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Do I need to crush biochar?

Handful of biocharWe've been collecting charcoal all winter by screening it out of the ashes in the bottom of the wood stove, waiting until I figured out how to crush it before putting the biochar to use.  Large-scale farmers powder their biochar so that it's easy to apply with their machinery, but on the small scale, crushing biochar is a pain.  In fact, from the multitude of questions about how to crush biochar on forums (and from my own experience), I suspect that the crushing step is holding a lot of gardeners back using biochar in the garden.  Is it really necessary to grind biochar?

The answer is "probably not."  Scientists found that biochar particle sizes ranging from a twelfth of an inch to three quarters of an inch showed the same effects on crops.  In fact, natural processes in the soil probably break those large particles down into small particles quite quickly due to freeze/thaw cycles and to plant roots and fungal hyphae penetrating the charcoal.  Unless you're an industrial-scale farmer who needs powdered biochar so that it can be applied with your existing machinery, grinding charcoal probably only speeds up the process by a couple of months to a year.

I even stumbled across a statement
on a biochar forum that made me think we'd be better off using chunky biochar in our soil:

"Although powder yields the most surface area for nutrient retention, chemical reactions, microbial habitats, and water retention, it also is more likely to become compacted, clay-like, and to cut off oxygen and airflow, which we know is critical for the rhizosphere and plant roots."


Charcoal in soilSince our soil is already high in clay and needs all the airflow it can get, it sounds like we should apply our biochar in chunk form.  Come to think of it, the healthiest part of our current garden is the area where the previous residents tossed their stove ashes, and the chunks of charcoal in the soil there don't seem to be causing any problems.  A few big lumps like this remain after fifty years, but most of the charcoal has broken down to soil-sized particles.

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This post is part of our Biochar in the Backyard lunchtime series.  Read all of the entries:





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This is some work, but being retired leaves plenty of time for garden problems. I purchase 20# bags of Cowboy Charcoal at Lowes. Add 10# to a 5 gallon paint pail. Sharpen your garden spade to a sharp edge with a tool grinder. Sprinkle about a quart of water to keep the dust and chips in the pail, and vigorously chop the charcoal into about one half inch chunks. Add water as necessary make a slurry. I add this to my tumbler- composter to mix into the compost. Add microbes if needed. It's best to let the charcoal, compost,and microbes combine for about a week; then when you add this to your soil, the microbes arrive in their new home. If you have good compost tea add this for a soil drench. RG Phoneix
Comment by Richard Sat Jul 6 18:05:40 2013