At the present time all areas of the worldwide economy are being incentivised and forced to innovate towards decreasing net carbon emissions. Net zero is the target and this means cutting carbon emissions to a level where remaining emissions are absorbed by trees and oceans. It’s suggested that for the planet to remain liveable, emissions must decrease by 45% by 2030, and be net-zero by 2050. And yet emissions continue to rise.¹ 

In 2007 the British entrepreneur Richard Branson announced the Virgin Earth Challenge Prize, a prize for the inventor of a technology to remove carbon from the atmosphere, with the stipulation that the technology will be expected to remove one billion metric tonnes of carbon per year for a total of ten years. The cash prize was $25m. Since then Branson has endorsed and invested in a variety of bids at sequestration, most recently backing Direct Air Capture technology in the UK.² Aviation is perhaps the most difficult sector to crack however, and Branson still continues to own a 51% stake in his flagship airline Virgin Atlantic.³ A carbon credit is defined as a piece of paper that allows a buyer to emit one tonne of CO2, and the price of a carbon credit in the EU Emissions Trading System (ETS) is taking steps towards €100 per tonne.⁴. 

We turn our minds now to the Amazon, and to a great civilization lost to the sands of time. When European settlers found Amazonia in the 1600s it was much to the misfortune of the then inhabitants who quickly died of disease, became nomadic or were enslaved. Although some tribes remained undiscovered even until the modern day ⁵ , almost all of their (agrarian) culture was lost. It is difficult to find information about exactly when, but at some point it was observed that in some areas of the Amazon there were dark soils with an exceptionally deep humus layer and brilliant horticultural value. The Portuguese named it terra preta meaning ‘black soil.’ 

The soil has been found in impressive plot sizes averaging 20 hectares right up to 360 hectares, covering a total of 0.1 to 0.3% of the Amazon Basin. This equates to 18,900 square kilometres or 1,890,000 hectares of land that was cultivated; illustrating the extent of original Amazonian civilization. 

It became apparent that terra preta was the result of application of charcoal to the soil. The fertility of terra preta stands in contrast to the more common Amazonian soil, which is remarkably low in fertility. The addition of charcoal should be distinguished from the slash and burn agriculture that is common in the basin. 

Normally when we burn wood we combust it in the presence of oxygen. The carbon leaves the process as carbon dioxide gas and the product that remains is ash. In contrast, charcoal manufacture pyrolyses the wood (through pyrolysis), in an oxygen free environment. The carbon is left as charcoal, which varies between 50% and 95% in carbon content.⁶ 

Charcoal was traditionally made by charcoal makers who used piles of wood covered carefully with earth to exclude oxygen. Kilns were also used and are still used today in some places, where the door of the kiln is closed up with stones or bricks to minimise oxygen ingress. Today very large industrial kilns are used however the amount of charcoal still made using old style kilns in third world countries may be surprising. 

Through archaeological work we know terra preta was made using a mixture of charcoal, bone, pottery and animal faeces. Biochar can be applied by hand but there are also methods to apply it at scale including broadcasting using a muck spreader. Biochar can also be ground to the desired granularity and applied mixed with water as a slurry. What quantity to apply is still  up for discussion and the mass of biochar can be misleading due to significant variation in water content. Application rates of 5-50 tonnes per hectare are suggested ⁷ but some research has shown that application of very large amounts of biochar may offer little additional benefit. 

Biochar consists mainly of organic (carbon containing) compounds; it is therefore surprising that its application has a considerable positive effect on the available nitrogen levels in soil. What’s even more conclusive is that it increases nitrates available in the soil after the application of manufactured nitrogen fertiliser by 50-95% ⁸. So our dependence upon nitrogen fertiliser needn’t stand in the way of biochar use. 

Soil depletion is a major problem with current agricultural practices.⁹ It is claimed that generating three centimetres of topsoil can take as long as 1000 years. Terra preta is reported to regenerate at a rate of 1cm per year. So if applied effectively biochar could be of serious use in fighting soil depletion. While unproven it is claimed that the microstructure of the biochar provides a habitat for types of fungi that are helpful for soil health and this may go some way in explaining its effectiveness. 

In Gabe Brown’s book Dirt to Soil: One Family’s Journey Into Regenerative Agriculture, he talks about how the soil he managed to regenerate took on the colour and consistency of chocolate cake; just like terra preta.¹⁰ Gabe Brown claims that using methods like mob grazing he managed to achieve several inches of topsoil improvement within as little as 20 years. Perhaps the ultimate solution is to combine mob grazing with biochar application. It is in fact possible to feed cattle biochar, although whether this constitutes a valid mode of application is unclear.¹¹ 

A key question is the effectiveness of carbon sequestration through applying biochar. A transatlantic flight emits somewhere in the range of 1 tonne of CO2. CO2 consists of two oxygen atoms and one carbon atoms. Oxygen and carbon atoms have atomic masses 16 and 12 respectively. Therefore CO2 is composed of 12/44 = 27% carbon by mass. A reasonable estimate for biochar is 70% carbon content. Therefore 1*0.27/0.70 = 0.39 tonnes = 390kg of biochar application is required to offset one tonne of CO2. In addition it may be the case that over time biochar causes increases in soil carbon content beyond that of the biochar itself.¹² 

A question is what the scale of the impact of biochar application could be. If we go beyond the absolute maximum and consider what would happen if all of the earth’s lumber was applied as biochar? Yearly world timber consumption is around 2000 million m3 

¹³. At a density of 0.5 tonnes per m3 this is 1000 million tonnes of wood. 1000/0.39 = 2564 million tonnes of CO2, or 2.5 billion tonnes.  Global CO2 emissions in 2021 were 33 billion tonnes. So even in the impossible circumstance we used world timber production in biochar application, we wouldn’t get near offsetting world carbon emissions. Alas biochar is no panacea. But it could be part of the solution. 

There are also some disadvantages of biochar. While biochar is claimed to have remediation properties for contaminated soils there is also the risk of biochar contamination with phytotoxic, cytotoxic, and neurotoxic substances; particularly if it is produced in the wrong way.¹⁴ Depending upon equipment and procedure there can be significant losses on application. If not combined with the soil correctly there can be wind loss in dry windy conditions. The substrate also has a liming effect, increasing the soil pH, so it may not be suitable for soils that are already alkaline. Another concern is fire safety, biochar is purported to have a flashpoint of 40C which, particularly with rising summer temperatures in the UK, is cause for moderate concern. 

There is uncertainty around whether biochar would be effective in non-tropical soils, as its traditional use has been in tropical areas. There is a paucity of information online regarding whether biochar has been used in the UK in a full agricultural setting. It would be a reasonable guess to assume someone has tried it somewhere. In the retail sector; as a product for people’s gardens it seems to have reached a considerable audience, with at least three companies selling biochar as a product for people’s gardens. 

And then there’s the cost. An estimate of the cost of biochar including application is £148 – £389 per tonne including delivery and application. At 20 tonnes per hectare and £300 per tonne this equates to £6000 per hectare. For a typical 200 hectare farm the cost is prohibitive at £1,200,000. There is another side to the coin however. With carbon credits valued at €80 (£68) per tonne ⁴, this takes the cost of biochar down to £232 per tonne. Carbon credit prices may continue to climb further. It’s also important to remember that biochar persists in the soil for hundreds or thousands of years, contrasting with periodic application of the usual pesticides and fertilisers. Indeed the Royal Horticultural Society advise gardeners that biochar need only be applied once.¹⁵ 

Applying one dressing of nitrogen fertiliser at £1000 per tonne and 0.1 t per hectare gives a rough cost of £100 per hectare. If nitrogen content in the soil is increased by 50% through biochar application it could be argued that this increase in nitrogen content is worth £50 per hectare each time nitrogen fertiliser is applied. Applying artificial fertiliser once per year gives us: £6000/50 = 120 years to pay the investment off. It’s still not looking any good. 

It is not entirely surprising that the cost of biochar is so high. The main feedstock for biochar is wood, a solid substance that takes many years to produce and is difficult to transport. In addition the manufacturing process for biochar is a batch process with a solid, irregular input. This stands in contrast to the continuous processes with liquid or gaseous feedstocks and intermediaries that characterise usual agri-chemical production. 

There is a byproduct of biochar production that may be of use, perhaps particularly in the fruit growing and forestry sectors. Pyroligneous acid, known as wood vinegar or liquid smoke, is the liquid condensed from the smoke of the charring process. The smoke leaves the furnace where the charcoal is made and travels up a diagonal pipe where it condenses due to the temperature gradient, the liquid then flows back down the diagonal pipe and is collected from an outlet around half the way down the pipe. When applied to the soil it is claimed to increase soil quality and soil growth. More advanced claims include that it changes the clustering properties of water molecules and therefore the physical and ionic properties of water in its uptake by plants. The insecticidal properties of pyroligneous acid stand out as being potentially the most useful and quantifiable. 

Given that the carbon sequestration effects are relatively modest in light of the available feedstocks and overall scale of CO2 emissions, we propose biochar principally as a solution to the problem of diminishing soil quality, rather than as a solve-all in the fight against global warming. Its widespread use becomes significantly more likely with more efficient industrial processes for its manufacture, as well as with possible production as a by-product of other large industrial processes, which could significantly decrease the cost of purchase. Recent development in the biofuel sector may provide opportunities for the production of large quantities of biochar which could then be used for agricultural purposes. A company called West Biofuels have developed a wood gasification process which produces biochar as a secondary product.¹⁶ A line of inquiry is whether furnaces like those used in glass production could use a gasification system like West Biofuels’ for heat, instead of oil or natural gas fired furnaces. 

References: 

1. UN. For a livable climate:
Net-zero commitments must be backed by credible action. https://www.un.org/en/climatechange/net-zero-coalition. Updated 2022.

2. Virgin. Virgin earth challenge. https://www.virgin.com/about-virgin/virgin-group/news/virgin-earth-challenge. Updated 2022.

3. BBC News. Branson’s virgin atlantic considers stock market listing. https://www.bbc.co.uk/news/business-58119588. Updated 2021.

4. Böhm S. There’s a massive bubble in the price of carbon – and yet it won’t bring down emissions any faster. https://theconversation.com/theres-a-massive-bubble-in-the-price-of-carbon-and-yet-it-wont-bring-down-emissions-any-faster-174821. Updated 2022.

5. Thomson H. John hemming: Encounter in the amazon. https://www.thewhiterock.co.uk/?page_id=2775. Updated n.d.

6. Ukranian Biofuel Suppliers. Chemical properties of charcoal. http://ukrfuel.com/news-chemical-properties-of-charcoal-23.html. Updated 2015.

7. Major J. Guidelines on practical aspects of
biochar application to field soil in
various soil management systems . https://www.biochar-international.org/wp-content/uploads/2018/04/IBI%20Biochar%20Application%20Guidelines_web.pdf. Updated 2010.

8. Zhang M, Liu Y, Wei Q, Gou J. Biochar enhances the retention capacity of nitrogen fertilizer and affects the diversity of nitrifying functional microbial communities in karst soil of southwest china. https://www.sciencedirect.com/science/article/pii/S0147651321009313#:~:text=Past%20research%20has%20figured%20out,restricting%20ammonia%20oxidation%20and%20nitrification. Updated 2021.

9. Arsenault C. Only 60 years of farming left if soil degradation continues. https://www.scientificamerican.com/article/only-60-years-of-farming-left-if-soil-degradation-continues/. Updated 2014.

10. Brown G. Dirt to soil: One family’s journey into regenerative agriculture. Chelsea Green Publishing; 2018:44.

11. Bettle J. Benefits of feeding cattle biochar. https://www.dorsetcharcoal.co.uk/post/benefits-of-feeding-cattle-biochar. Updated 2021.

12. Blanco-Canqui H, Laird DA, Heaton EA, Rathke S, Acharya BS. Soil carbon increased by twice the amount of biochar carbon applied after 6 years: Field evidence of negative priming. GCB Bioenergy. 2020;12(4):240-251. https://onlinelibrary.wiley.com/doi/abs/10.1111/gcbb.12665. Accessed Jul 25, 2022. doi: 10.1111/gcbb.12665.

13. Gresham House. Global timber outlook . https://greshamhouse.com/wp-content/uploads/2020/07/GHGTO2020FINAL.pdf. Updated 2020.

14. Han H, Buss W, Zheng Y, et al. Contaminants in biochar and suggested mitigation measures – a review. Chemical Engineering Journal. 2022;429:132287. https://www.sciencedirect.com/science/article/pii/S1385894721038663. Accessed Jul 25, 2022. doi: 10.1016/j.cej.2021.132287.

15. Royal Horticultural Society. Biochar. https://www.rhs.org.uk/soil-composts-mulches/biochar. Updated 2022.

16. West Biofuels. Bio-char & rotary gasifier. http://www.westbiofuels.com/biochar. Updated n.d.