Toxic Soil

Growing leafy greens and other edibles in toxic soil can make you very sick. In some cases, it can kill you. Toxic soil contains heavy metals and other poisons. Often found under landfills, junkyards, and factories, toxic soil is increasingly found in urban areas.

Toxic waste barrel (Elias Sch.) Pixabay

What makes soil toxic?

Healthy soil contains a balance of organic matter, air, water, and minerals that plants use as food. Some of those helpful minerals, such as boron or molybdenum, can reach toxic levels in the soil. Heavy metals can also make soil toxic. So can organic pollutants, such as creosote, excessive fertilizer, herbicides, industrial solvents, pesticides, explosives, and petroleum products. In some cases, radioactive materials, such as radon and certain forms of plutonium, can be in your soil. It ends up that fill dirt used to be brought in from questionable locations when building homes. [Hopefully, that doesn’t happen any more.] The problem is, without soil testing, you don’t know what you have.

Soil is the earth’s filtering system. Like our kidneys, it can only handle so much. Heavy metals and other toxins in the soil often leach into groundwater. They can also become part of the dust that you inhale and the foods you eat. Toxins can be absorbed through your skin and may even coat produce you grow or buy at the store. [Always rinse off your leafy greens and root vegetables, and wash your hands frequently, just in case.]

Is your soil toxic?

The first step to learning whether or not you have toxic soil is a soil test. Not those cheap plastic things. A real, lab-based soil test. They are inexpensive and extremely valuable. Especially if your soil is toxic.

If your soil test results indicate heavy metals, such as lead contamination, or other toxins, there are steps you can take to remove those dangerous materials. Traditionally, that meant simply digging up the toxic soil and burying it somewhere else. Today, many researchers are looking to plants for a solution.

​Put plants to work!

As plants absorb water and nutrients, they also take up nonessential elements, such as cadmium, lead, and mercury, which can contaminate soil. Using plants to remove toxins from soil is called phytoremediation. Phytoremediation uses plants to contain, remove, or render toxic contaminants harmless.

Phytoremediation processes (Townie) CC BY-SA 4.0

Phytoremediation plants can be classified as accumulators or hyperaccumulators. Accumulators (A) are plants that pull toxins out of the soil and up into their aboveground tissues. Beets, for example, will absorb and accumulate radioactive particles found in the soil. Hyperaccumulators (H) collect toxins particularly well, absorbing up to 100 times the toxins of accumulator plants. Sorghum is a hyperaccumulator of arsenic.

(Rona.fawzy19) CC BY-SA 4.0

How does phytoremediation work?

Accumulators and hyperaccumulators can reduce toxins in the soil through several different processes:

• phytoextraction - toxins are extracted from the soil by being absorbed through the roots and distributed throughout the plant; these plants are then removed from the site and disposed of safely [not in the standard trash bin - contact local waste authorities regarding their safe disposal]
• phytostabilization - these plants use root secretions to stabilize toxins in nearby soil, reducing runoff and leaching
• phytodegradation - root enzymes break down organic toxins into harmless molecules which are then absorbed by the plant and released into the atmosphere through transpiration; Canna lilies (Canna) work well in this application.
• phytostimulation - plants release acids and carbohydrates into the rhizosphere (root zone) which stimulate mycorrhizal fungi to break down organic toxins; this method works well on petroleum hydrocarbons, PAHs and PCBs.
• phytovolatilization - plants absorb toxins through their roots and cough them into the atmosphere through stems and leaves (transpiration); poplar trees are particularly good at this; selenium and mercury are often removed from soil this way.

There are advantages to using plants to clean toxins from soil: it’s inexpensive; it doesn’t harm the environment; and it preserves valuable topsoil. The disadvantage is that this is a slow process. It can take years.

(Rona.fawzy19) CC BY-SA 4.0

Several studies have demonstrated that specific varieties of certain plants are very good at dealing with toxic soil. While I understand that Latin plant names can feel tedious at times, different cultivars behave differently, so getting the proper plant makes a big difference. For example, not all willow species are useful at cleaning soil. Studies have shown that Salix matsudana and S. x reichardtii are far more effective than other willow species.

Many trees, including American sweet gum, larch, red maple, spruce, Ponderosa pine, and tulip trees are able to accumulate radioactive particles (radionuclides), such as radon and plutonium.

Uptake and dispersal of contaminants by phytovolatilization (Rona.fawzy19) CC BY-SA 4.0

Which plants remove which toxins?

I created the chart below from information provided by several studies on toxic soil and phytoremediation.

You can contact me if you would like a larger version of this chart,

Keep in mind that, just because a plant will absorb toxins, does not mean it is something suited to your garden or your region. Some nasty invasives have become firmly established that way. 

Did you know that some companies extract these toxic and sometimes valuable minerals from plants? This is called phytomining.

Now you know.