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Using charcoal and wood ashes in the garden

Sifting biochar"Both charcoal and ash are important soil amendments on my homestead. I was surprised to see your ash just dumped on the ground. I store mine out of the rain until I need to use it. Just this past week I spread three trashcanfuls on the back pasture and still need another 1/2 can full to finish the job. As for the charcoal, I soak it in urine prior to adding it to the compost. What do you do with yours?" --- Su Ba


I'll start out with the ash side of the equation. Wood ashes are a good source of garden nutrients...for some soil. Unfortunately, they're a bad match with our ground since wood ashes sweeten your soil and add a hearty dose of potassium...and our garden soil already veers almost too far toward the alkaline and definitely contains more potassium than it should. We are planning on raising the pH of some pastures this year, but those areas are also overabundant in potassium already, so we'll be purchasing lime instead of applying ashes. In the end, I highly recommend that gardeners perform a soil test before adding wood ashes to their soil willy-nilly or you may end up doing more harm than good. (On the other hand, you might find that your soil is a perfect fit for wood ashes! Either way, it's good to know.) That explanation (plus the fact that we're not soapmakers) is the reason why our ashes are simply sitting in a pile of waste on the ground.

The charcoal, though, we sift out with greedy little fingers to turn into biochar. Please do read the lunchtime series I've linked to in my previous sentence for more information, but the short version is that we tried activating our charcoal with urine just like Su Ba does, and didn't seem much effect. So last year's charcoal went down the composting-toilet hole to create a combination much more like the terra preta that modern biochar is trying to replicate. I won't be applying that compost until this coming fall and probably won't have any results for you until 2016, but that's the direction we're aiming for at the moment. Stay tuned for more information on biochar...in about eighteen months.



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I wonder if you might be able to make use of the ashes as a liming agent by leaching the potassium component out. The potassium should be mostly water-soluble (as KOH, K2CO3, etc.), while the calcium (as CaCO3 or CaO) should be mostly insoluble.

Of course, then you'd have a bunch of potassium-loaded water to deal with. Maybe you could rig something up so the rain would filter through, taking the potassium with it and leaving your lime behind. Sounds like a job for your inventor! :-)

Comment by Jake Tue Dec 30 08:59:30 2014