Heavy Metals in Wood Ash–??

The Question

So I'm doing my research on soil amendments, (still for the same article) and I'm reading up on wood ash, and I see this: "they can also be a source of heavy metals that you don't necessarily want in your garden." Damn straight. But was it true? (If you just want the answer, go to the end of this post, Results, Conclusions, and Celebrations.)

That quote came from about.com's gardening site, and in this short article Marie Iannotti ended up taking the reasonable position that "a small amount of wood ash will add some nutrients and be beneficial to most soils," but that "Large amounts should be avoided."

Still–heavy metals? They're almost as scary as cancer these days. And since I've been using wood ashes for several years, I have a real investment in this. Also, I'm doing research, ya know, and when I do research, I mean, I DO it.

(One of my rules is to cite only government and university sources. Not that they're infallible–ha!–but since I'm trying to promote organic techniques, I figure I'll convince noone if I cite green organizations.)

In Search of an Answer

I dug around some. Fingers a-tremble, I typed "wood ash fertilizer" and fearfully scanned the results. One, a study by the Finnish Forest Research Institute, said that "Migration of heavy metals into plants is reduced when their solubility is reduced. Increased soil pH prevents metals from dissolving. Wood ash increases the pH value of soil, reducing acidity." Ash means higher pH which means fewer heavy metals in crops. Whew. Furthermore, they concluded that, in the particular soils they worked with, "wood ash does not accumulate metals in edible berries and mushrooms" even fifty years later. This was what they call a long-term study.

Another Finnish article concluded that in long-term studies, concentrations of many metals sometimes dropped for berries and mushrooms where the soil had been treated with ash.

Now feeling bold and confident, I took a deep breath, typed "wood ash heavy metals," and tackled source the third, from the University of Georgia. Here I read that "Wood ash contains few elements that pose environmental problems. Heavy metal concentrations are typically low." Good stuff. When we're talking about heavy metal concentration, low is good.

Furthermore, (there's always a furthermore), in acidic soils "liming [adding alkaline amendments] improves crop growth by increasing the availability of nutrients such as phosphorus, providing calcium and magnesium to crops, and decreasing the availability of certain metals." Aha. Decreasing the availability of certain metals. I liked the sound of that.

And so it went. Lighthearted, positively gay, I perused further offerings. Purdue says that wood ashes may contain "trace amounts of heavy metals" but concludes that "Applying small amounts of wood ash to most soils will not adversely affect your garden crops" or, presumably, you.

A University of Wisconsin article called "Using Industrial Wood Ash as a Soil Amendment" (it's a pdf file, and I can't get the link to behave) goes even further than Georgia's, stating that "The concentration of heavy metals in wood ash is very low." Wow. From an academic publication, that's strong language.

Results, Conclusions, and Celebrations

It appears that Marie Iannotti was correct: wood ashes do contain heavy metals. However, the real question is, will my vegetables contain heavy metals? Am I going to be eating gobs of nickle and lead if I fertilize or amend with wood ashes?

And the answer to that is–No. The ashes contain metals. The soils treated with ashes contain metals. But the crops grown in those soils, and treated with those ashes, do not. The metals do not move into the crops. As long as soil pH is above 6, the metals will remain bound in the soil, chemically locked in place.

So am I going to keep using ashes in my garden? Yes–in moderation. More on that tomorrow. Meantime, I think I'll go fertilize my garden.

8 Responses to Heavy Metals in Wood Ash–??

  1. This, I would say, is ‘Serious’ gardening.
    Yeah–maybe too serious. I notice a profound (in all senses) silence in the room.

  2. Serious gardening, but thanks for passing along your conclusions — it's very interesting information.

  3. I have been putting smokeless fuel ash on my garden and some wood ash. It has helped the heavy clay soil break down. I am worried about the heavy metal problem. Whern I googled this before using the ash last year, I found nothing detrimental about using fire ash. I do not think wood ash is a problem. I now am thinking , after 25 years of no poisons and only organis products on the soil, I have made a big mistake. Will have to move the vegetable patch to a previous lawn area.

  4. Nancy–Thank you.
    Jacquelin–Either I’m misunderstanding you or I fear you may be misunderstanding me. Has this post made you MORE nervous about wood ash? Its intent was the the opposite–oh dear. Now this post is making
    me nervous.
    Yeah, I was misunderstanding you. It’s the smokeless fuel that’s the issue. See the post on this–

  5. Gute Arbeit hier! Gute Inhalte.

  6. Dankeschön, fussball, and willkommen.

  7. now – this is 10 years ago …. i was looking for an english version of a german conclusion to NOT USE WOODASH at ALL in the garden, because of the heavy metals. this is the link… in german :

  8. One has to bear in nine that there “heavy metals” are required for healthy plant growth. Just like water. Not enough and plants, people and most all life will cease to exist. Too much and the same thing can happen. It’s just a fact of life. So getting all worked up about heavy metals in general is unnecessary. The place to worry is where there have been chemicals spilled or disposed of and the like. Of course also avoiding certain things like using seashells or oyster shells in freshwater cultures as the carbonates will bind to many heavy metals such as cadmium, copper and zinc. All of which prove to have toxic effects to many freshwater species in constant applications. Though in occasional limited applications can be very beneficial. It’s just a matter of learning what to use where and when.

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