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.