Bio-Char Composting

So I’ve written about Bio-Char before – its the essentially pure carbon that is left over from burning wood in the absence of oxygen.  We make some while running our gasifiers and its a prime component in Terra Preta – the incredibly fertile soils found in bits of the Amazon.  It has also been in the eco-news a ton lately as a means of carbon sequestration.  There have even been some low tech trials (low tech are my favorite!) using it as a soil amendment.  What those trials have found is what I had suspected: that while long term fertility potential is increased due to the addition of carbon to the soil, all those free ionic bonds “fix” nitrogen out from the soil and initial fertility is lessened for a year or more.  The trail found that by saturating the bio-char first (in this case with human urine) the results on harvest were substantial.  I have dreamed of using bio-char from our gasifiers as a final filter in a aquaponics system – not only to clean the water, but as importantly to “pre-charge” the bio-char with nutrients for its use as a fertilizer.  Since I don’t have my aquaponic greenhouse yet, and my suburbanite neighbors wouldn’t take to kindly to me pissing in a bucket each morning in the backyard, I decided to use my compost bins as the nutrient source.

Compost bins leach out a significant amount of nutrients -either through runoff, or through ammonia gas.  My thought is that by mixing biochar into a pile, I would precharge the char, and the end result would be an even better soil amendment.  So, putting action to thought as is my wont – here is what I did this afternoon:

35#'s of hardwood charcoal from Whole Foods. $28 and likely a better use than grilling steaks.

We don’t have access to our own charcoal yet (next year I may make my own charcoal maker), so Whole Foods helped me out with an end of season sale.

Next I needed some material.  As I wrote about yesterday – that is no problem anymore.  400# of willow coming right up!

2 days old and free. The things people throw away these days!

Finally, I ran the willow and the charcoal through the chipper.  The thought was to make the char into itty bitty bits to increase surface area.  I figured dust would be an issue, but DAMN is dust an issue!  Carbon dust is quite dangerous – rather explosive and wicked rough on your lungs.  I had a dust mask on, and mixing the charcoal in with the leafy fronds helped, but soaking the char first might be better.  Its an issue, and you’ve been warned.  Here is the results:

Always amazing how much it reduces in volume once its chipped. Plan on soaking this a lot - that char is DRY.

The plan for this bin is to fill a raised bed for a side by side run next year.  My hypothesis is that the compost char will increase the fertility potential of the soil by preventing leaching and raising the overall carbon content of the soil significantly.  Bacteria LOVE to live on the incredibly rich surface structures of char particles, so soil life should explode.  This will hopefully be a major component of my pushing for 10# ‘ sq ft in some raised beds next year.

It will be interesting to see if the added char will affect the composting process at all.  It may inhibit it due to the char sucking up nutrients.  I have a good idea of the process with 4 brush piles in the works, so it should be readily apparent if more nitrogen is needed in a charified pile.

Much more to come!


29 Responses

  1. What about using kitchen sink grey water to precharge the charcoal or even the willow-char mix.

    • Grey water is a bit nutrient dilute, but it would work over time for straight bio-char. Compost+Char I would think it would leach out far more nutrients than it would gain. A nice combo in this scheme that I am mulling on is to make char from the trunks of the willow – hopefully in a gasifier powering/heating something else – and adding it to compost made using the smaller 1″ diameter wood with all their leafy goodness. Nice holistic use of the prunings or green coppice cuttings.

  2. Do you think this blog deserves a name change 🙂 I’m thinking enthusiastic compost man… errr Hyped up Humus?

    Keep it up though – it is interesting🙂

    viv in nz

    Ps wish I had you here to take my jungle away and reduce it to size.

    • I am certainly getting a bit narrow on posts of late, but so much of my thinking keeps coming back to soils and carbon. The carbon cycle is just so incredibly important – Climate Change is going to change everything. And the best way to build resilient soils is to build organic matter (carbon) and I am becoming convinced that the best way to do that on a large scale quickly is through coppicing for compost (as well as fuel). Pull the carbon from the air, sequester it in the soils, and literally reap the rewards whilst you make the world a better place for our kids.

      I’ve grown on gardens with a 18″ humus layer – it is almost unbelievable the difference it makes over even very high grade garden soils.

      When I get back from Australia / NZ in a few weeks I will likely have some other things to write on!

  3. Great to see this. No c21st gardening/farming without char!

    One thing I found out is that it helps when the char gets cooked: Then it absorbs more water.

    Outside I did this by flushing my garden fire place. Inside I used saw dust briquettes for heating, which I put into a bucket of water when red glowing (beware the mess).

    I learned this from a research paper by Lehmann who found that terra preta production is more effective in hot humid conditions. Here’s a simple experiment to understand this: Put char in bucket with water – it will float. Boil bucket for 15min – it will sink down.

    Greetings from Bavaria,

  4. Good web site and information on what you are doing.
    I’ve been making char for about three years now and adding it to my compost bins. There have been noticeable results in my crops the last two years.
    Not being a scientist, I have done some side-by-side experiments, and one can see the results. This is good enough proof for me that something is working. After all, the Amerindians were not scientist that we know of, and something worked for them too.
    Good luck on your experiments and keep us posted

  5. The newer type of charcoal briquets that are labeled “100% Hardwood Charcoal with starch binder” will degrade sufficiently in a compost bin if you just toss them in when you add/mix your compost.

    If you screen your compost when it’s finished the lumps that are still intact can be tossed into the next bin load where they will accelerate the composting of that batch.

    The most amazing thing about biochar-enhanced compost is it’s amazing water holding ability. You can pour water on it and it sucks it up like a sponge. Unlike peat or coconut husks it never has a hydrophobic stage.

  6. A good friend of mine put biochar into his compost and found that the worms absolutely loved it!

  7. ok, I am still such a novice at this. I have been reading about the terra preta. I am not sure how I would go about making my own char. Can I really just purchase the hardwood charcoal and mix it with my compost? Bury it in the soil? I don’t have a fireplace or anything and the amount that I can coppice is minimal in my urban lot. But it sounds so interesting. and btw, how would the neighbor know what was in the bucket if filled inside? silly neighbors!

    • Thersea,

      Yes I think you can just buy charcoal and mix it in. Several things to consider. 1) Kingsford is not the same as hardwood charcoal so read the label. 2) The making of charcoal is often done by cutting virgin hardwood timber, not a good thing ecologically so it is not something to make a habit of. Better to find someone with a bit more room and fabricate a charcoal maker, etc if you will be doing a fair bit of it.

      Burying it in the soil would also be of use. The thought of putting it in compost first is that the char will first soak up nutrients, so it will “deplete” your soil a bit for the first year if not precharged first by running it in a compost bin, mixing it with fresh manure, or soaking it in urine (serious). The pay offs are worth it in any case!

  8. How did the chipper blades hold up? Carbon can also be fairly abrasive, as can some of the mineral binders used in briquettes.

    Might I suggest a pre-compost of only green material plus intact chunks of charcoal? The organic matter would help a lot with dust, I think, by promoting the growth of biological adhesives along the interior pores.

  9. I should mention how I break up my charcoal:

    I moisten it, cover it in squishy organic matter, and pound on it with a de-limbed Christmas tree trunk. Very little dust rises that way, and when it begins to, I use a shovel to turn it over, or add a little more water.

    It leaves a coarser mix, which I think might be better than fine powder.

  10. I’ve done several batches (converting branch debri in a barrel that has a lid to regulate air flow – basically you snuff it our when the nasty brown smoke turns to white smoke). The way to break it up is to put it into a 50 lb feed bag (I save them from the chicken feed) and just run over it as you come and go from work. But I don’t live in a development. And I only did 3 loads, so it’s was not mass production.

    • I intend to build something similar next season – would like a way to channel the heat into something useful – like a char producing pizza oven or something. Will give it some thought this winter, but our gasifiers do the job so modifying one shouldn’t be hard. I think 50-60#’s of char per yard of compost (10% by weight) sounds good based on nothing but gut. This will actually work well, as the brush wood composting seems to work best on limbs of 1″ or less, the carbon in the trunks would be better used as fuel or perhaps mulch.

  11. Great idea that made me think of something I can do here (on a much smaller scale). We have some charcoal that got wet and next time I turn the compost (today, I hope) , I’ll layer it at the bottom of the pile. Whatever leaches out will be absorbed by it, and if the worms love it…

  12. I got a good run down on biochar at this year’s PASA conference. It covered pretty much what your post did. One thing the presenter mentioned was throwing char into animal pens, where the animals would walk on it and mix it into their own manure. Brilliant – break the char into small pieces without generating a huge dust cloud, control odor, and pre-charge the char with nutrients.

    Seems you could get some manure and just dump it on the char. After it has sit around for a while, maybe chop it with an icebreaker tool and let it sit some more, then repeat. You won’t get such fine pieces, nor instant results. But neither will you generate a dangerous cloud of dust. Barring that, I think you should keep a bucket filled with char near some discreet shrubbery.

  13. […] covered bin 3, so yesterday night’s rain won’t leach the compost too much. One Straw, by the way, has an interesting post on soaking up the leached nutrients with biochar. Following his advice on my own much smaller scale, I put whatever leftovers of charcoal at the […]

  14. Hi Rob,
    I don’t know when I’ll ever have enough time to absorb all the information you’ve shared on this blog – but I really appreciate it.

    I’ve nominated you for “The Versatile Blogger” meme award. Kind of like a chain letter but with no prospect of riches.


    • Thanks Bev!

      Currently on Holiday in Australia, and now New Zealand. Hopefully making a few contacts but more importantly gathering massive amounts of information for later processing – like on the 22 hour flight home. Mostly cultural insights, but rather a bit about ecology as well. Much more important will likely be the shift of my scope – this rock called Earth is a BIG ASS place. Something that can be personally understood on many levels, and very misunderstood by most of us Americans, myself included. Hubris is a perpetual opportunity with me – so this humility is welcome.

  15. May be too early to say without repeated attempts, but this pile hit a lower peak temp and cooled faster than my control groups. Crazy amount of vairables, but it also matches my hypothesis that the char would suck up nitrogen and slow the composting down.

    Will likely need to either add less initial carbon to get it to compost well, or to soak the char in comfrey tea, urine, or something first to charge it with nitrogen.

  16. I guess the best thing is to put the char below the compost. Mix it 50/50 with brown nitrogen-low stuff, for better aeration and pathways for worms/microbes/fungi when the stuff rots. Being at the bottom of the compost it can 1) absorb compost seepage (otherwise lost) and 2) perhaps provide the little “grinding stones” needed by some worms for digestion.

    But I did that only once. Alas I had no opportunity for comparative compostology when I had my garden: There was no much soil when I started, only granite sands and rocks below the grass. (That’s how I got started with char coal: Every boulder plugged out of the ground left behind a hole. As I had not enough material to fill the hole I threw in some BBQ char. Only later I learned this is actually a good idea.)

    Also I had no big on-time supply of char dust. I just added it to the compost when I got some from my garden fireplace (flushed with water, then grinded by hand) or from home heating (saw dust briquettes put in a bucket of water when red glowing).

    Soaking with urine is sure a good idea. But char plus urine gets extremely smelly (ammonia), even to a sickening degree. Make sure it is well covered with wet stuff that absorbs/filters the ammonia. Too much urine is poison to most compost fauna and chases away the worms – but they do return and the dead critters’ bodies constitute a good step in the composting cycles.

    Methinks comfrey tea would be a waste of work and comfrey.

    For real gardening men there’s Florifulgurator’s practical portable pissoir: You need a funnel, a bottle/canister, and a table tennis ball to automaticly close the funnel to avoid odor. The funnel needs to fit absolutely airtight to the bottle, for otherwise the ball gets stuck. (Kitchen-sink experiment: put ball in funnel cone, fill in water – the ball gets stuck down and no water flows off. Close funnel outlet with finger – ball floats up and when opening the outlet, waters flows out.) Peeing into that thing and watching the ball rotate is fun. Needs only a cup of water (or cold coffe, etc.) to flush.

    • Why would the comfrey be wasted in the tea? – when done, I just add to compost or as a mulch. We grow about 400#’s of comfrey per year here at the moment, so 5#’s for a tea batch wouldn’t bother me.

      Interesting funnel arrangement, btw.

      • Comfrey tea is great, of course. But I wouldn’t make it for the compost explicitly: There’s enough other nutrient sources. Comfrey is the perfect mulch plant, the crown jewel of mulch. I would leave it for the garden soil and worms and not carry it back to the compost. But well, I didn’t have so much of it like you.

  17. You said the char might slow the composting down. From the nitrogen perspective I would think the same. But I’ve seen more reports on acceleration, e.g.

    So, can it be that even without heat the char compost runs faster? What do you see?

    (Alas I had no opportunity for comparative composting – so I’m eager to hear what you report. All I can tell is that my composts always froze to the core in winter.

    At least, yesterday I could compare my “supercharged” char compost soil with “standard” compost soil in the garden of my permaculture expert friends.

    Their baby salad is growing much faster on my char. It contains more than 50% char – counter to all advice and evidence from terra preta scientists. But I also used extreme amounts of urine when creating it. Still, the main advantage seems my char soil holds much more water.)

    • Good to hear that your results are positive. On the composting end, I really need to do true side by sides; build 3-4 bins of the same material: 3 with varying degrees of char and one control. A friend suggested Sunchokes as a possible perennial composting material as the stalks are near ideal for hot composting once shredded and they certainly spring eternal.

      It could be hypothesized that the mesophilic bacteria may find the char rich environment attractive as the massive amounts of surface area of the char would increase their living space, but the thermophilic bacteria, which has a higher metabolic rate would be unable to pull in nitrogen fast enough if the char is also sequestering nutrients. This scenario could explain fast decomposition without heat, i.e. the significant increase in mesophilic populations off-seting their slower decomposition compared to the thermophillic bacteria. I would be surprised if the meso’s could outpace the the thermo’s, and I would hate to lose the pastuerization elements of the hot composting, but it sure owul dbe a great tool to have in the box, and the thought of putting that many “good bugs” into my soil has me all a-quiver.


  18. you must read this
    “The Biochar Revolution” with “The Biochar Solution”
    I want to call this book: “All about Biochar” because “The Biochar Revolution” collects the results and best practical advice that these entrepreneurs have to offer to the biochar community.

    • I have The Bio-Char Debate” by Jeff bughes in my Chelsea Green bag – but just because it was their book on bio-char. Will likely check at least one book on Bio-Char this winter. Thanks for the tip.

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