Most gardeners have heard of biochar, but you may have a multitude of questions. What exactly is it? Is it the same as charcoal? Will it benefit my plants? How do I use it? Can it slow down climate change?

Biochar is a high-carbon material that has been produced by ‘pyrolysis’ – incomplete combustion of organic matter in the absence of oxygen. It’s very stable, so can provide benefits to the soil over an extremely long period.

The idea of using biochar first came from observation of the terra preta (‘black earth’) soils found in the Amazon basin. In these soils, the high concentrations of stable carbon were created by indigenous civilisations thousands of years ago(1). The soil retained nutrients and water, and supported good levels of biological life, unlike many of the impoverished soils in the area.

How is biochar made?

The pyrolysis process of making biochar creates three products: oil and gas, which can be used as both as fuel, and the biochar, a very stable carbon-rich material that can benefit the soil.

Although, in theory, biochar can be generated in a pit, this method is difficult to get right and often generates a lot of smoke and impurities. One of the most popular small-scale devices are the top-lit updraft stove that can be used for both cooking and producing biochar. These are available for use in gardens, although they are quite costly – typically around £800 – so more likely to be used in a community setting. The process can also be made in much larger industrial set-ups, such as a rotary or barrel kiln, allowing a continuous input of woody material to produce biochar and harness the natural gas produced.

How can biochar benefit my plants?

Biochar is very stable carbon with a diverse range of pore sizes. It behaves like a sponge, with a surface that is negatively charged, making it good at holding onto water and nutrients. This brings many potential benefits:
• Stabilises the soil structure for better root growth(2).
• Increases water retention on sandy soils but decreases runoff on clay soils(3).
• Provides a favourable porous environment for beneficial microbes to thrive(4).
• Reduces loss of nitrogen through leaching (5,6), but increases availability of difficult-to-reach nutrients such as phosphate(7).
• Reduces soil acidity(8).

How to use biochar to improve the garden

1.    Although biochar can provide many benefits for the soil, these seem to be realised under quite specific conditions. In practice, positive effects are most likely to be observed on soils that are impoverished with low levels of organic matter and reduced nutrients, especially phosphate, as found in tropical regions(9).
2.    Results from trials suggest biochar had most effect when applied at high rates of 1-2kg per square metre(9). Over a larger area of garden, this could be quite pricey as a 10kg sack can typically cost around £20. One of the most promising uses is to improve the composting process.
3.    Mixing it into compost at a rate of 3% increased the rate of composting by 20%,10 reduced emissions and improved nutrient retention(11).
4.    Things to watch out for
5.    Although biochar is similar to charcoal, we would not recommend adding the charcoal that you get for your barbecue to your soil, as this contains additives such as binders and accelerants that could be toxic.
6.    Home-produced charcoal from wood can also be less beneficial than biochar made under more controlled conditions.
7.    Users should be aware that biochar needs to be ‘preloaded’ with nutrients first, or it can lock up nutrients in the soil. Many products available to gardeners come labelled as pre-charged, but if using another form of biochar (e.g. one that is produced on site), it’s best first to mix it with a nutrient-rich source, such as compost, a few weeks before applying.
8.    Some forms of biochar will make the soil more alkaline, which may not be a good thing for acid-loving plants and can lock up nutrients(8).
9.    Applying biochar at too high a ratio in a potting mix can also raise its conductivity, causing toxicity to the plants(12).
10.    Biochar can be used in certified organic systems but it must meet specific purity criteria, so if your land is certified organic, ensure you check with your certifying body before using.

Wider benefits of biochar

As compost and biochar seemingly bring similar benefits to the soil, how do they compare? A main benefit of biochar over compost is its stability. Compost is broken down in the soil slowly, releasing carbon dioxide, whereas biochar has been shown to remain stable over a long period(13). In practice, they can be added to the soil together; the compost provides nutrients and added microbiology, and the biochar, provides longer-term benefits and stability.

It’s also been proposed that biochar can help mitigate rising CO2 levels by locking up carbon in the soil. One study calculated that if all residual organic matter from agricultural, livestock, forestry and water treatments was converted into biochar, global CO2 emissions could reduce by 6%(14). Such benefits would require major changes to working practices and infrastructure to facilitate local production, while taking advantage of other benefits such as energy generation.

There’s plenty of research work that demonstrates biochar’s potential. However, if it’s going to expand beyond a niche soil amendment, there’s still work to be done to understand how best to use it in a range of specific settings.

References

1.    Glaser, B. & Birk, J. J. State of the scientific knowledge on properties and genesis of Anthropogenic Dark Earths in Central Amazonia (terra preta de Índio). Geochim. Cosmochim. Acta 82, 39–51 (2012).
2.    Islam, M. U., Jiang, F., Guo, Z. & Peng, X. Does biochar application improve soil aggregation? A meta-analysis. Soil Tillage Res. 209, 104926 (2021).
3.    Edeh, I. G., Mašek, O. & Buss, W. A meta-analysis on biochar’s effects on soil water properties – New insights and future research challenges. Sci. Total Environ. 714, 136857 (2020).
4.    Bolan, S. et al. Biochar modulating soil biological health: A review. Sci. Total Environ. 914, 169585 (2024).
5.    Borchard, N. et al. Biochar, soil and land-use interactions that reduce nitrate leaching and N2O emissions: A meta-analysis. Sci. Total Environ. 651, 2354–2364 (2019).
6.    Lebrun, M. et al. Manure-biochar blends effectively reduce nutrient leaching and increase water retention in a sandy, agricultural soil: Insights from a field experiment. Soil Use Manag. 40, e13135 (2024).
7.    Glaser, B. & Lehr, V.-I. Biochar effects on phosphorus availability in agricultural soils: A meta-analysis. Sci. Rep. 9, 9338 (2019).
8.    Geng, N. et al. Biochar mitigation of soil acidification and carbon sequestration is influenced by materials and temperature. Ecotoxicol. Environ. Saf. 232, 113241 (2022).
9.    Joseph, S. et al. How biochar works, and when it doesn’t: A review of mechanisms controlling soil and plant responses to biochar. GCB Bioenergy 13, 1731–1764 (2021).
10.    Sánchez-García, M., Alburquerque, J. A., Sánchez-Monedero, M. A., Roig, A. & Cayuela, M. L. Biochar accelerates organic matter degradation and enhances N mineralisation during composting of poultry manure without a relevant impact on gas emissions. Bioresour. Technol. 192, 272–279 (2015).
11.    Guo, X., Liu, H. & Zhang, J. The role of biochar in organic waste composting and soil improvement: A review. Waste Manag. 102, 884–899 (2020).
12.    Nocentini, M. et al. Effects of biochar and compost addition in potting substrates on growth and volatile compounds profile of basil (Ocimum basilicum L.). J. Sci. Food Agric. 104, 1609–1620 (2024).
13.    Gross, A., Bromm, T., Polifka, S., Fischer, D. & Glaser, B. Long-term biochar and soil organic carbon stability – Evidence from field experiments in Germany. Sci. Total Environ. 954, 176340 (2024).
14.    Lefebvre, D. et al. Biomass residue to carbon dioxide removal: quantifying the global impact of biochar. Biochar 5, 65 (2023).