Biochar in the Garden

Phillip small is developing this FAQ on the application of biochar to the home garden. Although a work in progress, as it must be with the current state of knowledge, It covers enough to give a new user a running start.

The evidence to date supports spending a fair amount of effort to produce a finely powdered product. In fact I would get the appropriate screen and simply use only the fines in the garden. This of course will prove a little difficulty with commercial wood charcoal were the fines have already been cleaned out.

Everyone is discovering that crushing wood charcoal is not easy or convenient.

If one actually has an efficient retort working on a pyrolysis gas fuel system, then we have the option of using non wood plant material as a feed stock which immediately solves the problem. The pollen evidence and the additional likelihood that the original terra preta was cooked up in earthen kilns formed out of corn stalks and their root pads informs us that the original biochar protocol did exactly this.

In the meantime, crushing wood charcoal is the available option. Laying down a ground sheet to capture the charcoal powder on a concrete slab, then a layer of charcoal and then a plywood sheet, creates a crush zone. Driving your car back and forth over the sheet may do some good. Making use of a drop weight while standing on the panel is the next option we may want to try.

What you will learn is that as fineness goes up, so does energy expenditure at a much faster rate. Welcome to the world of mining and milling.

The reason this all works is because the surface of elemental carbon grabs and holds nutrients until a root cell arrives with its biological entourage and extracts the nutrient away from the carbon. The nutrient so bound can not escape into the mobile drainage system. Obviously a root will have difficulty penetrating a large chunk of charcoal.

As yet no one is marketing powdered charcoal as such, although that can not be far off. That will be followed very quickly with fertilizer blending. In the meantime it is pure do-it-yourself.

In terms of application, I would blend five pounds of powder in ten to twenty pounds of soil with fertilizer and use that to set seeds in mini hills of the blend. That way you are not treating the huge amount of area that lies in between the seed beds.

This should maximize immediate results for the home gardener.

I would particularly like to see this tested in this manner on the clearly poorer soils. Fine loamy soils barely need the assistance. Former desert soils most certainly do. And there are plenty of urban lots in which removed topsoil was never properly restored. At least with this protocol, the home owner has a method for soil restoration that compliments and supports any thing else he may try.

A really interesting experiment would be to plant alfalfa in a very thin top dressing that included fifteen percent biochar on a subsoil base. It is the nature of alfalfa to run a root system both deep and broad while also fixing nitrogen. This penetrates the sub soil with organic material on an ongoing basis. The top dressing holds the soluble nutrients also needed. The question that we are really asking here is whether this protocol is able to produce a viable top soil quickly. While this is going on, it may be possible to harvest the alfalfa and perhaps aerate the top three inches. Obviously any now barren non productive field could be used for this experiment and I expect the carbon to counter even salinity by sequestering the salts into the carbon.

The important point is that the initial top dressing does not need to be very thick, although more will be clearly better. But if you have an impossible soil, getting anything to set up and establish itself is a blessing. The established root material will then start the process of rehabilitating the soil. After that it is a matter of how much of a hurry you are in. An established alfalfa field providing a steady and improving supply of fodder is at least nicely carrying itself.

Welcome to a 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 pyrolysis. Medium temperature pyrolysis produces a more traditional charcoal, high temperature pyrolysis produces activated charcoal. Ideally biochar is made in a way that achieves maximal woodgas condensate retention.

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 a chemical decomposition of organic materials by heating in the absence of 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 amounts of woodgas condensate and ash.

1.03 What temperature range is considered "low temperature" in the context of biochar?

The theoretical low end of the range approaches 120 deg C, the lowest temperature at which wood will char, (Reference) thus the temperature at the pyrolysis front. A more practical low end is to use the piloted ignition temperature of wood, typically 350 deg C. (Reference) The theoretical high end, between biochar and more traditional charcoal, depends on the process and feedstock used, but is seldom indicated in excess of 600 deg C.

1.04 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.

1.05 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.

1.06 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.

1.07 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. Bioenergy lists: Terrapreta -- Discussion of terra preta, the intentional placement of charcoal in soil.
  3. 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 20 pound bags by the pallet, about 600 pounds, for less than US $ 0.7/lb. For larger amounts, as in a shipping container, consider coconut shell charcoal, available for less than US $ 300/mt. 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, most gardeners will want to start with 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 can produce a fair amount of smoke and partially combusted gases. Out of concern for air quality, many gardeners may prefer not to use these approaches.

2.03 What are some less smokey approaches to making biochar for the gardener?

Choose your feedstock wisely. 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.

Inverted Downdraft Gassification. For a cleaner burning configuration, consider a Top Lit Updraft (TLUD) technique, also referred to as an inverted downdraft gassification. The technique looks simple but in reality it involves some fairly sophisticated physics (PDF). That doesn't prevent success using common materials and dead simple design. Take that same open barrel configuration, tweak the design per the afformentioned physics involved, and now light it from the top instead of the bottom. This takes a different skill set than lighting from the bottom but its also not that difficult to master. A little vaseline or ethanol on a cotton ball can work wonders for starting up. Once the fire gets going, the top layer of wood burns, creating charcoal, naturally. The heat from the charcoal layer burning heats the wood below it, and ignites it, but at a lower temperature sufficient for pyrolysis. The gases released by pyrolysis (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. (Source). The lack of oxygen below the combustion zone is impedes loss of the charcoal despite the high temperature flame immediately above it. This alows biochar to build up faster than it is consumed, at least until the pyrolysis zone reaches the bottom of the fuel column.

A handy TLUD fired Retort. The 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, small openings in the retort allow wood gas to escape, but restrict the flow of oxygen in. While capable of very high yield efficiency, 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 smaller retorts is that much of the wood gas generated from the retort can end up not being burned. Folk Gunther's hybrid TLUD/retort demonstrates a simple configuration that neatly addresses these concerns.

2.04 What are some higher volume but less smokey approaches to making biochar for the garden?

While TLUD's can get fairly large [Link needed], a large TLUD/Retort is less practical, than a large drum retort.

A Large Drum Retort. [Expand]

The Wood Vinegar Kiln. [Expand]

2.05 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]

2.06 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.

2.07 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.

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

2.09 What gases does pyrolysis produce?

2.10 How much heat does pyrolysis produce?

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

2.12 Is biochar worth more as a fuel than its value for offsetting greenhouse gases?

2.13 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. [Expand, obviously]

3.02 What size should the biochar be?

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 addition to the household kitchen scrap collector with a healthy layer of biochar.

3.07 Will biochar affect the compost process?

Casual observation indicates that adding fine, untempered biochar may 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?

5.01 Does biochar affect soil pH?

5.02 Does biochar increase soil CEC and Base Saturation?

5.03 Does biochar improve soil moisture characteristics?

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

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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