Composting biochar may unlock stronger benefits for soil and plant growth than simply mixing it with compost
image: Composted biochar versus compost with biochar: effects on soil properties and plant growth
Credit: Irina Mikajlo, Thomas Z. Lerch, Brice Louvel, Jaroslav Hynšt, Jaroslav Záhora & Bertrand Pourrut
A study finds that “preparing” biochar through composting can make it far more effective for improving soil fertility and boosting plant growth.
Biochar, a carbon-rich material made by heating biomass in limited oxygen, has gained attention as a tool for improving degraded soils and storing carbon. But using biochar alone does not always help plants. In some cases, it can even slow growth because fresh biochar may contain unwanted byproducts from pyrolysis or temporarily hold onto nutrients that plants need.
Researchers have long suggested that combining biochar with compost could solve this problem. But an important practical question remained: is it better to mix finished biochar with compost just before applying it to soil, or to compost biochar together with fresh organic material first?
A study published in Biochar directly compared these two approaches. The research team tested three different biochars made from different feedstocks, including beech wood, mixed hardwood, and digestate with cereal straw. Each biochar was applied to soil in three ways: alone, mixed with green waste compost, or first composted together with fresh organic material to create composted biochar. The researchers then grew lettuce for three months in two soils with different fertility levels and measured plant biomass, soil chemistry, and microbial activity.
“Our results show that biochar is not just a material to add to soil. How it is prepared before application matters greatly,” said lead author Irina Mikajlo. “Composting biochar before use can reduce some of the drawbacks of fresh biochar and create a more biologically active amendment for plant growth.”
The differences were striking. Biochar applied alone did not improve lettuce growth compared with untreated soil. Mixing biochar with compost increased plant growth by about three times. But composted biochar increased plant growth by almost six times compared with the unamended control.
The composted biochar also performed better in several soil quality indicators. It helped maintain a more neutral soil pH, while biochar alone or biochar mixed with compost tended to shift soils toward more alkaline conditions. Soils treated with composted biochar showed higher dissolved organic carbon and total nitrogen, both important indicators of improved nutrient cycling. Microbial activity was also higher in composted biochar treatments, especially in the less fertile Luvisol soil.
The study suggests that the composting process changes biochar in useful ways. During composting, biochar becomes coated with organic matter, gains more available nutrients, and develops properties that support microbial activity. The process also reduced differences among biochars made from different feedstocks, making the final products more similar in function.
This finding could help farmers, land managers, and soil restoration practitioners make better use of biochar. Rather than viewing biochar as a stand-alone soil amendment, the study suggests it may be more effective when biologically “charged” through composting before it reaches the field.
The benefits were most noticeable in the poorer soil, indicating that composted biochar may be especially valuable for degraded or low-fertility soils where improvements in organic matter, nutrient cycling, and microbial activity are urgently needed.
The authors conclude that composting biochar before soil application can be a more powerful strategy than simply mixing biochar with compost. For sustainable agriculture, this could mean healthier soils, better plant growth, and more efficient use of organic waste resources.
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Journal Reference: Mikajlo, I., Lerch, T.Z., Louvel, B. et al. Composted biochar versus compost with biochar: effects on soil properties and plant growth. Biochar 6, 85 (2024).
https://doi.org/10.1007/s42773-024-00379-2
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About Biochar
Biochar (e-ISSN: 2524-7867) is the first journal dedicated exclusively to biochar research, spanning agronomy, environmental science, and materials science. It publishes original studies on biochar production, processing, and applications—such as bioenergy, environmental remediation, soil enhancement, climate mitigation, water treatment, and sustainability analysis. The journal serves as an innovative and professional platform for global researchers to share advances in this rapidly expanding field.