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Saved by Philip Small
on May 21, 2008 at 8:05:28 pm

Welcome to the Gardening with Biochar FAQ!

... a work in progress...


When gardeners add biochar to garden soil, we are, in effect attempting to follow in the footsteps of the originators of Terra Preta. Because we don't know exactly how that process worked, nor how we can best adapt it outside its area of origin, we are left to discover much of this by experimenting with our own gardens and comparing observations within our own communities.


1.0 What is Biochar?

Biochar is charcoal formed by low temperature (350 to 450 deg C) pyrolysis.  Ideally it is made in a way that achieves maximal woodgas condensate retention.  Additionally, it can be infused with nutrient byproducts, such as nitrogen [and calcium? fact check needed] as achieved by EPRIDA.

1.01 How does biochar relate to agrichar and to Terra Preta?

Agrichar is a synonym for biochar. This material was fundamental to the creation of Terra Preta de Indio, as it is to creating its modern equivalent, Terra Preta Nova. Terra Preta "Classic" was made by adding charcoal, broken pottery shards along with the organic fertilizer amendments. This, in conjunction with the microbial ecology occurring in these soils, resulted in an incredibly fertile soil, and a reputation for self-regeneration.

1.02 What is pyrolysis?

Pyrolysis is partial combustion due to restricted oxygen.  This releases heat energy and yields combustable gases (aka syngas, wood gas, and producer gas) and charcoal.  The charcoal produced is a combination of black carbon, along with small amount of condensate and ash.

1.03 Can I substitute other forms of charcoal for biochar?

Yes, up to a point. The woodgas condensates in biochar give it considerable value, but that is not to imply that using simple charcoal, or charcoal made from other than plant materials, won't produce some, and even most, of the same benefits.  It is normally adviseable to avoid charcoal briquetttes because the binders used during manufacture can add undesireable constituents.

1.04 Why are the condensates valuable?

We believe this to be the case because higher temperature charcoal does not produce as much of an observed beneficial effect.

1.05 Is biochar made from hardwood best?

Biochar made from hardwood is richer in condensates when compared to biochar made from softer wood, from bamboo and from less woody vegetation.  The fact that hardwoods were readily available to the originators of Terra Preta de Indio has not escaped the attention of Terra Preta enthusiasts.

1.06 Where can I join in with this community of Terra Preta enthusiasts?

  1. Bioenergy lists: Terra Preta: the intentional use of charcoal in soils.
  2. Hypography Science Forums: Terra Preta

2.0 How do I Get Biochar?

You can purchase biochar, purchase a charcoal substitute, or you can make it yourself.

2.01 Where can I purchase biochar?

Currently manufactured biochar is in short supply and is needed for research projects. The alternative is to purchase charcoal and use it as a biochar substitute.  Cowboy brand hardwood charcoal is available in the United States in 8.8 and 20 pound bags by the pallet, about 400 pounds, Cowboy brand hardwood charcoal is available in the United States for about 70 cents per pound.  For larger amounts, as in a shipping container, consider coconut shell charcoal. Worth repeating: It is normally advisable to avoid charcoal briquettes because the binders used during manufacture can add undesirable constituents.

 2.02 How do I make biochar?

While colliers the world over normally use either a covered pit or a covered mound (earth kiln) to make charcoal, gardeners will want an easier method that works at a smaller scale.  Home pyrolysis is pretty easy to accomplish and a bottom lit burn barrel is the common starting point. Make sure the openings at the base of the barrel are large enough. Light it off, give it an occasional shake to settle the fuel, and, when done,  pop a cover on it or douse it with water.  The burn in all of these approaches is fairly inefficient, often producing a fair amount of smoke and partially combusted gases.  Out of concern for air quality, we hope you'll consider one of the more efficient approach to making biochar.  

2.03 What are some more efficient approaches to making biochar?

No matter what technique you use to make charcoal, choosing uniformly sized, dry woody material produces the highest yields. Uniformity is one reason that colliers will routinely use coppiced hardwoods.



For a cleaner burning configuration, consider a Top Lit Updraft (TLUD) technique, also referred to as an inverted downdraft gassifier.  Take the same open barrel configuration, but light it from the top instead of the bottom.  This takes a different skill set than lighting the bottom but its also not that difficult to master.  Once this gets going, the top layer of wood burns, creating charcoal. The heat from the charcoal layer burning heats the wood below it, and ignites it. The gases (carbon dioxide and water) flow through the charcoal layer. Glowingly hot charcoal has a wonderous ability to strip oxygen molecules from of anything that passes over it, so it converts the water into hydrogen, and the carbon dioxide into carbon monoxide. These two gases are flammable and they are ignited once mixed with air coming into the top of the open barrel above the charcoal layer. The result is a scrubbed gas-fed flame that is much more controlled, and which burns substantially cleaner and hotter than can be achieved with the bottom lit burn barrel. The lack of oxygen below the combustion zone is impedes loss of the charcoal, so that biochar builds up faster than it is consumed until the pyrolysis zone reaches the bottom of the fuel column.   That's the difference between a gassifier and an open fire


Competing with open flame production of biochar for use the retort.  A retort process works by restricting the air supply to the target feed stock for the duration of the burn.  An outside heat source pyrolizes the retort contents, wood gas can escape the retort but the openings are too small to allow much in the way of oxygen to get in.  While capable of very high yield efficiencies, the open flame used to fire the retort is not as clean as can be achieved with an inverted downdraft gassifier.  A common further inefficiency with a retort is that much of the wood gas generated from the retort can end up not being burned. Folk Gunther's hybrid TLUD/retort uses a simple configuration that neatly addresses these concerns.

2.04 How much charcoal yield can I expect?

On a dry matter weight basis, as well as an energy basis, between 20 percent, for the top lit open barrel approach, and 60 percent, for a retort under ideal conditions. 50 percent is a reasonable goal. [Sources needed]

2.05 What can I burn to make biochar?

Any reasonably dry and clean burnable feedstock will work.  Woody plant material is the primary candidate.  Bones are also a traditional component in Terra Preta, but one we don't know as much about.  Other materials can be used conditionally.

2.06 What do I need to consider in making biochar from other than woody plant materials?

The two considerations are, what additional contaminants are being carried off as pyrolysis gas during the burn, and what contaminants are present in the ash component of the charcoal produced.

2.07 What refractory materials can I use to make a kiln? a retort?

2.08 What gases does pyrolysis produce?

2.09 How much heat does pyrolysis produce?

2.10 Is biochar worth more as a fuel than as a soil amendment?

2.11 What do I do if I make more biochar than I can use?


3.0 How do I prepare the biochar once I've made it?

You can use it as is, especially if it is a small amount. For larger amounts, the choices are to crush, screen, add liquids, add dry materials, and to compost it.

3.01 Why would I need to prepare the biochar, as opposed to applying it as is?

There are several reasons that might apply to your situation.

3.02 What size should the biochar be?

Once charcoal is fully involved in soil processes, larger pieces tend to break down, so the value of pulverising can be diminished.  Some folks will reduce it regardless in the hopes of seeing faster results. Others believe that a range is size distribution is a healthy situation.

3.03 What are some ways to crush and screen biochar?

[For crushing, I am leaning to a mortor and pestle approach: a 5 cm dia hardwood trunk 2 m long and a 20 liter bucket with a plywood insert in the bottom.

For screening, I think a sloped screen works better than a horizontal screen for higher volumes.]

3.04 What can I do to make the biochar easier to crush?

Wetting and drying it seems to help.  Crushing it with a little moisture in it helps to control dust.

3.05 Besides water, what else can I soak the biochar in?

Yes.  Compost tea, MiracleGro (TM), fish emulsion, urine,  ....

3.06 Can I add biochar to compost?

Yes. This will help temper the biochar. For the added benefit of odor control, consider topping off each additin to the household kitchen scrap collector with a healthy layer of biochar.

3.07 Will biochar affect the compost process?

Anecdotal accounts indicate that it will accelerate the composting process.

3.05  Will biochar harm the worms in my compost?

Anecdotal accounts indicate that worms tolerate up to xx% charcoal, above which reduced worm activity can occur.

3.08  Can I use biochar in my composting toilet?

Yes. Again, the added benefit of odor control is compelling.


4.0 How do I apply Biochar?

4.01 What materials combine well with biochar for application?

4.02 How is biochar generally used

4.03 What is the normal application rate for biochar?

4.04 Are there benefits to deeper placement?

4.05 Are there benefits to using biochar as a mulch?


5.0 What happens after biochar is in the soil?

5.02 Does biochar affect soil pH?

5.03 Does biochar increase soil CEC and Base Saturation?

5.04 Does biochar improve soil moisture characteristics?

5.0x Can biochar have a harmfull effect on my soil or on my garden?

That is a remote posiibility.  A more likely effect is that

5.05 Does biochar affect soil ecology?

5.06 Does biochar improve plant growth?

5.07 How much improved plant growth can I expect?

5.08 How much carbon dioxide does sequestered biochar offset?

5.09 How much nitrous oxide formation does biochar prevent?

Soil scientist Lucas Van Zweiten has observed a 5 to 10 fold reduction in nitrous oxide emmissions with some of the biochars he is working with in an agricultural setting. Generally, soil with elevated soil nitrate levels in the presence of sufficient moisture and robust soil organic matter will have higher nitrous oxide production, and thus will be more likely to benefit at the levels observed by Van Zweiten.





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