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Coal ashes in the garden?

I have been an organic gardener for more than 30 years now, and I have a question about using anthracite coal ashes in the garden. I have heard they are "good," "bad" and "not sure" from a few people. My grandmother threw all her ashes in the garden, and it was always bountiful. A friend actually gets truckloads of ash for his garden and says his potatoes are great. Would the garden still be considered organic, or is there something potentially dangerous or harmful in the ashes?

I've seen the same kind of mixed blessings but no "official" or scientifically researched information on whether using coal ashes in the garden is a good idea.

Part of the issue is that the "bottom ash" from the home burning of coal is different from the more researched "fly ash" that's produced by power plants.

I've heard two main reasons against using home-burned ash in the garden: 1.) it may contain heavy metals, such as arsenic and cadmium, and 2.) it could raise the pH of the soil to a level that's too alkaline for plants.

I suspect there's validity to the part about soil alkalinity. But that would be a function of how many ashes you add over time and how acidy the soil was in the first place. If the soil is too acidy, the ashes would help. And crops that don't mind soil pH on the high side (such as the potatoes you mentioned) would be fine. Acid-lovers such as azaleas, dogwoods, blueberries and most evergreens probably wouldn't benefit from ashes.

As for the heavy metals, I've heard that can vary by coal source... and that the levels aren't high enough to be worried about.

One verdict: the U.S. Environmental Protection Agency doesn't consider coal ash to be hazardous waste, and it's allowed to be recycled into such uses as paving material, concrete blocks and bricks.

So far as benefits in the garden, coal ash can help break up compacted clay, improve drainage and probably add at least small amounts of nutrients (although not as much as wood ash). It's also a convenient way to do something with coal ashes other than trash them.

What I don't know is whether those factors outweigh any possible contaminants and detrimental effects on your particular soil. So I guess I'm in the "probably-OK but really-don't-know-for-sure" camp.

Either way, coal ashes would qualify as being "organic," "natural" or at least "non-synthetic." The coal was mined from the Earth and burned, so it's akin to lime, greensand and similar minerals used in gardening.