Friday 12 April 2013

From terra preta to playing with charcoal

Amazonian terra preta black-earth soils have existed for up to 2500 years. They were man-made by ancient charcoal making communities. Old settlements have been rediscovered buried under ancient rainforest and confirm the observation of early 16 century explorers that there was at that time widespread human amazonian habitation. It would seem that ancient potters mixed their broken shards, charcoal, plant and animal waste in ancient middens and for uncounted generations added the fertile residues onto their soil. These terra preta soils even now, are up to two metres deep, fabulously fertile and high yielding. 

Charcoal provides within it’s deep pores a wonderful substrate for arbuscular fungi. These are the fungi that provide rich mycorrhizal associations with plants. The mycelium of these fungi is rich in glomalin, the recently discovered long lasting organic matter that comprises a significant component of the world’s soils.

Charcoal in the soil has superb agricultural properties and holds plant nutrients, water and air. It reduces soil emissions of greenhouse gases such as methane and nitrous oxides. It is generally kind to all manner of soil life. Plants love it. In South America a native species of worm mixes it intimately with the soil. Best of all, carbon can be stable for millenia when sequestered in the ground!

In recent years there has been intensive research into the agricultural uses of charcoal. They call it biochar and is made by a slow burning process known as pyrolysis. There is huge potential for farm wastes to be charred and returned to the ground. It’s a win-win situation. It can lead to huge increases in soil fertility and at the same time sequester a significant amount of carbon.

charcoal kiln
Traditional  Oxfordshire charcoal production
Using charcoal at home

For six years now I have been burning my bonfires to make charcoal. I do it by the simple expedient of dousing my fire with water and not letting the embers burn through. I have an annual fire to burn my bulky prunings at both cemetery gardens and on the village plot as well at at home and generate about six barrow loads of char each year. 
Char varies considerably depending on it’s source and method and temperature of its production. I cannot make any special claim for the black stuff made by my unconventional method but that has not stopped me having fun.

Embers immediately turn black

Black gold
Best practice is to shallowly cultivate biochar into the ground. As a none digger I have limited myself to applying it as a mulch on my vegetable garden, in the knowledge that  future sowing and planting operations will  work it in. I cannot, of course, claim any valid conclusions from my play. I have recently tentatively started using char as an ingredient in potting compost and as a soil amendment when planting ferns and other special plants that normally do not thrive in my soil.

Fresh charcoal, like wood ash, often contains lime. It strongly absorbs nutrients and can initially deplete the soil. This can be corrected by mixing in multi-nutrient fertilizer into an outdoor weathering pile of char. I also suggest that char be tried as a constituent of a compost heap and that the gardener recreates the ancient midden to charge the charcoal with fertility. Such compost will have some permanent bulk and will not, like normal compost, all eventually decay away.

With regard to  my own efforts at composting I frequently have difficult choices of how to recycle the copious amounts of herbaceous tops that my garden generates each winter.  Especially so, for plants with coarse leaves like miscanthus and other tall grasses with fibrous foliage.
Fifteen months ago I prepared a heap composed of succeeding layers of fresh charcoal and large amounts of this strawy debris. It was supplemented with a few handfuls of a nitrogen-rich general fertilizer and a little dolomitic lime. I also added liberal amounts of aquatic vegetation, skimmed out when cleaning my pond. Still not completely decayed I have recently applied the product of my heap  as a thick and fairly coarse mulch over my asparagus, rhubarb and soft fruit.

The bottom of the heap will be a great place to establish my marrows and butternut squashes. 

The soil surface of my vegetable plot after six years. With an old brick I have inadvertently  added my version of shards of native indian pottery!

More about terra preta from wikipedia

I have been writing more about char, glomalin and humus here


  1. Now you just have to wait 2500 years to see if it's any good! :)
    Roger, I have to admire the degree of science you apply to your gardening. I learn something new from every one of your posts!

    1. What a lovely comment Mark, and thank you for your very amusing remark. I am glad you put the smiley in though. I really appreciate it coming from a skilled gardener like yourself.

  2. Like ash, I’m sure that charred organic material may also be beneficial to a garden. What you are making is not charcoal. Making charcoal requires three things: dry organic material, heat, and control of the amount of oxygen. Pyrolysis reactions start once the temperature of the wood reaches about 250°C the, first with the hemicellulose breaking down, followed by the cellulose above 300°C, and then lignin above 320°C. Garden fires just seldom get hot enough, and more importantly, nor can you regulate oxygen.

    1. Thanks for this Rick I tried to cover myself when I called it 'my black stuff' and I realise it's not the real Macoy. When I examine it closely it does appear to be carbon and I am interested in what its long term effect will be Even if it's just a pale shadow of genuine biochar I will be pleased. I wonder if the original indian source of terra preta was made by a true pyrolysis process. I have speculated to myself what temperatures are generated in my pile of burning embers before dousing with water. In my ignorance I had even wondered whether it was too hot and had driven off too many volatiles!
      I also wonder about the recognised benefits about charring when it takes place in the fields as a quick burn in tropical agriculture.
      You have been very kind to me Rick, in saying it might be of some benefit but if it really is not much better than wood ash I might lose heart! I would be grateful to hear other opinions about my black stuff even if they are to stop being silly!

  3. Hi, Not sure if I can put a link in here, but a bit later I will try - I am fascinated by this whole biochar thing and it seems to be in my inbox from every angle at the moment. I saw an article on the soil association FB page a few days ago and got thoroughly absorbed in how make a biochar kiln in your own back yard from 2 different sources. (I was so absorbed in fact that I burnt the Brioche I had been making.) The first I saw was from Australia, and the second from California. Each was a wonderful reflection of the culture from whence it came. It's amazing how much we have forgotten, it seems to me, in the diverse methods of replenishing the soil. I don't think I will be building a biochar kiln in my own backyard. (Too much opportunity for a disaster and my neighbours are thatched!), but seriusly thinking of splashing out on the commercial stuff, and afterwards going non-dig. This is the year of the new garden for us, and it seems like a good time for taking on new ideas and getting stuck in!

  4. A new garden is a new adventure Jane. The history of terra preta is absolutely fascinating! - as you say.

  5. This is an Australian method - a bit ramshackle, but if you have the space, why not! This is the Californian version - rather more upmarket, and well beyond my means!

    Perhaps, Roger, you have the space/means to try one of these methods?

    1. Thanks Jane for this. I expect many readers will be interested. For myself I will pursue my unconventional method and although Rick correctly says its not pucker charcoal my plants seem to love it.

  6. Have you ever tested the pH of this black stuff, or have you just gone for try a little bit, then try a bit more? It might not be true charcoal, but I don't think it won't be the same as ash either, it will still have much more carbon in it.

    I create huuuuuuuuuuuuuuge piles of ash with my all weekender ponticum bonfires, and I'm never sure what to do with them. I did once test the pH of the ash (just for fun, the soil sampes were tediously hard to read from the scale) and it was obviously alkaline, but amazingly strong. I tend to just leave it to leach in the rain (where to I don't know) and then put it in a pile behind the shed.

    I do sometimes give a dose to the Horsetail zone, as I read it's an acid lover. I doubt it cares, it probably is way out of reach somewhere near the centre of the earth, but it makes me feel better.

    1. Wood ash is a completely different ball game to biochar Sarah and vastly superior. I now astonish myself how I used to waste it when burning my fires to ash!
      But yes ash from a wood fire is alkaline, it has half the liming value of regular lime and as a fertiliser does contain some potash.
      Looking back I note Rick's comment that my stuff is not charcoal. We know Rick well now because he adds valuable comments to this post. And of course my biochar is not proper charcoal. I now think my answer should have been, its not complete charcoal but it is BETTER for my purposes

    2. My guess (after my curiosity took me to my favourite wiki site) is you are making charcoal, mixed with ash and perhaps a residue of unburned wood. Because you can't control the oxgen, so you have all stages of the burning process present. So I'm agreeing with Rick about the oxygen, but not the temperature. I found this fire information. A bonfire should be hotter than a ciggy!

      Smoldering cigarette:
      Temperature without drawing: side of the lit portion; 400 °C (750 °F); middle of the lit portion: 585 °C (1,100 °F)
      Temperature during drawing: middle of the lit portion: 700 °C (1,300 °F)
      Always hotter in the middle.

      I'd like to try this (biochar, not smoking), but maybe not until I've got a bit less to deal with. Shame it has to cool down, otherwise it would be great to get rid of the pesky mulch digger cats! (sorry cat's a just a little joke!)

    3. Sarah, I have been looking back at this blog from the future as there seems to be a flair up of interest in it at the moment (wow, flair could be a pun). Somehow I had missed your fascinating reply, and reading my previous comment I note my bad grammar has reversed what I meant to say! Terra preta is a million miles superior to woodash!


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