Terra Preta experiment

Ok, y’all know about terra preta, right? This is where you add charcoal to soil, and through the magic of microporus carbon and those wonderful, wonderful soil microorganisms, the soil retains fertility on almost unheard-of timescales. You put compost in a bed and you need more compost in…a year? Maybe two? Amazonian natives used terra preta a thousand years ago and that soil is still noticeably more fertile than the soil around it. So yeah, gotta try this. Since everyone who knew how to do this is now dead, the method is still being pieced back together. In its simplest form…you make charcoal, crush it, and mix it into the soil. I don’t really know how fine it’s supposed to be, or if it even works in temperate zones, but what the heck. Let’s give it a shot. I know folks in New England spread fireplace ashes on fields, so it probably won’t hurt, right? Lots of pics after the break…

Hardwood charcoal. Don't use those pressed briquets!

I started with hardwood (oak) charcoal. This is really charcoal, not BBQ briquets. I got a big bag at the home and garden store that sells fancy-schmancy grills and smokers. Don’t remember the price, but it was fairly cheap. (If this works, I’ll consider learning to make charcoal, but let’s make sure it works first, shall we?)

Charcoal before smashing

I dumped maybe half the bag into a tub. Then I used the ancestral pestle of my people (a wooden baseball bat) and smashed and smashed. It made a lot of dust. Do it when there’s a breeze and stand up wind! (Even so, your Kleenex will frighten you for six or eight nose blows afterward.)

Charcoal being smashed

About halfway done

I had no idea how fine to make it. The biggest lumps I left were the size of big marbles or really small chicken eggs. This left a lot of pure dust in the bottom of the tub, and lots of pea-sized pieces. It took maybe 20 minutes.

Charcoal in my bed

Charcoal in my bed

This bed, in the greenhouse, is about 30″ wide. I put a bunch of the charcoal in and spread it around with a rake, then dug it in with a digging fork. The whole thing got covered with a few buckets (maybe 4-6″?) of garden blend soil (compost + topsoil) and raked smooth. I didn’t mix the two layers a whole lot, though. Probably should have…

Finished bed

Finished bed

Then planted I tomatillos. We’ll see how it goes. I admit I don’t have a really good “control” for this experiment…if I add the soil to the north bed of the greenhouse, it’ll be close, but I already notice that that bed doesn’t grow as well as the south bed…shadier, I think. Maybe when the peas and tomato come out of that corner, I’ll just put the same soil in but not the charcoal and see if we can tell a difference. And you know, looking at the soil itself may give some clues, too. Come back in a year or five and we’ll see!



  1. Matt said,

    July 7, 2010 at 8:55 pm

    Very cool! Let us know how it goes

  2. Heather said,

    July 8, 2010 at 11:04 am

    Color me fascinated. You do cool mess in your garden.

    • Emily said,

      July 8, 2010 at 12:01 pm

      “Cool mess” is probably the best explanation I’ve ever had for what I do in the garden!

  3. UrsulaV said,

    July 8, 2010 at 2:46 pm

    Fascinating! I’ll definitely look forward to an update on this one.

  4. RecycleBill said,

    July 12, 2010 at 7:30 pm

    I’ve been adding ashes and coals to the compost pile for as long as I can remember. It makes sense as the charcoals contain many of the same chemical elements as the wood they’re made from and wood waste is added to compost in almost every commercial composting operation in the world.

  5. RecycleBill said,

    July 12, 2010 at 7:58 pm

    Just a note: Ashes and charcoals contain Potassium hydroxide, AKA, Lye, as in Lye soap and oven cleaner, a caustic chemical that could harm plants and people if used in excess. That said, Potassium is necessary to all life and is an ingredient in commercial fertilizers so unless you’re dumping ashes several feet deep I wouldn’t worry about its caustic effects.

  6. Don Hennick said,

    July 18, 2010 at 1:03 pm

    I’m with a group in Seattle studying biochar . you may get better results making your own biochar. due to the fact that fuel charcoal is intentionally made at lower temperatures so as to retain more voc’s biochar is made up in the 1000 F range and fuel charcoal is made in the 600F range . We have a small test plot garden on our local community college campus and are experimenting with village scale carbon sequestration concepts. It is my belief that all gardeners will be called upon to bank carbon with biochar . soil contains and has the capability to store much larger amounts of carbon than the atmosphere and forests combined Also you can control the dust by wetting your char

    • Emily said,

      July 18, 2010 at 5:24 pm

      Don- I got malware/virus warnings when visiting your web site. You sound like a real person…probably better go check your site.

      Do you have instructions for making biochar?


  7. new_biochar_land said,

    February 1, 2011 at 3:40 pm

    You want to know all the secrets about biochar ?
    This book will help !


    Here practice and theory merge under a single cover of “The Biochar Revolution” and reveals hidden secrets of science called Biochar

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