizing or removing metals from surrounding areas through their roots, thus decontaminating groundwater.

Contaminated soil can be excavated from the ground and treated in a lab, or a large plastic cover can be placed over the contaminated soil to prevent runoff or direct contact with other plants and organisms. High temperature treatments (which produce a granular soil that won’t leach minerals) are among the most commonly used methods to treat soil, as are solidifying agents (which cement the soil,) and soil washing.

These treatments are expensive, however, so an expert will probably suggest a course of action that involves raising the pH of the soil, draining wetlands, applying phosphates, and choosing plants that are less susceptible to toxic uptake (which means planting fruits and seeds over leafy vegetables).

Some plants are capable of stabilizing or removing metals from surrounding areas through their roots, thus decontaminating groundwater (after the Chernobyl disaster, scientists planted sunflowers in contaminated areas for this very purpose).

So why should you pay someone to test the dirt in your yard? For one thing, decontamination helps identify the pH and nutrient levels of your soil, which clues the gardener in to any deficiencies that may be compromising their crops. The process also helps identify potential planting hotspots, especially when tested in conjunction with major nutrients like Phosphorous, Potassium, Calcium, and Magnesium.

Knowing this will be useful if you ever decide to purchase fertilizer with soil amendments (like added phosphate, limid, or dolomite) or if you combine soil with other additives that are already enriched.

Here’s How To Test Your Soil

  1. Using a spade or trowel, take small samples of soil from three to ten random spots in your garden. The Environmental Sciences Analytical Center at Brooklyn College recommends sampling 6 to 8 inches deep for vegetables.
  2. Thoroughly mix the soil in the container, taking care to remove any pebbles, leaves, or roots you might find. Then, transfer at least one cup of the soil mixture into a plastic bag and seal it (don’t fill the bag; it’ll be flattened if you mail it in an envelope and you don’t want dirt spilling out). Make sure the sample is dry; do not dry soil on a stove or radiator as this may affect the readings.
  3. Mail the bag to your preferred testing site. Many of the colleges and universities who operate soil testing programs offer testing services at a low cost (most tests cost between $10-20: be sure to ask for heavy metal testing in addition to the standard panel). If you live in the East or South, try the University of Delaware College of Agriculture and Natural Resources. Folks in the Midwest should try Midwest Laboratories (a nationally-renowned testing center), or the K-State Soil Testing Lab at Kansas State University. West-Coasters should check out Perry Laboratory and Wallace Laboratory, two California-based greats who work specifically with farmers and gardeners. New Englanders need look no further than the University of Massachusetts, Amherst Soil and Plant Tissue Testing Laboratory, which boasts a 3-6 day turnaround time for most samples.
  4. Follow-up with whoever conducts your soil test on how to proceed with your gardening plans. This video from the University of Delaware’s Soil Testing Program offers a nice introduction on how to decipher the numbers that will come back with your results.


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