Biochar on trial: what scientific research tells us so far

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Posted Jul 25 2019 by Andrew Rollinson of Blushful Earth
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Working wonders or potentially harmful? Dr Andrew Rollinson examines the scientific research surrounding biochar to date, including his own personal experiments.

In any gardening catalogue one will likely find a number of biochar products. Their selling points are the capacity to improve plant growth and provide long-term soil health. One recent academic author listed 26 possible benefits of this substance, such as: suppressing CH4 (methane) and N2O (nitrous oxide) release, increasing water and nutrient retention, countering land degradation/land reclamation, balancing pH, sequestering and storing atmospheric carbon, reducing aluminium toxicity, supporting nitrogen fixation, reducing plant uptake of pesticides, even as a means of processing plastics and refuse (though specifically how this is done is not clear) (1). The truth however is that knowledge remains at the research stage, and there are a large number of studies which report that biochar can also range from being totally useless to potentially harmful.

The name ‘Biochar’ is a modern invention. But, the substance is not the product of some new technological process. It is in fact merely charcoal – nothing more. Charcoal is given the name of “biochar” when it is applied to soil with the aim of improving said soil.

Charcoal is formed by pyrolysis, a process of slowly heating woody organic matter in an atmosphere of limited oxygen such that the wood does not combust. It results in the release of a complex mixture of volatile organic carbon (VOC) molecules as gas and oils, leaving behind the char. The char is formed from what is scientifically known as the “fixed carbon” component of the wood – the lignocellulosic parts which the plant used for its rigid structure and which do not split and volatise under high temperature alone. Many of these VOCs and other polycyclic aromatic hydrocarbons (PAHs) also become retained in the charcoal, along with incombustible elements such as metals which the plant has taken up during its lifetime.

Example of biochar in the Amazon rainforest

Example of biochar soils in the Amazon rainforest.

The history of biochar and its potentially amazing properties is an interesting story. It starts in the 1870s when European explorers visited the Amazon rainforest. There they discovered widespread patches of extremely rich soil isolated within regions where fertility was otherwise poor (2). They called these rich soil patches the terra preta do indio, translated from the Portuguese as “Black earth of the Indian”. In the 1950s and 60s they were brought to a modern audience by the pioneering work of Dutchman Wim Sombroek, and others have continued his work since then.

It was found from these 20th century investigations that the terra preta do indio were extremely long lasting (perhaps up to 7000 years old), and that they contained three times more nitrogen and phosphorous than surrounding land. It was also identified that they contained seventy times more carbon, from the principal ingredient – charcoal (2). Interestingly, similar ancient black earths have been found in different sites across the world.

As with most good stories there is also a mystery: currently no-one knows how the terra preta do indio were made, nor how it can be recreated. Human activity is responsible (either intentionally for agriculture, or as a by-product of land occupation); but – and for the sceptics of marketed biochar products – it is known that high crop yields do not just come by applying charcoal to soil (3). Therefore, the terra preta do indio were something more than a product of conventional shifting cultivation “slash and burn” activity. Current suggestions about their origin range from the burning of green vegetation under rainfall, to the inclusion of numerous ingredients such as human and animal excrement (part of ancient midden heaps), algae, local vegetable or aquatic animal oils (1,3).

Tests have been made to determine whether, during the char production process, the beneficial properties can be recreated by changes in temperature, residence time, initial feedstock type, and heating rate; but results have been inconsistent or inconclusive (3). Even where initial fertility improvements have been noticed, no long-lasting effect has been reproduced (4). There is something else still to be discovered.

Charcoal with PAH molecules

Charcoal with PAH molecules.

Evidence points to two properties of charcoal that seem to be beneficial: surface morphology – which is believed to influence water retention and encourage inhabitation by soil biota, and the presence of PAHs – which gives the biochar longevity and facilitates nutrient retention. But, there is plentiful evidence that biochar can have a negative impact through its harmful effects on soil biota such as earthworms, and because some PAHs are carcinogenic. This therefore gives concern that applying biochar to soil could pose a risk to the food chain, adversely disrupt soil ecology, and reduce rather than improve crop yields. On this there remains a scarcity of proof in research studies, with controversy even as to whether PAHs become bio-available or not. What is known is that PAHs are very resistant to leaving char – they can remain over geological time periods. Even tests to assess whether PAHs in charcoal can affect plant growth have been inconsistent or inconclusive, with generally an equal number of positive to negative effects (5).

Unsurprisingly, only a few countries have attempted to offer suggestions for legal limits of PAH in char when used as a soil amendment. The recent European Biochar Guidelines (6) have taken one of these and made a first attempt, but the scientific basis for this is not strong. Despite stating that the risk of PAH contamination is “…considered to be low, even if higher thresholds would be taken into account”, the text goes on to admit that “most standard methods for testing PAHs are unsuitable for biochar”.

Charcoal (biochar) produced in a downdraft gasifier

Charcoal (“biochar”) produced in a downdraft gasifier.

I have personally trialled homemade ‘biochar’ created under different pyrolysis conditions, and I have observed no improvements in plant growth. I did observe that there were no earthworms in the test area for many years afterwards. Concerning? For me, yes. Based on this and the other evidence which is listed above, the reader can hopefully make a better decision on biochar for themselves.

(1) Barrow, C.J. Biochar: potential for countering land degradation and for improving agriculture, (2012), Applied Geography, 34, pp. 21-28.
(2) Marris, E. Black is the new green, (2006), Nature, 442, pp. 624-626.
(3) Schimmelpfennig, S., Glaser, B. One step forward toward characterization: some important material properties to distinguish biochars, (2012), Journal of Environmental Quality, 41, pp. 1001-1013.
(4) Glaser, B. Prehistorically modified soils of central Amazonia: a model for sustainable agriculture in the twenty-first century, (2007), Philosophical Transactions of the Royal Society, 362, pp. 187-196.
(5) Rollinson, A.N. Gasification reactor engineering approach to understanding the formation of biochar properties, (2016), Proceedings A of The Royal Society, 2016, 472 (2192).
(6) EBC. European Biochar Certificate – guidelines for a sustainable production of biochar, version 8.2E of 19th April 2019, (2019), European Biochar Foundation: Arbaz.

(1) Sourced under Creative Commons by Simon on
(2) Sourced under Creative Commons from Wikipedia
(3) Author’s own image
(4) Author’s own image

Andrew RollinsonAbout the author

Dr Andrew Rollinson, author of our Gasification book, trained as a thermal decomposition engineer and specialises in small-scale gasification technologies. A member of the RSC and technical advisor for UKWIN, he works independently to encourage sustainability, promote energy independence and protect nature.