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Saved by Philip Small
on May 27, 2008 at 3:16:30 am

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.  Higher temperature pyrolysis produces a more traditional charcoal.  Ideally biochar is made in a way that achieves maximal bio-oil condensate retention. When used broadly, the term biochar simply refers to charcoal made from any biomass waste, and may or may not have a significant bio-oil condensate component.  In this broader context biochar is simply charcoal used for agricultural purposes


1.01 What are the benefits of using biochar in the garden?

The following benefits occur with additions of biochar

  • Enhanced plant growth
  • Suppressed methane emission
  • Reduced nitrous oxide emmision (estimate 50%)
  • Reduced fertilizer requirement (estimate 10%)
  • Reduced leaching of nutrients
  • Stored carbon in a long term stable sink
  • Lowered soil acidity
  • Lowered aluminum toxicity
  • Increased soil aggregation due to increased fungal hyphae
  • Improved soil water handling characteristics
  • Increased soil levels of available Ca, Mg, P, and K
  • Increased soil microbial respiration
  • Increased soil microbial biomass
  • Stimulated symbiotic nitrogen fixation in legumes
  • Increased arbuscular mycorrhyzal fungi
  • Increased cation exchange capacity

1.02 How much biochar do I need to apply to achieve these benefits?

This is the subject of ongoing studies. The degree of benefit clearly increases with the application rate.  If you are satisfied with a very rough estimate, we would venture that a target application rate of 5 kg/m2 (1 lb/ft2) would be sufficient to achieve these results in most gardens. However, there are substantial benefits related to soil biology at rates well below 1 kg/m2. This FAQ provides information on how to use small amounts of biochar in your garden to best advantage. [peer review requested of this statement]


1.03 How long does it take for these benefits to become apparent?  How long do they persist?

Some effects, such as lowering soil acidity, occur immediately.  Other effects depend on soil biology and take time to develop.  Increased cation exchange capacity will take several years to develop fully. The good news is that these effects are very persistent. The effects of adding biochar in Terra Preta de Indio have persisted for millenia.


1.04 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 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.


1.05 What is pyrolysis?

Pyrolysis is the chemical decomposition of organic materials by heating in the absence of oxygen.  This yields combustible gases (called syngas), tars and charcoal. The charcoal produced is a combination of black carbon, along with small amounts of bio-oil condensates, tars and ash.


1.06 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. This temperature range is more relevant to woody charcoal than to charcoal made from bamboo, or other high cellulose fuels.  Woody charcoal has an interior layer of bio-oil condensates that microbes consume and is equal to glucose in its effect on microbial growth (Reference - PDF) High temperature char loses this layer and consequently may not promote soil fertility as well. (Source)


1.07 Can I substitute other forms of charcoal for biochar?

Absolutely. While the bio-oil condensates in biochar definitely play a role in soil fertility, charcoal without bio-oil condensates has been demonstrated to produce excellent results.  It is normally advisable to avoid industrial charcoal briquettes because the binders used during manufacture can add undesirable constituents. On the other hand, briquette binder can be innocuous See below (5.08) for information on how to receive some standardized rice-hull charcoal to conduct your own home research pot trials, and compare your results with others. 


1.08 Does charcoal break down in soil?

Charcoal is highly stable, however soil microbes do break it down, although at a very slow rate.  (More...)


1.09 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 charcoal from a biochar manufacturer, you can purchase any of a wide range of charcoal products suitable for amending soil, or you can make charcoal yourself. Hopefully when you do, you can pick up the knack of making charcoal which retains that condensate goodness.


2.01 Where can I purchase biochar?

Currently manufactured biochar is in short supply and is fully utilized for academic research projects.


The alternative is to purchase charcoal safe for use in the soil, which by broader difinitions, can also be regarded as biochar. In Britain charcoal is widely available in nurseries. In Australia, you can ocassionally buy "redhead" brand bamboo charcoal from supermarkets, or small bags of Horticultural Charcoal. Much cheaper is to ask your Charcoal BBQ Chicken shop for a 20K bag of Mulga charcoal for about $(AUD)30. This will need to be ground a little in a motar and pestle before use. It is excellent mixed in rough chunks in native orchids potting mixes.  It usually has a pH of 6 so can be used on acid loving plants.  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 What can I grow to make my own charcoal?

In Britain commercially available charcoal and is made from fuel produced by "coppicing" as has been done in British forests for more than 2,000 years. This is an ecologically sustainable use of forests and may contribute to the health and longevity of some British  forests.


2.03  Can I burn to bones to make charcoal for my garden?

Yes.  It appears that char derived from bones, along with char derived from other types of food wastes, was a component in Terra Preta de Indio.


2.04 How do I make my own charcoal?

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 simple burn barrel is a common starting point. A bottom ventilated, bottom lit burn barrel is a popular variation. If you have some basic tools for cutting metal, you can make barrel into a higher yielding kiln [Need Link].  These approaches can produce a fair amount of smoke and partially combusted gases.  Out of concern for air quality, many gardeners may prefer a less smokey approach.  

Covered pit: [Example1], [Example2]

Covered mound: [Example1], [Example2]



2.05 Isn't making a lot of smoke kind of un-neighborly?

Smoke can be a nuisance when it isn't your own. The considerate option when you are close in to a neighborhood is to use a (nearly) smokeless approach. In some jurisdictions, generating smoke is more than un-neighborly, its illegal.


2.06 What are some nearly smokeless approaches to making charcoal 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 principles (PDF). That doesn't prevent success using common materials and dead simple design. Take that same open barrel configuration, tweak the design per the aforementioned sophisticated principles, 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 top layer burning warms the wood below it releasing combustible and noncombustible gases which flow up into the charcoal layer. Glowingly hot charcoal has a wondrous 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.  They join with the other flammable gases released from the fuel.  These ignite as they mix 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). Insufficient oxygen below the combustion zone impedes loss of the charcoal despite the high temperature flame immediately above it.  This allows charcoal to build up faster than it is consumed, at least until the pyrolysis zone reaches the bottom of the fuel column. The downside is that, while wondrously clean burning in even the simplest configurations, a TLUD won't achieve its 20-30% charcoal-to-fuel yield potential without tricking it out to pre-heat and damper the intake. Thankfully, Folke Günther has come to our rescue. 


Folke Günther's simple TLUD-fired Retort.  A retort works by restricting the air supply to the target feed stock for the duration of the burn.  An outside heat source pyrolyzes the retort contents, small openings in the retort allow wood gas to escape, but restrict the flow of oxygen in.  While retorts are capable of very high yield efficiency, the open flame used to fire the retort is not as clean as can be achieved with a gasifier.  In small retorts, a further inefficiency is that wood gas generated from the retort can end up blowing by the combustion zone without being burned. Folke Günther's elegant solution is to combine a TLUD with a retort.  This is easily the simplest, cleanest burning and highest yielding method we know of to make garden-sized batches of charcoal.




2.07 What are some higher volume but less smokey approaches to making charcoal for the garden?

While TLUD's can get fairly large [Link needed], nobody appears to have tried a large TLUD/Retort.


A Large Drum Retort. [Expand] Use a drum with a fairly tight lid.  Place it on a stand over the hearth, and perforate the bottom of the drum to use the volatile gasses to fire the retort. [Example] The alternative is to run a piece of perforated pipe from the top of the drum to the firebox underneath.  [Example1] [Example2] [Example3] With the right fuel choice a large drum retort adapted to use the smoke will not only have a higher overall yield, it will also cut back dramatically on the smoke produced. 


The Wood Vinegar Kiln. [Expand] Not sure this is going to be a low smoke opacity alternative, but I would hope so.


2.08 How do I make charcoal that achieves biochar structure and chemistry?

Structure is a mostly of a function of the fuel type.  Hardwoods are currently preferred in this regard, but the understanding in this area is in flux.  The chemistry is better defined. The burn process should be controlled to retain condensates.  The tools to achieve this in a home-based setting are limited but also blessedly simple.  In all approaches it means restricting the air supply to slow the burn rate and achieve a low enough pyrolysis temperature that all the tars and volatiles produced don't gas off. Being willing to tolerate the inefficent combustion of smoke production, even if only near the end of the burn, certainly makes it easier to retain this volatile liquid component.  Being willing to dowse or damping off the burn before it makes the transition from a wood gas fire to a coal gas fire also helps. That can result in some brown char mixed in with the black char, but when it comes to garden soil, its all good. 


2.09 How much charcoal yield can I expect?

On a dry matter weight basis, as well as an energy basis, between 20 percent, for a TLUD with low superficial velocity (Source - PDF), and 60 percent, for a retort under ideal conditions. 40 percent is a reasonable goal. [Sources needed] 


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

2.11 What gases does pyrolysis produce?

The dominant combustible gases produced are carbon monoxide and hydrogen, along with a small amount of methane.  Carbon dioxide is also produced, especially with higher fuel moisture content. (Source)


2.12 How much heat does pyrolysis produce?

Pyrolysis itself is endothermic, thus requires an input of heat to be sustained. Heating value of the gas produced is 5,000 - 5,900 kJ/m³. (Source)  Comparatively less than the heating value natural gas, 33,320 to 42,000 kJ/m³ (Source - PDF), it is still substantial.


2.13 Is charcoal worth more as a fuel than as a soil amendment?

This can certainly occur. Its value as a soil amendment is highest when it is used in small amounts for carrying inoculate, or side dressing with starter fertilizer.  It is also of high value on those high value crops that are responsive to high fertilizer inputs. A basic spreadsheet can help in evaluating this.


2.14 Is charcoal worth more as a fuel than its value for offsetting greenhouse gases?

Maybe yesMaybe no.


3.0 What do I do with the charcoal once I've made it?

You can use freshly made charcoal as is, especially in 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 tree branch and something like 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 fill the biochar with biology and humic substances. 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, freshly made biochar may accelerate the composting process.

3.05  Will biochar harm the worms in my compost?

Composting worms have been observed to be unaffected below 50% charcoal content, above which reduced worm activity could 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 is the target application rate to achieve the effects of biochar?

From the data available to date, it appears that crops respond positively to biochar additions up to at least 50 Mg C ha-1, provided sufficient fertilizer is provided to prevent charcoal induced stalling (see 5.04). This is equivalent to 5 kg/m2 (1 lb/sf) and works out to a loose charcoal depth of about 5 cm or 2 in. (Calculation) Crops may show growth reductions at higher applications. For most plant species and soil conditions studied to date, this growth reduction did not occur even with 140 Mg C ha-1.

4.01 What materials combine well with biochar for application?

4.02 How is biochar generally used?

[normally , mixed in much the way you would prepare a planting bed by mixing in compost and other bulk organic amendments]

4.03 What is the normal application rate for biochar?

This is not well established

4.04 Are there benefits to deeper placement?

[better prevent leaching loss, mycorrhyzal highway below normal cultivation]

4.05 Are there benefits to using biochar as a mulch?

[better prevent denitrification loss of nitrous oxide, methane emmisions.  Heat up seedbed in spring]

4.06 I have a very limited supply of biochar, what is its highest and best use?

[Expand. seedball, sidedress with starter fertilizer, fungi innoculate]



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 adding biochar cause stalled growth?

Adding charcoal to soil can cause growth to stall where soil nitrogen levels are very low. That is probably not the case in most garden situations which have the advantage of compost, manure and kitchen scraps. Here's one tak

 The combination of returning bio-chars with high C/N ratios and abiotic buffering of mineral N may in some situations lead to low N availability to crops (Lehmann and Rondon 2005). In experiments in northern Sweden, however, increased nitrification and decreased ammonification was found after the addition of activated C to a pine forest (Berglund et al. 2004). It appears that the effects of bio-char on N dynamics in soils is not entirely understood. In a greenhouse study in Colombia, leguminous plants were able to compensate for low N availability with increased biological N2 fixation which is actually stimulated by bio-char additions (Rondon et al. 2004). Non-legumes, however, may require additional N fertilization to compensate for the immobilization. This is an undesirable effect as more N applications require more production of N fertilizers which is very energy-demanding (West and Marland 2002). (Source - PDF)



5.05 What can be done to prevent stalled growth ?

Three solutions are possible which are not mutually exclusive: (i) bio-chars are only applied to leguminous plants until sufficient N has built up to allow economically satisfactory production of non-legumes without a net increase of N fertilization; (ii) bio-chars are fortified with N for example in a composting step or during the production of bio-char in an energy production process (Lee and Li 2003); (iii) the amounts of applied bio-char are adjusted at a sufficiently low level to allow for N to accumulate and plant productivity to optimize. (Source - PDF)


5.05 Does biochar affect soil ecology?

The structure of the charcoal provide a refuge for small beneficial soil organisms from large grazers like earthworms.

Charcoal increases activity by mycorhizal fungi. It doesn't appear that this effect changes with the manufacturing temperature of the charcoal.

There is a long tradition in Japan of using charcoal as a soil improver. Nishio (1996) states “the idea that the application of charcoal stimulates indigenous arbuscular mycorrhiza fungi in soil and thus promotes plant growth is relatively well-known in Japan, although the actual application of charcoal is limited due to its high cost”. The relationship between mycorrhizal fungi and charcoal may be important in realising the potential of charcoal to improve fertility. Nishio (1996) reports that charcoal was found to be ineffective at stimulating alfalfa growth when added to sterilised soil, but that alfalfa growth was increased by a factor of 1.7-1.8 when unsterilised soil containing native mycorrizal fungi was also added. Warnock et al (2007) suggest four possible mechanisms by which biochar might influence mycorrhizal fungi abundance. These are (in decreasing order of currently available evidence supporting them): “alteration of soil physico-chemical properties; indirect effects on mycorrhizae through effects on other soil microbes; plant–fungus signalling interference and detoxification of allelochemicals on biochar; and provision of refugia from fungal grazers. (Source - PDF)

Low temperature woody charcoal (more so than grass or high cellulose) has an interior layer of bio-oil condensates that microbes consume and is equal to glucose in its effect on microbial growth (Christoph Steiner, EACU 2004) (Source)

Steiner et al (2008) observed that basal respiration (BR), microbial biomass, population growth and the microbe's efficiency (expressed by the metabolic quotient) increased linearly and significantly with increasing charcoal concentrations (50, 100 and 150 g kg-1 soil).  Application of smoke condensates (pyroligneous acid, PA) causes a sharp increase in all these, plus in substrate-induced respiration (SIR), as well as an exponential increase in population. We suppose that the condensates from smoke contain easily degradable substances and only small amounts of inhibitory agents, which could be utilized by the microbes for their metabolism. (Source)

Aggregation is improved:

The presence of bio-char in soils actively promotes the formation of aggregates through a greater abundance of fungal hyphae. Bio-char is able to serve as a habitat for extraradical fungal hyphae that sporulate in their micropores due to lower competition from saprophytes (Saito and Marumoto, 2002). (Source - PDF)


5.06 Does biochar improve plant growth?

5.07 How much improved plant growth can I expect?

You can expect that harvested weight will be, in most cases, observeably higher with a combination of char+fertilizer than you will achieve with the same amount of fertilizer alone. In some cases, the observed effect will be dramatic. Steiner (2007) reported a doubling of maize grain yield with fertilizer+char compared to fertilizer alone. Yields subsequently declined over the course of four cropping cycles, however, the decline was less with char than with without. Significantly, soil P, K, Ca, Mg remained higher in the char amended soil despite greater harvest removal. (Source - PDF). Considering the few places that biochar has been tried, it should not come as a tremendous surprise to find that your actual results may turn out to be less than dramatic than this.


Data on the effect of charcoal on crop yields is still rudimentary – only a limited number of crops grown on a limited number of soils have been investigated. The interactions between crop, soil type, local conditions, and biochar feedstock, production method and application rate will have to be studied in far more detail before large scale deployment of biochar as a soil amendment can be contemplated. Nonetheless, there is evidence that at least for some crop/soil combinations, addition of charcoal may be beneficial. (Source - PDF)



5.08 Is there a way for me to perform my own yield studies in a way that will be useful to others?

Certainly: CharDB, the international online open-source database of biochar soil amendment trials.

You will now be able to register your biochar soil amendment trials in a uniform format "CharML" that should facilitate comparisons between the different entries. This will hopefully lead to interesting new conclusions and a better knowledge on the fascinating world of biochar! (Source)


5.09 How much carbon dioxide does sequestered biochar offset?

5.10 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. However,


The effect of biochar production on nitrous oxide emissions is largely an unknown factor. Although there is a possibility that biochar additions may reduce N2O direct emissions from soils, and may also reduce indirect N2O emissions by reducing nitrate run-off, neither of these possibilities has been adequately demonstrated under a range of different agricultural conditions. (Source - PDF)





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