Readers’ Questions: Arsenic from pressure-treated wood, Part I

(My most recent absence can be attributed at least in part to the fact that I, along with most of Bozeman's Symphonic Choir, got up at an appallingly early hour on Saturday morning to catch the bus to Billings for a joint concert. We acquitted ourselves honorably (three standing ovations, though one being for Bach's 4th Brandenburg Concerto which features not a note for singers, it's hard to take credit for all three. I'll give it a try, though: we were eloquent even in silence? No? Anyway, after the reception some of us sang all the way home, arriving at those homes about nineteen hours after we'd left them. Such excursions require a day or two to catch one's breath.)

Last week Jen in Maryland left the following comment on my January post “Compost vs. Arsenic: And the winner is--compost!” about dealing with arsenic in garden soil:

As a gardening novice, I placed a small veggie garden next to an old telephone pole in my backyard. It then dawned on me that maybe the soil could be contaminated with whatever they treated the pole with. The house is 60 years old, so I don’t know, but now I’m tempted to give up, for fear of contaminating my family. Advice?

The stuff we are talking about here is the copper chromate arsenate (CCA) that was used as a wood preservative from the 1930s on. By the 70s it had became the outdoor wood-treatment of choice across the United States until concerns that matched Jen's led to its being banned by the EPA, save for a few rare exceptions, in 2003.

Compost vs. Arsenic: And the winner is--compost!

Several posts back I mentioned in passing that compost can help fix arsenic in soil, and it seems reasonable to explain what the hell I was on about there. To "fix" arsenic is get it to bond with other, stable molecules so that it can neither leach from the soil into the water supply, nor migrate into your vegetables, and thence into you.

I got onto this originally because the news articles about the great Tennessee coal ash spill of '08 kept mentioning arsenic. Arsenic, I learned, occurs in several different forms, but the one that turns up most commonly in coal ash is arsenate(V), the same form that leaches into soil from telephone poles and fences treated with copper chromate arsenate (CCA).

And yes, at least one recent study conducted at the University of Florida found that when carrots and lettuce are grown in soil that's contaminated by CCA, both vegetables absorbed less arsenic when the soil received plentiful treatments of compost. 

Er--coal ash as a soil amendment?

I always knew the Brits were crazy.

Given the justifiable panic over the Tennessee fly ash spill here in the States, it’s almost impossible to believe that people might put anything remotely resembling coal by-products in their gardens on purpose, but they do.

Sounds nuts, I know, but experiments have been ongoing in experiments have been conducted in India, Australia, China, and the United States, Poland, Thailand, and who knows, maybe the North Pole, growing vegetables (or in China, trees) in soil amended with bottom ash. Since ash is almost always quite alkaline (pH>9), this only makes sense in soils that have low pHs, but in those it can raise pH, improve water retention, improve mineralization, and increase nutrient availability. And at normal agricultural application rates (20 tons or more (!) or so on each hectare, which is about 2.5 acres) virtually all of experiments I’ve looked at show that these unlikely amendments are not accumulating at dangerous levels in plants.

Coal ash in the garden?

Coal ash has been coming at me from all directions; by an odd coincidence, a recent commenter on last summer’s wood ash in the garden posts wrote about coal ash, even as I worked on the post about the great ash spill at the Tennessee power plant.

Jacqueline had been putting her ashes from wood and solid fuel on her vegetable garden for a couple of years, though she did the responsible person’s internet search first, finding only sites touting the benefits of ashes in general. Then, just a week or two ago, a response to a letter in the Guardian cited the smokeless fuel folks as saying that these ashes should under no circumstances be put in gardens.

Gulp. So—how bad is this bad news?

Hot compost anyone? Read it and weep

Here’s a photograph, taken on Friday, Nov. 28:

That, folks, is a thermometer. It’s the thermometer in my compost. It reads (in case you can’t see it) a hundred and twenty-three degrees Fahrenheit. (123°F.)

Others may be grateful for family, friends, turkey, jobs, whatever; I’m grateful for  the compost heap (which I mis-typed as “heat,” a serendipitous error).

I built this heap on Tuesday the 18th, the day before surgery, and in the days just after, husband Steve brought me progress reports: 120° on Thursday, 140° on Friday. On Saturday I hobbled out to see for myself: 140°.

Soil + sand = cement? Not so much.

They say adding sand to clay soil yields cement. Maybe sometimes. But based on a highly unscientific experiment in my back yard, not always.

The warning about sand pops up here and there all over the internet, often coming from university Extension offices. The one in Colorado for instance, says flat out, “Don't add sand to clay soil,” predicting a consequence that would make any gardener quake in her gardening boots: “this creates a soil structure similar to concrete.” A specialist with Ohio State University goes even further, saying that the result is usually “a disaster.”

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.

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.