Biochar Size?

Does the size of biochar matter? Does it have to be chunks, crumbles, or can bacteria find plenty of places to inhabit on powdered biochar?

Generally, the finer your biochar is ground the better it is for soil microbes. I don't go to great lengths to crush it, though some do. I find that it breaks down over time due to weathering, freeze/thaw cycles, and working in the soil.

Powdered biochar still has massive internal surface area. The micropores are way smaller than the particle sizes even when you get to dust or flour consistency, and about the only way to do that is with a mill (unless you have a mortar and pestle and limitless patience). So by crushing it you expose more of the structure and make it that much more enticing to the microbiota.

I crush most of mine with a machine comprising a pair of steel drums driven by a big electric motor. The end result is a mix of sizes from fines to about 5 mm chunks. Sometimes I sieve out a particular grade if I want potting mix, concrete aggregate, chicken grit, or plaster darkener. And we've discovered that cattle prefer bigger chunks...it seems that they enjoy something to crunch on, about 2-5 cm diameter.

What I would think, it would be interesting to look at bacterial compositions in and around biochar, at different grades of sizes, as I would expect there to form a differential. I have read at high levels of ashing like 163 tons per ha acre, that certain spore forming alkaline loving bacteria take over, less composition of types, and would wonder in inside alkaline charcoal would be more similar to that circumstance, but have no qualified research on such. Back to what I was thinking, I would believe that big chunks equal more moisture in soil with drought, small or powdered, more moisture or more moisture for longer, when relatively moist.

Phil, any chance we can see the that crusher design?

Sure thing! Here is a "front" view of the beast:



And here is a clip of it at work:

Youtube link

You can see how much water is in the biochar as it clings to the rollers.

[edit] I will take some more photos next time I go up to crush a load. You can just see the edge of the motor frame on the left. It's a 5 HP 3-phase motor with a triple belted pulley transferring power to the closer of the two drums. There is a chain and sprocket assembly that drives the second drum from the first and you can see the big preloaded springs that allow that drum to move. This is great, because it means that a rock or chunk of partially charred wood does not ruin the day.

It was built to crush bottles at a recycling facility, then did a stint reducing volcanic scoria to sand-like consistency. After that it sat idle for a few years until the owner heard what I was doing. He had been a personal friend of Dr Peter Read, who is credited with coining the word "biochar" in 2005, and was eager to donate the machine with the knowledge that it would be working for a good cause.

Thanks for the photo and video!
That thing disintegrated the char without even slowing down!

Thanks for the replies! I did get a reply to this very same question, with a very interesting answer.

In short, the person noted that the biochar acts as a sort of refuge of bacteria from nematodes, etc. So having some larger sized bits, like pebble sized maybe can be helpful to allow soil life to quickly restore when predators and inclement conditions strike.

Personally, I think maybe a 50/50 mix of dust and pebbles is the way to go. The dust still provides a home and a little bit of refuge, while being involved in more of the soil, while the pebbles act as mega cities, a safe guard against soil life depletion.

Could someone use activated charcoal the same as one uses biochar, just soak in a compost tea, incorporate? Wouldnt be cheaper, just curious.