Introduction to Biochar in Agriculture

David Yarrow spreads biochar from white cart onto a garden bed he is preparing in Colrain, Massachusetts. The biochar was produced the day before in a homemade burner.

David Yarrow spreads biochar from white cart onto a garden bed he is preparing in
Colrain, Massachusetts. The biochar was produced the day before in a homemade burner.

Most readers of this journal have heard of biochar. It is hard not to, if you are an informed citizen concerned about contemporary issues. Biochar use has been heralded as a breakthrough development in soil health, crop production, carbon sequestration, environmental cleanup and pollution control, among other positive social purposes. Sales of the product have been tripling every year since 2008 and powerful players on the world stage, including China, have been studying its potential for solving varied growing problems.

We thought it might be time to take a closer look at biochar, especially its applicability to agricultural soils and enabling them to provide more and healthier food.

History of Char

Charring is produced by pyrolysis, which is heating biological material – agricultural residues, wood chips, virtually any organic matter – in a low-oxygen environment. The lack of adequate oxygen prevents normal burning, but enables volatile or semi-volatile materials to off-gas. Depending on the material and the temperature, other materials may be driven off and chemical changes will occur, potentially leaving just a porous carbon structure.

Black carbon is the generic term given to all solid chemical-thermal conversion products of carbonaceous materials, including charred residues. Biochar has been recently added as one material in the black carbon spectrum.

Char has an ancient history. The use of charcoal in cave paintings has been dated to 30,000 BC. Its use as a fuel began over 7000 years ago in the smelting of copper and more recently in smelting iron and producing glass. The Egyptians were pyrolyzing biomass 5000 years ago to form pyroligneous acid (composed of wood vinegar, tars and smoke condensates) for embalming.

Wood pyrolysis has been used for extracting valuable gases and oils for commercial purposes for hundreds of years. At its peak a standard distillation apparatus or retort could process 10 cords of wood within a 24 hour period. For these “industrial” purposes the gases and oils were the product, and char a secondary byproduct. In the 1800s, however, coal burning began replacing charcoal as an energy source and by the 1920s petroleum refining replaced biomass pyrolysis as the leading source of chemicals, distillates and volatiles.

One of the earliest examples of the agricultural uses of char is terra preta, the rich, fertile dark soils of the Amazon Basin which the indigenous people created by adding the remains of fire pits and middens full of bones and trash to the normally light tropical soils. Other similar char uses in agriculture date back to the early 1600s in Japan and potentially earlier in China. These, along with the natural deposition of char from forest fires, prairie fires, volcanoes, etc. have resulted in the widespread presence of it in soil organic matter.

Historically, char has been a waste product because the primary purpose of making it had been on optimizing the liquid and gas products of biomass for energy conversion and other uses, not on biochar for carbon sequestration. Thus despite the long research history of pyrolysis, more study is needed to optimize yields of biochar itself, and standardize its properties.

Biochar in agriculture

A number of studies have taken place concerning the effectiveness of biochar in promoting yields. According to those studies the positive impacts of biochar seem to outweigh the negative ones. A 2011 meta-analysis found an overall average yield increase of 10%, rising to 14% in acidic soils, from use of biochar. Approximately 50% of the compiled studies observed short-term positive yield or growth impacts from use of the material, 30% reported no significant differences, and 20% noted negative yield or growth impacts.

Results of some studies have found that biochar’s greatest potential might be in places where soils are weathered or degraded and fertilizer scarce, in part because it helps the soil to better retain any nutrients that it does have. One study of such degraded soils in western Kenya suggests that farms using biochar averaged 32% higher yields than controls.

According to a 2014 World Bank report biochar probably holds the most potential for small farmers in developing countries, not just because they are working with infertile soils most likely to benefit, but because biochar may be a key element of ‘climate-smart’ agriculture — practices that both help to mitigate climate change and reduce vulnerability to its effects.

Biochar additions to infertile soils have also been found to improve soil cation exchange capacity (CEC), the ability of soils to hold onto crucial nutrients. Applying biochar has been demonstrated to improve availability of potassium, for instance. But studies have found that not all biochar–soil combinations cause an increase in CEC. Experts suggest ways to “charge” biochar with fertility before applying it so it doesn’t absorb nutrients and water already in the soil, slowing plant growth. Many farmers soak it for a day or two in compost tea, fish emulsion, aged manure or other nutrients before application. They also recommend inoculating it with microbes before use.
The mechanisms by which biochar reduces vulnerability to climate change effects are somewhat speculative so far. Some researchers suggest the most important feature of the substance is its porosity and the presence of many connected spaces. Clays tend to be composed of flat grains and sand tends to be of circular grains. Even though clays can hold large amounts of water, that moisture has a hard time moving through the grains and reaching plant roots. But biochar is very amorphous so it creates many convoluted pathways that help to slow down drainage in sand and speed it up in clays. These same spaces provide protection for microbes from predation by larger creatures.

Section through a charcoal pile showing wood and soil cover.

Section through a charcoal pile
showing wood and soil cover.

Other scientists are exploring how biochars can mitigate climate change by cutting emissions of nitrous oxide, a greenhouse gas. According to a Chinese study, after biochar had been applied to corn and wheat fields once, nitrous oxide emissions declined over the following five crop seasons, a period of three years. Recent studies have also indicated a complex biochar and fertilizer interaction with respect to yield response. Others have observed that some biochars raise pH, particularly useful in regions like the northeastern US.

Many analysts report, however, that biochar’s effect on agronomic crop yield is variable, with crop production improvements ranging from negative to more than twofold, compared to controls. Some suggest that this inconsistency is because feedstocks, production methods and temperatures have not been standardized. Grass and nonwoody biomass biochar is more easily mineralized than wood-derived biochar, for instance, resulting in longer predicted soil residency times for wood biochar. From a soil fertility perspective, this increased mineralization from non-woody feedstocks could provide nutrient resources to plants. On the other hand, food waste biochar and that with high volatile matter contents have also suppressed plant growth.

Others feel that the material itself changes as it ages. Soil nutrient improvements may take some time to be observed. Delays may occur if the particular element is enclosed in a chemical ring structure that only slowly decays. Most of the existing studies have been limited to less than 3 years, which may not be enough time for the soil nutrient cycle to be fully affected.

Lastly, it is not clear whether biochar use will be economical in agriculture. Recommended application rates vary from 0.1 pound to 1 pound per square foot, or 2 to 22 tons per acre. At $1000 to $3000 per ton, that is a lot of expense for many growers. In the Third World particularly, poor soils and poverty often go hand-in-hand. Studies in Africa have found that few local farmers are willing to buy biochar at the price necessary to create it.

Environmental and Industrial Uses

Biochar’s capacity to bind to heavy metals in soil can keep them from reaching plants or entering water supplies. This interests governmental and other groups concerned about reclaiming land that has been destroyed by mining. The product’s large and numerous pores also give it a large surface area which enables it to tie up contaminants in polluted water and could remove chemical wastes and provide low-cost water treatment where little funding is available.

Scientists are just beginning to explore biochar’s potential for treating fluids in industrial settings. Oil and gas drilling fluids, print toners and paint products all have been suggested as markets for its ability to clean and process materials that flow.

Biochar Production

Plant responses to biochar soil additions are the net result of production conditions (feedstock types and pyrolysis methods) and postproduction storage or activation activities. These processes can confer unique properties on each batch of biochar, even from the same pyrolysis unit and biomass feedstock.

Reactor pyrolyzes biomass yielding charcoal, from which heat has driven off volatile gases. Some gases are condensed for oil, some used to predry the biomass or burned elsewhere.

Reactor pyrolyzes biomass yielding charcoal, from which heat has driven off volatile gases. Some gases are condensed for oil, some used to predry the biomass or burned elsewhere.

The raw feedstock biomass characteristics impart specific properties to the resulting biochar, such as ash content and elemental constituents like density and hardness. Currently char is being made from such diverse materials as chicken manure and nut hulls, wood scraps and plant residue. Such various sources of organic matter will provide quite different kinds of char.

Specific biochar nutrient concentrations may be greater or lesser than what was in the original feedstock nutrient concentration. Occasional volatilization and loss of nutrients during pyrolysis may be linked to higher production temperatures. The large range of operational maximum temperatures common to slow pyrolysis processes determines the extent of volatilization taking place and therefore the final composition of the resulting biochar.

There are also important differences in biochar quality not only as a function of the production process but also linked to postproduction storage or activation. Surface oxidation of black carbon in storage, even at ambient conditions, alters surface chemical groups, which correspondingly influences potential interactions with soil nutrient cycles. Activation can occur by simply cooling the biochar with water or exposing hot biochar to atmospheric oxygen during cooling.

However you make your char, you should cool it afterward – either with water or air. This cooling process, sometimes called “priming”, significantly alters the chemical and physical properties of the char. Unfortunately, researchers are still unclear how to best prime different chars to maximize their benefits.

Whether you own a large farm or tend a kitchen garden, you can produce your own biochar. The key is to heat waste biomass in a low or zero oxygen environment at temperatures ranging 200 to 800˚ C. Many small farms or homesteaders can only afford low-tech systems that generally produce less than a yard of char per burn.

The International Biochar Institute provides free open-source instructions for constructing many low-tech, small-scale biochar production systems. Materials range widely in cost. You could build a simple burner out of scavenged materials, spend $300 on a cone kiln, or pay upwards of $5,000 plus labor for an Adam Retort. Instructions and designs are available at as well as

Biochar in the Future

The possibility of engineering “designer biochars” for improving a specific soil deficiency is one direction in which biochar could develop. There have been efforts, for example, to impregnate biochar with various inorganic fertilizers to serve as diverse slow-release nutrient sources. Biochar could also be blended with specific composts, which could increase its value for both fertility and microbial inoculation.

Biochar is expensive as a carbon sequestration agent or as a soil supplement for crop yield improvements. The high production cost for biochar, however, could be offset if these specialty or boutique markets are more fully developed. Also, if biochar applications to other sectors besides agriculture are expanded, it could result in reduced costs of production.

Critics of biochar are quick to point out, and rightly, that the ultimate future of biochar depends on the sustainability of the feedstocks. In a completely ecological world there is no waste and every product or process must be evaluated for completing cycles efficiently. We need to carefully analyze use of biochar by this standard and see if there are more efficient uses for organic matter than charring and burying them.