A Lighter take on Heavy Metals

Given that my "Heavy Metals in Wood Ash" post appears to have gone over like a lead balloon (yuk, yuk), I thought I’d try a shorter version and see if that flies.

To start at the end: a fair amount of research has been done, all of it concluding that it is safe to use wood ashes in gardens in moderation. The ashes contain only trace amounts of heavy metals, and the metals do not move into whatever is grown in them. And yet, on-line question and answer gardening sites often do not mentioned this research or its conclusions.

That makes me mad. I expect people handing out advice to know their stuff. Silly me.

Wood ash does contain heavy metals. (For a complete breakdown comparing ash to lime, go here.) So does soil. It had better, since several heavy metals (copper and zinc, for instance) are essential plant and animal nutrients.*

I don’t mean to be dismissive about pollution, including soil contamination, by heavy metals. And these metals—lead, cadmium, mercury and others—do cause very serious health problems.

But it’s not enough to know that there are metals in ash. We need to ask which metals are present, and how much, and whether these metals move from the soil into the plants grown in it. Lead, for instance, is present in wood ashes at about the same level as in lime—and I never heard anyone warn against using lime as an amendment because they might get lead poisoning.

All of the studies I looked at (and this was an extensive project, conducted from my living room couch and taking all of two hours) concluded that ash contains only trace quantities of dangerous metals. Even more interesting, these metals do not move into the mushrooms, berries, and pine trees studied in these articles.

There’s an elegantly simple reason for this: In order for ANYthing to move into a plant root (and from thence to the berry, or bean, or whatever) from soil, it must be dissolved in water. At a pH above 6, most heavy metals don’t dissolve. They remain chemically bound in the soil. Since  ashes are alkaline, they raise pH, making it even less likely that the metals they contain will move into the plants.

There’s a recent article (June ’08) in the on-line organic magazine Dig It about heavy metals in soils. It talks about pollution sources from the past (including car exhaust and some fertilizers) and in the present (treated wood, some commercial fertilizers and amendments). It does not mention wood ash.

*Technically, these are light metals, not heavy ones, but they're frequently included in discussions and definitions of heavy metals, so I'll follow the crowd here.

6 Responses to A Lighter take on Heavy Metals

  1. You’ve made me realise just how much of my lovely university education I’ve forgotten…
    How sad. That was, of course, my intent.
    –Kate

  2. I thought your full blog on the heavy metals was fine, but then again I read Science New for fun.
    Ah, well, that explains everything. Us science/research nerds need to stick together.
    Thank you for th kind words.
    –Kate

  3. Hi Kate
    I loved your post on the heavy metals but didn’t think I could add anything intelligent enough to adorn the comments! We garden on limestone here, so adding the wood ashes to the already-alkaline garden probably doesn’t make a lot of sense, although I did read somewhere that you can make a paste of them with water and put it round the trunks of apple trees to repel pests, so I might try that. Isn’t it great that you can do all that research from home nowadays?
    Hi, Amanda. Yes, it’s incredible what one can do from the living-room couch, research-wise.
    UC Davis, whose word on horticulture is next to the word of God in the US, agrees with you about ringing plants with ashes, though they didn’t mention apple trees specifically. They did say it’s a good way to keep slugs and snails at bay. I hope you’re getting more sunshine than we are–
    –Kate

  4. Nothing to say about heavy metals, Kate, but the last post reminded me of the time I tried circling plants with wood ash to keep off the slugs and snails.
    Didn’t work. At all.
    Nor did eggshells, nor sand, nor sharp stones. Either none of these things really work, or else we have Terminator molluscs in these parts.

  5. Yes, too much sun here in Windsor – horribly hot (and I know the weeds will be multiplying back at home). I was thinking more of a wood-ash paste that you paint on the bark of the trees. I can’t find my original reference (may even have been in a book!).

  6. Curiouser and curiouser.
    Soilman, you have put me in the untenable position of having to choose between believing that UC Davis is wrong, and believing that you have Terminator mollusks. Under the circumstances, I strongly suspect you have Terminator mollusks.
    Actually, I’m perfectly willing to believe that this particular little document by someone affiliated with the UC Davis extension program could be wrong in this particular.
    And from what I remember of slugs in England–oy. Sumo slugs. Out here in the high, dry West, we have nary a slug. The downside is a growing season about long enough to blink twice.
    Amanda, I’m going to leave this for you and Soilman to hash out. If you stumble on that reference, let me know.
    In the meantime, enjoy your sunshine.
    –Kate

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