Can plants be used to clear toxins from soil?

Question: I’ve heard of some plants being used to target and remove specific toxins from soil. Is there a list somewhere of the plants that can be used for these purposes?

Answer: There are highly specialized and exceedingly rare plants that are able to grow on metal-contaminated sites by taking the contaminants up into plant tissues. They are referred to as hyperaccumulator plants.

One hyperaccumulating tree species, identified in the South Pacific, takes up so much nickel that the sap runs blue. Other hyperaccumulator plants have been identified that take up zinc, copper and nickel. No hyperaccumulators have been found that can take up lead. A fern that takes up arsenic, the brake fern, has also been identified.

The thought was that these plants could be grown on metal-contaminated sites as a way to clean up the sites over time. There was also a potential to grow crops on these sites as a new form of mining–a green ‘ore’. But the challenges to making this idea a practical reality have been significant.

For example, if a soil contains 30 parts per million arsenic and the top 15 cm of soil weighs 2000 tons, that means that the arsenic in that soil weighs 60 kgs. If a hyperaccumulating plant contained 1000 parts per million arsenic, that means that every kg of dry plant tissue would have 1 g of arsenic.

So in order to get those 60 kgs of arsenic out of the soil, you would need to grow 60 dry tons of plant matter. Yields of ferns are low, so you are talking about growing annual crops of ferns for well over 20 years to clean the soil. And that’s assuming the plants can continue to take up arsenic at really high rates even as the soil arsenic goes down. Also, unlike crops like corn and wheat, these are wild species. So it isn’t clear if it would even be possible to grow them like a conventional crop.

In other words, hyperaccumulators are amazing plants. Can we use them now to clean our soils? No.

–Answered by Sally Brown, University of Washington

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