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.