A note on soil contamination

Cet article en français.

Louise:

 While reading a lot about gardening, sooner or later we become conscious about the consequences of soil contamination regarding the culture and consumption of edible plants. A lot of substances can contaminate the soil. I will skim the subject here and give a few references at the end of the article on which this text is based.

The possible presence of lead surrounding the foundation of my house didn't prevent me to garden, as long as I followed some important rules.

To my knowledge, the most common type of soil contamination around old houses made from painted wood planks is lead, since the paints, in a not-too-distant-past, have had lead in them. When paint must be redone, people generally scratched the old paint away, leaving it where it fell, getting slowly buried throughout the years.

Today, one of the modern types of contamination is arsenic, because the wood used for building porches, patios and decks is being treated with it. There's also the contamination from creosote, contained in old railway beams now used to create halfwalls, garden bed borders or walkway lining, amongst other things. And let's not forget accidental contamination, for instance in the case of an old oil reservoir that leaks.

Furthermore, the soil used to burry the basement of a newly built house could hold potentially unpleasant surprises since not every construction company thinks about the ecological impact of spreading contaminated soil practically on your doorstep.

 Finally, proximity to factories or major roads where circulation is dense can bring heavy metals via the dust carried by the wind.

When this old arch fell apart 2 years ago, we
chose not to rebuild it since the space receives a lot
of sun, which now allows me to grow some
vegetables. But since we had used wood
that was treated with arsenic, I have to grow
them in containers deposited over the ground.

In case of doubt, you can have your soil analyzed. Of course, decontamination is not always affordable for everyone (it's a pricey enterprise). But you can still garden, using techniques that circumvent the contamination problems. The easiest solution and the most radical one is to just garden in containers with new, bought soil.

Personnally, living in a beautiful, centennial victorian, I expected that old scratched-out paint to still be on site, right where it fell, sprinkling the soil with lead. But there's two good news: generally, plants don't tend to absorb a lot of lead. Furthermore, this heavy metal is stable and stays where it fell. It will not go elsewhere or spread. When plants do absorb some, it concentrates in the stalks and leaves, not in the fruits. Regarding roots and tubers, lead generally concentrates on the surface (more accurately in the soil stuck on these roots and tubers).

However, the quantity of lead contained in the fruits would be minimal if nonexistant. The warnings given by governments and universities are aimed at leaves, roots and tubers of vegetables. The plants that give fruits are thus considered fine for humans to consume. Don't forget that, in the botanical sense, a fruit is the part of a plant that bears the seed(s) and that comes from a flower that's been fertilized: tomato, squash, pepper, cucumber as well as grape, strawberry, or apple.

The University of California considered there is more risk for the gardener to inhale dust contaminated with lead while gardening than there is while consuming the vegetables if they are well cleaned. For the harvest of leaves, roots and tubers, this University recommends the following measures: 

- Use new soil in raised garden beds to prevent vegetables from getting in contact with the lead.

- Make sure the soil used is rich in organic matter, since this type of soil can imprison lead more efficiently and prevent plants from taking it.

- Add a compost rich in phosphorus and keep the pH neutral or slighlty alkalin (not higher than 7,5 however, else it may harm the plants). Avoid acidic soil since it stimulates the plants' capacity to absorb lead.

- Clean the leaves well, brush the roots to take out all traces of soil and peel the skin of tubers.

Just at the foot of my rock stairs, a tiny
watermelon of the variety Sugar Baby managed
to give me 3 melons. In the background, we can
spot some eggplant green leaves.

Personnally, I'd rather take a few more measures of safety. So, in a radius of 10 feet (3 m) around the house, besides ornementals, I only harvest plants that give fruits: tomatoes, beans, peas, groundcherries, peppers, strawberries, for instance. I avoid any kind of root vegetables even if the recommendation is to simply peel or brush them. However I'm not particularly concerned about consumming the edible flowers that surround my house. And of course, nothing prevents me from growing leaves, roots and tubers in containers.

Hélène :

I read that some mushrooms can actually absorb lead. The research on this is still young, but some scientists have figured out that mushrooms have the capacity to clean certain pollutants, not only chemical toxins but also heavy metals. This area of research is called mycorestoration and mycoremediation (you may want to consult the book I read on the subject and a Ted talk from the author, Mr. Stamets).

This cute morel grew just at the foot of a side-entrance to the house. I don't know if it absorbed any lead or other heavy metal, but I didn't take the risk. I contented myself with a picture of it...

For more details or to further your own research on the subject, here are the following references:

Documents on soil contamination (Governement of Canada) 

Lead poisoning (Wikipédia)

Home Gardens and Lead (University of California)