As you may already know, biochar in compost has major benefits.
We’ve written about it in depth before, but in short these benefits include:
- speeding up compost
- improving the final quality of the compost
What we haven’t known until now is when to add it!
However, a recent study has tested two different timings, leaving us with clear guidance as to the best time to add biochar to compost.
What the study examined
The study noted one key problem with composting green waste: it’s slow to break down due to high lignin, cellulose, and hemicellulose content.
The authors wrote that the process can take 120 days.
(However note that in reality, for many home composters, it would take longer than this.)
They also noted that in the process the compost loses many nutrients.
Biochar is often used as a solution to this problem.
This porous material helps retain nutrients, improve microbial activity and speed up the breakdown of organic material.
However, research hasn’t indicated at what point we should add biochar to compost.
The two solutions tested
The researchers created compost mixes with the following ingredients:
- 48% green waste
- 35% Food Waste
- 13% sawdust
- 2% phosphate rock
- 2% biochar
In the first mix (T1) the biochar was added at the start of the compost process during the mesophilic (cool).
In the second mix the biochar was added during the thermophilic (hot) phase.
What were the results?
The results clearly showed that the best time to add biochar is in the mesophilic (cool) phase before it gets hot.
The benefits of adding the biochar early included:
- higher temperatures (52.5°C vs. 47.7°C)
- a longer thermophilic (hot) phase (4 days vs. 2 days)
- faster overall composting (but only by 2 days)
- increased organic matter breakdown (58% vs. 53% in T2)
Both the higher temperature and longer hot phase should help reduce germs and kill more weed seeds.
The T1 compost also had better nutrient value:
| Nutrient | T1 (Biochar in Mesophilic Phase) | T2 (Biochar in Thermophilic Phase) |
| Total Organic Carbon (TOC) | 28.3% | 15.8% |
| Total Nitrogen (TN) | 1.91% | 1.17% |
| Total Phosphorus (TP) | 4.69% | 4.29% |
The T1 compost also had improved microbial activity.
Key bacteria for breaking down the compost (Streptomyces and Nonomuraea) were found at a higher level in the T1 pile.
These bacteria are known for breaking down tough materials like lignin and cellulose.
Meanwhile, the second pile contained microbes (Chloroflexus and Roseiflexus) that are linked to nitrate reduction, which in turn can lead to nitrogen loss.
The T1 pile was also more fertile and proved to be significantly better for germination.
Takeaways for composters
You don’t need to add biochar to create compost, but if you do want to optimize your composting with biochar, this study gives clear guidance as to when to do so.
If you do decide to use it, for best results add 2% biochar at the start of the composting process for faster composting and improved compost results.
Read more
Source
Oviedo-Ocaña, E.R., Soto-Paz, J., Parra-Orobio, B.A. et al. Effect of Biochar Addition in Two Different Phases of the Co-Composting of Green Waste and Food Waste: An Analysis of the Process, Product Quality and Microbial Community. Waste Biomass Valor (2025). https://doi.org/10.1007/s12649-024-02878-6