In biocharBiochar is a carbon-rich material created from biomass decomposition in low-oxygen conditions. It has important applications in environmental remediation, soil improvement, agriculture, carbon sequestration, energy storage, and sustainable materials, promoting efficiency and reducing waste in various contexts while addressing climate change challenges. More discussions, particularly in online groups and practitioner circles, I often see strong statements such as: “Everyone knows you should never apply raw biochar,” or “Uncharged biochar will lock up nutrients,” or “Biochar is an empty sponge”. These comments are well intentioned and arise from a real concern that some biochars, particularly fresh, low-nutrient woody biochars, may temporarily immobilise nitrogen or reduce nutrient availability if applied to the wrong soil, at the wrong rate, or without considering crop requirements.

However, as a blanket statement, the idea that all raw or uncharged biochar is harmful is too simplistic.

Something that everyone seems to agree on in the industry is that biochar is not a one-size fits all approach and biochar need to be tailored for its end-use.  I agree with this but also think the phrase should be applied consistently. If biochar is not one-size-fits-all, then neither is the advice that all biochar must be charged before use.

A nice analogy I came across the other day related biochar to wine and pairing wine with different dishes. It is the same with soil types. An alkaline biochar may be valuable in an acidic soil where low pHpH is a measure of how acidic or alkaline a substance is. A pH of 7 is neutral, while lower pH values indicate acidity and higher values indicate alkalinity. Biochars are normally alkaline and can influence soil pH, often increasing it, which can be beneficial More can limit plant growth. The same biochar may be unsuitable for an already alkaline or saline soil. A high-ash biochar may be useful where potassium, calcium or phosphorus are limiting. The same material may be risky if the receiving soil already has high salinity or nutrient imbalance. A low-nutrient woody biochar may perform best when combined with fertiliser or compost, especially in the first season. But that does not mean every uncharged biochar behaves the same way.

Will uncharged biochar really lock up nutrients?

I find the phrase “biochar locks up nutrients” is also too broad.

In some cases, fresh biochar can contribute to short-term nitrogen immobilisation, particularly where the biochar has high carbon content, low nutrient content and is applied to a soil that is already nutrient-limited. This is a real risk and should not be dismissed. It is one reason why combining biochar with compost, fertiliser, manure, etc can improve early crop responses.

But nutrient immobilisation is not a universal property of biochar. Many biochars are not nutrient-empty. Manure-derived, poultry-litter, sewage-sludge and high-ash biochars can contain potassium, calcium, magnesium, phosphorus and other mineral nutrients.  This point has been made clearly in the scientific literature. El-Naggar² and colleagues emphasise that biochar nutrient content depends strongly on feedstockFeedstock refers to the raw organic material used to produce biochar. This can include a wide range of materials, such as wood chips, agricultural residues, and animal manure. More type and pyrolysis conditionsThe conditions under which pyrolysis takes place, such as temperature, heating rate, and residence time, can significantly affect the properties of the biochar produced.   More, and that biochar can be a valuable source of plant nutrients if the production process preserves those nutrients. That is important because it directly challenges the idea that an uncharged biochar is automatically nutrient-poor or agronomically inert. Therefore, whether biochar supplies, retains or immobilises nutrients depends on the specific biochar and soil context.

The nutrient “lock-up” argument also overlooks the liming effect of many biochars. In acidic soils, poor crop growth is often linked to low pH and reduced nutrient availability. In these systems, alkaline or high-ash biochars can raise soil pH and improve the availability of nutrients that were already present in the soil but not readily accessible to plants. In my own research on acidic soils, a high- pH biochar applied without additional fertiliser still improved tomato growth and yield compared with the control, although the combined biochar plus fertiliser treatment performed best overall¹. This is an important distinction. It shows that combining biochar with fertiliser can enhance performance, but it does not support the idea that biochar alone is inherently harmful. If biochar is not one-size-fits-all, then neither is the advice that all biochar must be charged before use.

Do We Need to Add the Microbes First?  Does biochar really pull in microbes from the soil?

Inoculated biochar is different from nutrient-charged biochar. Nutrient charging is mainly about adding fertiliser, compost, manure or other nutrient sources before application. Inoculation is about adding beneficial microbes to the biochar before it reaches the soil.

This is why I am cautious about statements that describe fresh biochar as “biologically dead.” Fresh biochar may not contain many microbes immediately after production, but that does not mean it is biologically useless. Once applied to soil, biochar can become colonised by soil microbes and may act as a habitat or refuge for microbial communities.

Applying biochar without microbial inoculation simply means that this colonisation happens gradually in the soil. That is not necessarily a disadvantage. The microbes that colonise the biochar will often come from the surrounding soil and may already be adapted to local conditions, including stresses such as heat, drought or salinity. In this sense, uninoculated biochar may become colonised by indigenous microbial communities that are naturally selected for that environment.

By contrast, deliberately inoculating biochar with beneficial microbes is not as simple as adding a microbial product to a porous material. The strains need to be compatible with both the target soil and the biochar itself. Biochar properties such as pH, electrical conductivity, ashAsh is the non-combustible inorganic residue that remains after organic matter, like wood or biomass, is completely burned. It consists mainly of minerals and is different from biochar, which is produced through incomplete combustion.   Ash Ash is the residue that remains after the complete More content, moisture content and pore structure can strongly influence microbial survival. Inoculated biochar can be highly valuable, but it requires careful strain selection, formulation and shelf-life testing. It should not be treated as a universal requirement for all biochar applications.

Evidence

There is evidence that biochar can deliver benefits without always needing to be pre-charged or inoculated. Meta-analyses show that crop responses to biochar are variable, but often positive, especially where soil constraints are present. Jeffery et al. found an overall increase in crop productivity from biochar application, with stronger responses in acidic and neutral soils and in coarse- or medium-textured soils. Ye et al. also showed that biochar combined with fertiliser can increase yields, supporting the idea that biochar can improve fertiliser use efficiency rather than simply “lock up” nutrients.

I have also seen that biochar alone increased lettuce yield by approximately 20% compared with the control in the first season in alkaline, arid field conditions, This does not mean biochar will always work on its own, but it does challenge the idea that uncharged biochar is automatically harmful.

The point is not that charging or inoculating biochar is bad. Both can be extremely valuable. Nutrient enrichment, co-composting and microbial inoculation can all improve biochar performance when used for the right purpose. My concern is with blanket statements. Biochar should not be treated as one material with one behaviour. The priority should be engineering biochar through feedstock selection and pyrolysisPyrolysis is a thermochemical process that converts waste biomass into bio-char, bio-oil, and pyro-gas. It offers significant advantages in waste valorization, turning low-value materials into economically valuable resources. Its versatility allows for tailored products based on operational conditions, presenting itself as a cost-effective and efficient More conditions, then matching it to the soil type, crop need and intended function.

References

¹ Nicholas, H.L., Devine, A., Robertson, I. and Mabbett, I. (2023) ‘The effect of faecal sludge biochar on the growth and yield of tomato (Solanum lycopersicum L.) cultivar Micro-Tom’, Agronomy, 13(5), 1233. doi: 10.3390/agronomy13051233.

El-Naggar, A., El-Naggar, A.H., Shaheen, S.M., Sarkar, B., Chang, S.X., Tsang, D.C.W., Rinklebe, J. and Ok, Y.S. (2019) ‘Biochar composition-dependent impacts on soil nutrient release, carbon mineralization, and potential environmental risk: A review’, Journal of Environmental Management, 241, pp. 458–467. doi: 10.1016/j.jenvman.2019.02.044.

Hale, S.E., Nurida, N.L., Jubaedah, Mulder, J. and Sørmo, E. (2020) ‘The effect of biochar, lime and ash on maize yield in a long-term field trial in a Ultisol in the humid tropics’, Science of the Total Environment, 719, 137455. doi: 10.1016/j.scitotenv.2020.137455.

Hossain, M.Z., Bahar, M.M., Sarkar, B., Donne, S.W., Ok, Y.S., Palansooriya, K.N., Kirkham, M.B., Chowdhury, S. and Bolan, N. (2020) ‘Biochar and its importance on nutrient dynamics in soil and plant’, Biochar, 2, pp. 379–420. doi: 10.1007/s42773-020-00065-z.

Jeffery, S., Verheijen, F.G.A., van der Velde, M. and Bastos, A.C. (2011) ‘A quantitative review of the effects of biochar application to soils on crop productivity using meta-analysis’, Agriculture, Ecosystems & Environment, 144(1), pp. 175–187. doi: 10.1016/j.agee.2011.08.015.

Ndoung, O.C.N., Figueiredo, C.C. de, Ramos, M.L.G. and Leandro, W.M. (2021) ‘A scoping review on biochar-based fertilizers: Enrichment techniques and agro-environmental application’, Heliyon, 7(12), e08473. doi: 10.1016/j.heliyon.2021.e08473.

Ye, L., Camps-Arbestain, M., Shen, Q., Lehmann, J., Singh, B. and Sabir, M. (2020) ‘Biochar effects on crop yields with and without fertilizer: A meta-analysis of field studies using separate controls’, Soil Use and Management, 36(1), pp. 2–18. doi: 10.1111/sum.12546.