What is biochar and how does it improve growing media?

Biochar is a carbon-rich material produced by heating organic matter such as wood or plant residues at high temperatures in a low-oxygen process called pyrolysis. In growing media, it improves water retention, nutrient availability, and microbial activity, making it a valuable amendment for professional greenhouse cultivation and home gardening alike. The sections below address the most common questions growers have about biochar, from how it works to how much to use and what role it plays alongside peat and other substrate components.

How does biochar actually work in growing media?

Biochar works in growing media by contributing a highly porous structure that increases the substrate’s capacity to hold water and nutrients while improving aeration. During pyrolysis, organic matter is heated at temperatures typically between 450 and 600 degrees Celsius in the absence of oxygen, converting carbon into a chemically stable, charcoal-like form. The resulting material decomposes very slowly and can remain active in a substrate for extended periods.

The physical structure of biochar is central to how it functions. Its porous surface provides a large area for nutrient adsorption and exchange, and the functional groups present on that surface give biochar a measurable cation exchange capacity (CEC). This means it can bind and retain positively charged nutrients such as potassium, calcium, and magnesium, making them available to plant roots rather than allowing them to leach out of the substrate.

Biochar also influences the chemistry of the growing medium. It can raise the pH of acidic substrates, which in turn affects how readily nutrients are available to plants. Additionally, its porous environment supports beneficial microbial activity, including mycorrhizal fungi, which contribute to a more biologically active substrate. The combination of physical, chemical, and biological effects is what makes biochar in growing media a multifunctional amendment rather than a single-purpose additive.

What are the main benefits of biochar for plant growth?

The main benefits of biochar for plant growth include improved water retention, stronger nutrient uptake, greater resilience to drought stress, and support for beneficial microbial communities in the root zone. These effects result from biochar’s porous structure and its chemical properties, and they are most consistent when biochar is matched to the right crop, substrate, and inclusion rate.

Water management is one of the most practically significant benefits. Biochar’s porosity is broadly comparable to that of perlite, but it holds water more effectively, an advantage in substrates where moisture consistency is critical. Research into biochar-amended substrates has shown improved resilience to water stress in horticultural crops, including tomato, which is relevant for professional growers managing irrigation efficiency in greenhouse environments.

Nutrient retention is another key advantage. By increasing the CEC of a substrate, biochar helps keep nutrients in the root zone rather than allowing them to wash through. This can reduce the frequency and volume of fertiliser applications needed, which is a practical benefit for commercial growers aiming to optimise input use.

Disease suppression is a reported but dose-dependent benefit. Studies have found that biochar addition at low concentrations can help suppress fungal pathogens including Botrytis cinerea and Fusarium species in crops such as strawberry and tomato. However, this effect is not universal, it is influenced by the type of biochar, the concentration used, and the specific pathogen involved. Higher concentrations do not necessarily produce stronger suppression, and in some cases may be ineffective. Growers should treat disease suppression as a conditional benefit rather than a guaranteed outcome.

One further property worth noting: because biochar is produced at high temperatures, it is sterile at production, free from weed seeds, plant pathogens, and pesticide residues, a practical advantage when introducing a new material into a controlled growing environment.

What types of biochar are used in horticulture?

The most common types of biochar used in horticulture are produced from lignocellulosic feedstocks such as wood chips, tree bark, straw, and agricultural residues including rice husks and nut shells. The feedstock and the conditions under which pyrolysis is carried out both determine the physical and chemical properties of the finished biochar, which is why there is no single universal horticultural biochar.

Wood-based biochar is particularly well suited to professional horticulture. It offers a high surface area, low ash content, and good structural stability, which makes it compatible with peat-based substrates without dramatically altering their pH. Research comparing different feedstocks has found that wood-derived biochar generally provides the greatest surface area, which directly supports its water and nutrient retention functions.

Agricultural residue biochars, such as those produced from rice husks or corncobs, tend to have a higher CEC than wood-based biochars, which can be an advantage for nutrient retention. However, they may also carry higher electrical conductivity (EC), which can create salinity risks in sensitive crops if the biochar is not pre-treated or washed before use. This is particularly relevant when sourcing biochar from non-woody feedstocks.

In professional horticulture, feedstock selection can also be guided by circular principles. Kekkilä-BVB’s circular raw materials programme explores how materials such as biochar, reed canary grass, sphagnum moss, and wood fibre can be incorporated into substrate blends, reflecting a broader approach to sourcing ingredients that complement peat and coir rather than replacing any single component wholesale.

Quality standards are also relevant here. The European Biochar Certificate (EBC) is a voluntary industry standard that provides quality assurance for biochar used in agronomic systems across Europe. Under EU Regulation 2019/1009, pyrolysis materials including biochar are recognised as a component material category in CE-marked fertilising products, provided they meet specified criteria for nutrient content and absence of harmful substances.

How much biochar should be added to a growing substrate?

For most professional greenhouse applications, an inclusion rate of around 10 to 20 percent biochar by volume is a practical starting point. This range improves water retention, nutrient availability, and substrate structure without introducing the salinity or pH risks associated with higher rates. The optimal amount, however, depends on the specific biochar type, the crop being grown, and the other components in the substrate blend.

Research consistently shows that low to moderate inclusion rates produce the most reliable results. Studies in peat-based container media found that around 5 percent by volume produced meaningful improvements in early plant growth, with limited additional benefit from increasing to 15 percent. Kekkilä-BVB’s guidance aligns with the broader research consensus, noting that growing media may contain approximately 10 to 20 percent biochar-containing material by volume, and that because biochar is highly stable, its effects accumulate over time rather than diminishing after a single season.

Higher rates carry real risks. When biochar was mixed with peat at a ratio approaching 70 percent by volume, osmotic stress from elevated EC reduced plant growth and flowering in some crops. Alkaline biochars can also raise substrate pH beyond the 5.4 to 6.5 range that suits most greenhouse crops, making nutrients less available. Mixing alkaline biochar with acidic components such as peat helps manage this, but it reinforces the importance of testing before scaling up any new biochar blend.

The practical takeaway is that there is no single universal rate. Biochar type, feedstock origin, particle size, and the target crop all influence the outcome. Starting conservatively and adjusting based on observed plant response is the most reliable approach for growers introducing biochar into an existing substrate formulation.

Does biochar replace peat or other substrate components?

Biochar does not replace peat or other substrate components outright. It functions best as a complementary addition to existing blends, improving specific properties such as water retention, CEC, and biological activity. Research supports its use as a partial substitute for peat at rates up to around 25 percent by volume, but it is not a like-for-like replacement, and peat remains the primary constituent of professional growing media across Europe.

This distinction matters practically. Peat provides the structural consistency, reliable water-holding capacity, and predictable pH that professional greenhouse cultivation depends on. Biochar adds value by enhancing specific properties within that matrix, it blends well with peat, coir, and wood fibre, improving the overall performance of the substrate without requiring a complete reformulation. The EU CAP Network consistently describes biochar’s role in growing media as a partial, not full, substitute function.

Kekkilä-BVB’s research and innovation work reflects this framing. The company’s circular raw materials programme positions biochar as one of several complementary materials being explored alongside reed canary grass, sphagnum moss, and wood fibre, each contributing different properties to substrate blends depending on the crop and growing context. This approach treats substrate development as an optimisation challenge rather than a substitution exercise.

One practical consideration worth noting: when biochar partially replaces peat, the carbon stored in that biochar is not released back to the atmosphere as quickly as it would be from decomposing organic matter. This is a measurable property of biochar’s stability, though any specific carbon claims should be grounded in verified, product-level data rather than general assumptions about biochar as a category.

What is the environmental impact of using biochar in horticulture?

The environmental impact of using biochar in horticulture is primarily associated with carbon stability: the carbon locked into biochar during pyrolysis is converted into a form that can persist in substrates and soils for centuries rather than returning to the atmosphere as carbon dioxide within years. This makes biochar a recognised carbon dioxide removal technology, as noted in IPCC assessments of negative emission approaches.

The carbon benefit is real but context-dependent. Kekkilä-BVB describes the mechanism clearly: the carbon in biochar originates from atmospheric CO₂ captured by plants during growth, and pyrolysis stabilises it. When incorporated into growing media, part of that carbon can remain stored for extended periods. However, the scale of this benefit depends on the feedstock used, research indicates that waste wood biochar offers the strongest carbon sequestration potential, followed by crop straw biochar, with livestock manure and sewage sludge biochars lower on the scale.

Environmental outcomes are not uniformly positive across all conditions. Some research has found that biochar application can influence soil gas emissions in ways that are feedstock- and application-rate-specific. This means environmental performance should be assessed at the product and application level rather than assumed to be beneficial by default. The Growing Media Europe Environmental Footprint Guideline provides a methodology for life cycle assessment of growing media constituents, which offers a framework for making these comparisons in a structured and comparable way.

From a regulatory standpoint, claims about biochar’s environmental properties must be specific and verifiable. Under EU Directive 2024/825, enforceable from September 2026, vague terms such as “eco-friendly” or “carbon neutral” applied to biochar products without substantiated, product-level evidence are prohibited. Growers and producers working with biochar-amended substrates should ensure that any environmental claims they make are grounded in measurable, verifiable data from their specific products and processes. For growers looking to understand how biochar fits within a broader responsible production approach, the focus should be on documented performance rather than general category claims.

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