Welcome to a Gardening with Biochar FAQ!

... a work in progress...

When gardeners add carefully made charcoal 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?

From a scientific paper:

Biochar is ... plant biomass derived materials contained within the black carbon (BC) continuum. This definition includes chars and charcoal, and excludes fossil fuel products or geogenic carbon.

From the International Biochar Initiative:

Biochar is a fine-grained charcoal high in organic carbon and largely resistant to decomposition. It is produced from pyrolysis of plant and waste feedstocks. As a soil amendment, biochar creates a recalcitrant soil carbon pool that is carbon-negative, serving as a net withdrawal of atmospheric carbon dioxide stored in highly recalcitrant soil carbon stocks. The enhanced nutrient retention capacity of biochar-amended soil not only reduces the total fertilizer requirements but also the climate and environmental impact of croplands. Char-amended soils have shown 50 - 80 percent reductions in nitrous oxide emissions and reduced runoff of phosphorus into surface waters and leaching of nitrogen into groundwater. As a soil amendment, biochar significantly increases the efficiency of and reduces the need for traditional chemical fertilizers, while greatly enhancing crop yields. Renewable oils and gases co-produced in the pyrolysis process can be used as fuel or fuel feedstocks. Biochar thus offers promise for its soil productivity and climate benefits.

One hypothesis is that the best biochar is formed by low temperature pyrolysis, ideally at about 500 deg C (see Figure 1), with higher temperature pyrolysis producing a more traditional charcoal. This is a high enough temperature to achieve maximal surface area but also low enough temperature to achieve some 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 which could be used to improve soil quality.

Biochar enthusiasts generally agree that raw biochar needs to be processed further prior to being added to the garden. Composting, or soaking with compost tea, is commonly used to charge the pore volume with beneficial organisms and nutrients. Soaking in a nutrient rich solution (examples are urine or fish emulsion) prior to composting is accepted practice.

Figure 1: The properties of biochar greatly depend upon the pyrolysis temperature. Temperature effects on carbon recovery, CEC, pH and surface area. Lehmann (2007), Front. Ecol. Environ. 5:381-387. [1]

1.01 Should garden biochar have an activated carbon component?

Seems like a good bet that this will turn out to be the case. Activated carbon has a higher active surface area, sometimes an order of magnitude higher, compared to unactivated biochar. This is achieved at high temperature with the introduction of steam and/or chemicals in the absence of oxygen. There is growing interest in this potential characteristic of biochar. Attempts to achieve activated charcoal in a garden-scale kiln involve quenching, putting steam in contact with the charcoal at greater than 500 deg C. While steam activation is workable in an industrial setting [1], further study is needed. Activated carbon has yet to be confirmed as achievable in garden-scale pyrolysis, or even as a characteristic relevant to achieving Terra Preta.

1.02 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 emission (estimate 50%) (see 5.10 below)

Reduced fertilizer requirement (estimate 10%)

Reduced leaching of nutrients

Stored carbon in a long term stable sink

Reduces soil acidity: raises soil pH (see 5.01 below)

Reduces 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 .03 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 includes information on how to use small amounts of biochar in your garden to best advantage. [peer review requested on target application rate statement]

1.04 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.

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

Biochar is sold under a range of brand names such as the well-known global brand name and US registered trademark Agrichar™ which relates to Biochar produced from the BEST Energies proprietary slow pyrolysis process. Biochar 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. The effects of adding biochar in Terra Preta de Indio have persisted for millenia. Initial studies indicate we should not expect biochar to instantly recreate the full effect of Terra Preta de Indio, however, persistent partial effects are readily apparent for the duration of longer term studies. The degree to which Terra Preta de Indio is dependent on a community of soil biology unique to the Amazon is not known. In some ways, this is what you, the gardener, is going to attempt to discover. It is conceivable that the full effect of biochar in your garden will be seen by your grandchildren, but not by you.

1.06 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.

1.07 Do charcoal properties vary with source and temperature? What properties are important to the gardener?

Charcoal's chemical properties do vary with source and temperature. In the opinion of this author the single most important quality of charcoal to the gardener is the ability to lower acidity, also termed liming capacity or effective neutralizing power. This is easily measured in an agricultural laboratory as calcium carbonate equivalent (CCE). If you are growing acid-loving plants you will want a charcoal with negligible CCE, and purportedly this is true of Mulga (Acacia) wood, bamboo, and pine needle derived charcoal. If you are combating low soil pH and aluminum toxicity you will want a charcoal with substantial CCE. Oak and maple hardwood charcoal appear to have substantial CCE. Apparently Amazonian hardwood derived charcoal shares this characteristic. Raising soil pH has been identified as biochar's most important contribution to influencing soil quality in the context of Terra Preta. (Source)

1.08 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 (Christoph Steiner, Energy with Agricultural Carbon Utilization (EACU) Symposium, June, 2004) High temperature char loses this layer and consequently may not promote soil fertility as well. (Source)

1.09 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.

1.10 Does charcoal break down in soil?

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

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

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 ? How much should I pay?

Currently manufactured biochar is in short supply and is fully utilized for academic research projects. BEST Energies is purported to have a target price of AUD $200 /Mg (Mg is the same as a metric tonne) for Agrichar™. This is equivalent to USD $0.06 /lb, and would be very competitively priced.

The alternative is to purchase charcoal safe for use in the soil, which by broader definition, 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 20 kg bag of Mulga (Acacia species native to the Australia bush country) charcoal for about AUD $30 (about USD $0.30 /lb). 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 purportedly has a pH of 6.0 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. On sale, individual 20 pound bags have been available for about USD $0.50 /lb. For larger amounts, as in a shipping container, consider coconut shell charcoal, at times in mid-2008 available for less than USD $300 /Mg (about USD $0.14 /lb). 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 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 charcoal derived from bones, along with charcoal derived from other types of food wastes, was a component in Terra Preta de Indio. Bones are an excellent source of phosphorus, an element in limited supply in cellulosic charcoal. Initial impressions are that bone charcoal will have higher ash and CCE (See 1.07) than cellulose-derived charcoal.

2.04 How do I make my own charcoal?

Colliers the world over normally use either a covered pit [Example] or a covered mound (earth kiln) [Example] to make charcoal. This approach is about 10% to 15% efficient. 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. This usually involves a torch and snuff (or douse) routine. This approach is about 5% to 10. [Example] If you have some basic tools for cutting metal, you can make barrel into a higher yielding kiln [Example].

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. Gases produced from dry feedstock is easier to flare, a highly desireable feature. Flaring off the flammable smoke produced during charcoal production is a good practice for eliminating smoke as well as the methane (CH4) produced (See 2.09 and Gases).

An alternative use of the smoke produced is to condense it and retrieve it. (More information) Returning this liquid to the biochar, or using it alone, can have a stimulating effect on soil biology. (Source) This liquid is comprised of hundreds of compounds, ranging from thick tars to lighter alcohol, phenol and acid compounds. Terms applied to this liquid are bio-oil condensates, creosote, liquid smoke, pyroligneous liquor, pyrogenous acid, and wood vinegar. (More information)

If you want to use the heat generated to cook with, consider Robert Flanagan's Biochar Stove. [Example1] [Video] or Folke Günther's simple two barrel system. The inner can in Folke's system acts as a retort, 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. Retorts are capable of very high yield efficiency.

2.05 What are some higher volume and more efficient approaches to making charcoal for the garden?

A Large Drum Retort. Use a drum with a fairly tight lid. Place it on a stand over the hearth. A common, albeit questionable, adaptation is to 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 burn the smoke will not only have a higher overall yield, it will also cut back dramatically on the smoke produced.

A Masonry Retort Kiln. Check out the Adam-retort aka Improved Charcoal Production System (ICPS). At 3 cubic meter capacity, this is a tad too big to be considered a garden-level option but maybe someone can come up with a smaller sub-1M3 capacity size. If a smaller size can also achieve the 3M3's 35% to 45% efficiency, use the flare to help sustain the process, and be modified to capture some wood vinegar, it would be a huge boon to the individual biochar using gardener.

Adding a pyrolysis gas burner to a wood gas stove inpired approach. In a wood gas stove inspired approach (see TLUD page), wide separation between the pyrolysis front (and the charcoal accumulating upstrem from it) and the flare is desireable for retaining the charcoal. Work on a passive double chimney configuration looks very promising. [1] [2]. Active systems also show great promise. Adding a fan to the primary intake is a fairly common adaptation to control the rate of gas production in the range needed to achieve flare. More sophisticated approaches are being tried also:

For about [USD] $700 you can have an automated system that controls air supply to optimize combustion and minimize emissions. It can control a damper or an appropriately sized variable speed fan that injects air into the pyrolysis gases. This system is based on a Bosch wide band sensor which monitors stack oxygen levels and offers feedback control vis a programable logic controller (PLC) to maintain an excess air regime required for complete combustion. If the geometry is right products of incomplete combustion will be minimized and negligable. (Source)

2.06 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.

2.07 How much charcoal yield can I expect?

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

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

Steel and/or bricks. Nothing unusual really, and can use readily available materials. You can make a TLUD-esque design from standard 24 gauge snap lock black stovepipe. The heat will tend to defeat the snap lock so use a couple of sheet metal screws, or move up to welded seems. One advantage of the snaplock is that you can join sheets to get a larger diameter. Steel cooking oil cans and oil jober drums are particularly adaptable. For larger kilns, brick becomes essential. I'll see if I can dig up some information on designs and the refractory requirements of the brick and the mortar.

2.09 What gases does pyrolysis produce?

The dominant combustible gases produced are carbon monoxide and hydrogen, along with a small amount of methane (CH4), a powerful GHG, which comprises 2-3% of the produced gas. Carbon dioxide is also produced, especially with higher fuel moisture content. (Source) Flaring the producer gas eliminates the methane component. (More).

2.10 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 of 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 yes. Maybe 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. (Photos)

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. Fresh charcoal is hydrophobic, as well as reactive in terms of chemistry and nutrient profile. It also may be dominated by a size fraction that is larger than desired for the specific application.

3.02 What size should the biochar be?

Finer is better from a soil and plant views, but finer is worse from a dust control and air quality view. Considering that charcoal naturally degrades in soil to a fine size, the level of effort exerted to make fine charcoal is really driven by the application sytem employed: banding fine charcoal down the seed row will require fine sand grain sized particles, but there is no fundamental reason to prevent incorporating gravel sized pieces.

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. Be aware that soaking separates the soluble ash largely responsible for the calcium carbonate equivalency (CCE, the liming effect) and the salinity which can be a net benefit to acidic soils. Crushing it with a little moisture in it helps to control dust without removing the soluble ash content.

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

You would want to choose materials that would mitigate stalling [ See 5.04]: Compost tea, MiracleGro™ (Calculation), fish emulsion, urine, more on 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?

Raising soil pH is biochar's most important contribution to influencing soil quality. (Source) Soil pH mostly influences the relative availability of nutrients. At low pH, aluminum toxicity is particularly harmful to plant growth. Aluminum toxicity is an extensive and severe soil problem and biochar is the most available and obvious solution that we have to combat it. Soil phosphorus availability is highly dependent on soil pH range, and thus biochar can be used to substantially increase phosphorus availability in soils that are below the ideal pH range of 6.5 to 7.0. (More on biochar and 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 low. That is probably not the case in most garden situations which have the advantage of compost, manure and kitchen scraps.

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, Energy with Agricultural Carbon Utilization (EACU) Symposium, June, 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 How much carbon dioxide does sequestered biochar offset?

One kg charcoal, at 5% ash, offsets 3.3 kg carbon dioxide. On a home and garden scale, assuming that a gallon of gasoline releases 2.4kg of carbon and assuming that one wants to get to a negative carbon value then utilizing a 4kg bag of Cowboy brand charcoal as biochar comfortably offsets the carbon dioxide produced, with room to accomodate tha carbon footprint of delivering the charcoal to the garden. The production of the charcoal itself has no carbon footprint assuming the fuel used to make the charcoal was diverted from fate of decomposition. (Nod to Pangolin)

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 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)