A thought-provoking question came into Ask Gardenerd from Bill Thompson in New Hampshire this week about heavy metals. Let’s dive in.

Ask Gardenerd

I enjoyed your interview on The Garden Lady and have ordered your book.

In your interview you mentioned that your soil contains a lot of lead. Our soils has high concentrations of naturally-occurring arsenic. Trying to grow greens in a hoop house will protect our vegetation from freezing, but we lose the benefits that rainfall might provide in reducing the arsenic. We don’t know what arsenic uptake is for a number of varieties of greens and have not found AI queries to be of much help.

Do you have insights, suggestions, or resources you might recommend on how we might address this situation? We’re pretty leery of consuming these greens until we can get a better idea of how to proceed.

Thank you.

-Bill Thompson

Thanks for writing in, Bill. Let’s tease this apart a bit first.

What Studies Say about Heavy Metals

I read a couple scientific studies on the uptake of lead and arsenic into edible crops. The results show that uptake of heavy metals depends a lot on a few factors:

• the concentration of those heavy metals in the soil (ie. the more there is in the soil, the higher the levels were in the veggies grown in that soil);
• soil pH (more alkaline soil makes heavy metals less available to plants); and
• whether we’re eating the plant or a fruit of a plant (tomatoes don’t take up nearly as much arsenic and lead as lettuces do because the tomato plant provides a barrier preventing it from accessing the fruit).

One study showed that except in the highest levels of soil contamination, vegetables were generally below the EU threshold for heavy metal content (if you peeled the carrots, for example), but not in all cases. You can jump down to the conclusion for a summary of the results.

I’m also aware that there is a difference between naturally occurring arsenic and chemical arsenic contamination from fungicides and wood treatments, but not completely clear on what that difference is in terms of its availability to plants.  I have always assumed that any chemical combined with copper (as in horticultural dormant oils and copper sprays) has more staying power because copper doesn’t dissipate in soils. 

Solutions

I found one study that showed that growing a particular type of fern (patented by a now defunct company) called the Chinese Brake Fern (Pteris vittata) showed effective removal of arsenic from contaminated soils. Plants grew much better after growing and pulling the brake fern. Some studies have also shown that biochar can help reduce levels in soils. I haven’t tried this myself. 

If you have had your soil tested and know the level of concentration of Arsenic in your soil, that number, compared to the article data I shared above might give you a better sense of how susceptible your crops are to the heavy metals in your particular soil. You could also try phytoremediation, though it can take years to extract heavy metals. If you are able to keep your soil pH closer to alkaline, that will also help bind up the metals. Beneficial microbes lend a hand in binding up heavy metals as well. 

I hope this helps!