Tuesday, March 18, 2014

Terra Preta

I've been experimenting with making my own version of Terra Preta ever since I first saw the BBC documentary The Secret of El Dorado about ten years ago.   I started by burning piles of tree limbs and then putting out the fires to create charcoal, which worked but wasn't an ideal way to make bio char.

Then, I discovered the TLUD (top lit up draft) method of making bio char, and I was able to make a higher quality bio-char in a more efficient way.  The pie-in-the-sky plan is that by using a TLUD, I can scale the process up and eventually have a small field of terra preta instead of just a large garden of terra preta. 

To start, I found a 55 gal barrel with a removable lid (pull out the plastic sealer around the inside of the lid or it will smoke like a son-of-a-gun until it burns away).
The next thing I did was drill a bunch of 3/8" holes in the bottom.  After I drilled the holes, I drove a tapered punch into each hole to flare them inward so it looked like a barrel with a bunch of bullet holes.   Besides making it look like I had shot all those holes by practicing my quick draw, I thought it might stiffen up the bottom and keep the bottom from warping or burning out as quick.
After I found another lid and a piece of 8" diameter stovepipe about 4 foot long, I started making my chimney by plopping the stove pipe on top of the lid and marking out an eight inch circle before using a grinder with a cut-off wheel to make eight pie cuts.   Make sure to cut a little past the 8" diameter circle (I'll explain later)
After bending those cuts up (it might take a few times bending them and unbending them to get the pipe to fit right), the chimney can be screwed to a few of the sharp-pointy-bent-up parts.  Those eight little slits are needed to feed air to the chimney to get the afterburner effect when you actually start making bio-char with this thing so don't forget to include them.
Besides a few bricks to put the barrel on if you want to get all fancy or a few sticks to put the barrel on if you want to go all minimalist with your design, that's basically all here is to making my version of a TLUD. 

When it comes time to make some bio-char, I put the barrel on three bricks or sticks, fill it up with wood, light a fire on top, and put the chimney on.  It smokes a little at first, then flames start up the chimney, and it roars to life for about an hour.   After about an hour of so, the chimney is taken off, the air is blocked off by throwing dirt all around the bottom, and the lid is put back on tight (I usually throw some concrete blocks on top of the lid to make sure it is tight). 

The next day, I end up with some more bio-char to add to the garden.  A little muss and no fuss, and I'm on my way to creating some terra preta that will last for hundreds or thousands of years. 

Next time, I'll show the actual burn process.


  1. Yes!! I've wanted to see your setup, and hear about how it works. I fully intend to make some here, as I've got plenty of debris.

    When I saw a different version, they make a gap between the lid and barrel to give air for the afterburner. But I like the idea of having the lid on tight, and just making the slits a bit longer.

    I like the idea of punching the holes to make the bottom stiffer. It makes sense, but I wouldn't have thought of it.

    1. I try to keep things simple with a lot of options, so if it starts smoking too much and it needs some more air, I just lift the lid up a little with a pitchfork to get the fire hotter, then I can let it down again. If I built that gap into the design, I'd always have that gap (and it would be extra work to design a gap in).

  2. It is nice to see your setup after years of hearing about it! Someday I hope to get back into gardening like you and Ron and start doing stuff like this to improve it. I always dream of having well amended soil that grows just about anything.

    1. Technically, this is my second version, I originally had a little 16 gal. barrel with a little satellite dish for the chimney lid, and a 3 foot piece of 6" pipe for my chimney.

      You could make one out of a 5 gal metal bucket if you wanted too.

      The older I get, the more I try to tell people not to put stuff off for a better time in the future, start doing it yesterday.

      Find a 10x10 area in your yard and dig it up with a shovel or a spading fork (better yet, get the kids to help do it while you tell them outlandish stories), and make it the best 10x10 area you can. Grow some cover crops for a few years, throw some bio-char out there, compost or fertilize it. Since it's only 10x10, it shouldn't take more than 10-15 minutes to do anything to it.

      Add onto that area each year by squaring up the edges and before you know you won't have to dream about a garden anymore.

      Make a tiny 5 gal. TLUD and amaze the kids with your wood powered rocket and stories of conquistadors finding cities of gold built on foundations of terra preta in the Amazonian jungle.

      Did I mention that I don't think anything should be put off until a better time later in the future?

  3. So how do you use the biochar in the garden itself?

    1. The greatly simplified theory behind biochar is that the microbes and mycorrhizal fungi in the soil grow in the pores of the biochar.

      When I first started using biochar in the garden, I'd mix it with something like compost, manure (chicken or cow), or even something like a water-soluble fertilizer like Miracle-Gro to "charge" it so that the microbes and fungi can colonize the biochar and start doing all their beneficial business. Then I'd spread it over part of the garden and till it in.

      After years of applying biochar and building up the fertility, I usually just spread plain biochar in the fall before planting a cover crop of something like winter wheat.

      It shouldn't matter what type of wood or organic material was used to make the charcoal, once it's turned into charcoal it's basically just carbon.

      I'd suggest starting out by applying to only part of the garden at first (just to be safe), because it might take a year or so to get it to the point where it's working right. Although some people claim they've had problems with getting the biochar charged at first, I never had any real problems and have reached the point where I've applied over 100 tons per acre on parts of the garden and am convinced that there's a benefit to using biochar.

      As always, results may vary.