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?

First of all, a couple of definitions and distinctions, during which I will try to stick to the topic, keeping Star Wars allusions and other tangents to a minimum.

“Solid fuel,” my fellow natural gas Americans and close Canadian cousins, is apparently a whole variety of processed coal products, many of which have been developed as “smokeless” fuels, a requirement in parts of the U.K. designated “Smoke Control Areas.” (Isn’t it great where gardening research can take you?) (BTW, if I got this seriously wrong, I hope one of my British Blogging Buddies will set me straight.)

Secondly, though coal ash did spill all over a big corner of Tennessee, it was not the same sort of ash that Jacqueline put on her garden. Yes I know, “ashes to ashes, dust to dust” implies that ashes are ashes are ashes, but apparently they’re not, at least in the world of coal combustion. In that world there are two types of ashes and a couple of other sources of waste that need not concern us here, as we are discussing ash, and deviating not.

Home stoves and furnaces can capture only bottom ash, which is what most of us mean when we talk about ashes: the stuff that’s left at the bottom of the stove, or grate, or flue after any solid fuel is burnt. What spilled from the holding pond in Tennessee, however, was fly ash, which will just go whirling up the chimney into the atmosphere unless special equipment captures it.

All solid fuels produce both types of ash, but only very large industrial furnaces like municipal waste incinerators and power plants capture and store (with varying success) the fly ash. So, Jacqueline, that’s not what you’ve been putting on your garden, unless you have absolutely the most sophisticated stove in all Great Britain.

Unfortunately, bottom ash also contains significant quantities of heavy metals, including aluminum, arsenic, beryllium, cadmium, chromium, iron, lead, nickle, sodium, selenium, vanadium and zinc.

Despair may seem the only rational response to such a list, but there are actually several bright spots here. First, to look at just one of these elements, virtually all the arsenic (~85%) in both fly and bottom ash is the less toxic of two common forms. Furthermore, this form (the As(V), if you’re curious, as opposed to As(III)) is the one that can be successfully immobilized, or locked in soil by the addition of compost.

Now, arsenic is only one on a list of twelve, and that list is incomplete. But it suggests that despair may be premature.

Second, various people in numerous places are experimenting with using bottom ash as a soil amendment, both on agricultural land and for reclamation of highly acidic soils such as those near mines. This requires a post in itself, but no matter how suspicious one might be of the methods or motives involved, it’s surely encouraging that plants don’t instantly throw up their leaves and die the minute a speck of coal ash lands nearby.

I can hardly believe I’m saying this, but I’m beginning to suspect that a year or two worth of home-furnace ashes—some of them wood ashes, at that—on a home garden might not introduce toxic levels of anything.

Why, then, would everyone say otherwise, including industry representatives such as the one cited by the Guardian? First, these ashes are indeed toxic in high doses. And I'd guess that the industry needs to protect itself by making the blanket statement that one shouldn't put them on one's garden, ever. It's good advice, but the question at hand isn't whether one should put coal ashes on one's grden, but what one should do if it's already there.

If it were me, I’d probably try to figure out the application rate—how much of the stuff went onto how much land—and I’d get a complete soil test, one that looks at minerals. Then I’d ask a real expert, a soil scientist who works with this sort of thing every day.

If I decided to keep a vegetable garden on that site, I’d add plenty of compost and I’d test my pH regularly, since a near-neutral pH gives the best over-all chance of immobilizing the majority of toxic elements introduced by coal ashes. And I'd consider sending all plant wastes to the local landfill, rather than composting them at home, to avoid recycling the toxins back into my soil.

As for the rest of you--what would you do?

Comments

[Susan Tomlinson]

Thanks for clearing all that up.We don't burn anything in our household to get ash, but I know I'd probably be spreading it around already if I did, since I hate to waste anything.

[jodi]

I know nothing about coal; not something anyone burns around here--but we do have a wood stove in which we burn both hard and softwood. I put some ashes in the compost, but many of them I dump on the goutweed areas in an effort to suffocate them out. I think it might be working; not real pretty to look at in the spring, but if it kills off the damn goutweed (aegopodium) then I can plant something nicer there. That's my story and I'm stickin' to it!

[Helen/patientgardener]

There is often debate here in the UK about this and an amusing debate on Gardeners Question Time last summer with a lady who said she couldnt source ash anywhere as there were no chimney sweeps in her area. I have always been lead to believe that ash and soot from fires - whether wood or coal was Ok for the garden and particularly good for breaking up clay.

[themanicgardner]

Well, Susan, you at least are safe.

Interesting, Jodi --esp. as I had to look up goutweed. (Snow on the mountain.?) Are you trying to just alkaline it to death? Apparently defoliating it in spring and early summer does it serious damage.

I've read one shouldn't put ashes in uncovered compost, as many of the nutrients (it contains a number of trace nutrients as well as potassium) will leach out.

Helen Well, I'll be. Every on-line source I've seen says don't put coal ashes in the garden because of heavy metals contamination. Yet another Brit e-mailed me to say she'd had soot (via a chimney sweep) put in her garden for years.

I wonder if anyone in the UK has done an analysis of garden soil where coal ashes have been dumped for a century or so?

--Kate

[VP]

Kate - I haven't been putting it on my garden for years, it's the guys up at the allotment who have. Getting a definitive answer on whether there's potential harm from heavy metals in soot is proving hard to come by.

However last night when I was researching something else, I came across an exam question from last year's RHS Level 2 Certificate (the basic level horticulture qualification here in the UK) asking which items from a given list were single source fertilisers. Soot was included in that list! And yes, it counts as a single source fertiliser!

[Gerald Kusse]

All modern residential coal stoves and furnaces that function with forced combustion air produce large quantities of fly ash. Nothing I will put on a food producing plot.

[themanicgardner]

Well, VP, Gerald, um, it would appear that your opinions and information directly conflict. I am by this time thorougly befuddled on the question. I still haven't seen an actual analysis, either of the soot intself, or of soil where it's been applied, and I wish someone knowledgeable would produce one!

--Kate

[chimney liner]

Really enjoyed reading your blog post. I will have to bookmark your site for later.

[John Morris]

Thank you very good and a healthy writing. I will definitely keep track of posts and the occasional visit. Looking forward to reading your next post.

[Robert Howes]

Most of the soil on this place (in Llansamlet, Swansea, Wales, UK) is poor and probably acid, but some soil I took from the bank overlooking the road seems to be mostly old coal fire ash plus all the splashings from the road and whatever the weeds and creatures have given it over many years, minus whatever has leached out. Anyway it is growing stuff like billio (well). I must get a kit and analyse it and the poor soil. I plan to put wood ash on the poor soil as well as compost in the hope of fixing it.

Cheers,

Bob
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