Biochar is Not Identical

You bought a bag labelled biochar, added it to your grow mix composition (what you might know as potting mix or substrate), and expected it to do what the label and that smooth talking influencer promised: hold moisture, buffer pH, feed your roots for years. Unfortunately, in a peer-reviewed comparison of four commercial biochars sitting on real garden centre shelves, researchers measured pH readings running from 6.58 to 8.78 and electrical conductivity levels eight fold apart, all from products sold with virtually identical bullet points.

If you read my earlier breakdown of charcoal in soilless mix, you already know biochar's slow-release nutrient trick mostly doesn't survive when used with a typical indoor mix. Developing real cation exchange capacity takes months of microbial colonization your grow mix will most likely never provide.

This is a different problem though. It isn't whether biochar's chemistry eventually activates indoors. It's whether the bag you were convinced to buy is even going to be made or created in a way that gives you a chance. It usually isn't, and the difference shows up in your mix's pH and salt levels the day you add it, long before any microbe gets involved.

Let's Get You Up to Speed

This UG article will help you understand:

• Why "biochar" describes a production process, not a single ingredient, and what that means for what's actually in your bag
• How feedstock (what got pyrolyzed) and pyrolysis temperature change pH, salinity, and porosity
• Why moderate biochar rates have consistently outperformed heavy substitution across the research
• Why biochar research has exploded since 2020, and why that growth doesn't mean the product is standardised
• What to actually check before adding a new bag of biochar to an existing mix
• Where UG's charcoal verdict on indoor activation ends, and where the standardisation problem begins

Got Things to Do? This is For You!

Biochar isn't one standardized product. It's a name for whatever comes out of pyrolyzing organic material (cooking the crap out of in near-zero oxygen so it breaks down chemically instead of burning), and the feedstock and temperature going in determine what comes out. A 2017 comparison of four commercial biochars from Switzerland and Germany measured pH between 6.58 and 8.78 and electrical conductivity between 55 and 439 µS/cm, a spread of nearly eightfold among products sold as interchangeable.

Research on Viola production found that 10% to 25% biochar by volume improved growth, while 50% reduced it unless fertilizer rates were adjusted to match. A 2026 scientometric review counted 973 published studies on biochar in growing media between 2011 and 2024, with a peak of 169 in 2024 alone, growth that reflects more untested combinations entering the literature, not a settled recipe. None of this changes my previous charcoal verdict that biochar's slow-release chemistry mostly stays dormant in a sterile indoor mix. It does mean that if you're adding biochar for its immediate structural or chemical effect, feedstock and inclusion rate matter more than the brand on the bag. Start at 10 to 20% by volume, and check your mix's pH and any visible salt crust on the surface within the first few waterings before adding more.

Does This Contradict What I Already Said About Charcoal Indoors?

No, and it's worth being specific about why not. The charcoal breakdown makes a mechanism-specific claim: biochar's cation exchange capacity , the property that lets it act as a slow-release nutrient reservoir, mostly develops after months of microbial oxidation forming carboxyl and phenolic groups on the carbon surface. An inert indoor grow mix, kept free of the soil biology that does that oxidizing, never gets there. Biochar sitting in a peat and perlite mix indoors is not going to become the nutrient sponge the marketing and popular influencer posts describe.

That's a claim about what biochar becomes over time. This article is about what biochar already is on day one. pH and electrical conductivity aren't things biochar develops slowly through microbial activity, they're baseline properties set during production, present in the bag the moment you open it. A biochar that ships alkaline because it was made from a mineral-rich feedstock will nudge your mix's pH the day you add it, with or without a single microbe ever colonizing it. A biochar carrying a heavy salt load from its feedstock will raise your mix's electrical conductivity immediately, putting stress on the same root zone that over-fertilizing does, for reasons that have nothing to do with slow-release chemistry.

So the two verdicts sit next to each other without conflict. Don't buy biochar for indoor plants expecting the long-term microbial nutrient-holding story, that part I already covered. But do pay attention to what you're adding to your grow mix right now, because that part varies by manufacturer far more than most product listings ever list on a bag or product marketing page.

What Actually Changes Between One Bag of Biochar and the Next?

Think of pyrolysis the way you'd think about roasting coffee. Same basic process, wildly different result depending on what bean you start with and how long you leave it in the roaster. A light-roast Colombian and a dark-roast robusta are still both technically coffee, but no one substitutes one for the other in a recipe and expects the same cup. Biochar works the same way. What went into the furnace, and how long it sat there, changes the finished product just as much as the fact that it went through pyrolysis at all.

Biochar isn't one ingredient. It's a name for whatever comes out the other end of a furnace, and the furnace conditions decide almost everything that matters. Feedstock sets the mineral load: manure, poultry litter, and other high-ash materials carry more calcium, potassium, and sodium into the finished product, which raises soil salinity risk in a way that cleaner woody feedstocks don't. Pyrolysis temperature does separate work: higher temperatures generally push up porosity and surface area, and also drive pH upward as base cations concentrate in the ash fraction.

The 2017 comparison mentioned above put numbers on this. Four commercial-grade biochars, two sourced in Switzerland and two in Germany, were tested side by side. One, made from woodchips, came in mild: pH 6.58, electrical conductivity of 55 µS/cm. The other three, made from bark and wood screenings, husk and paper fibre, and mixed forest wood, averaged pH 8.78 with electrical conductivity of 439 µS/cm, roughly eight times higher. These four products were marketed for the same use.

Stat: Among four commercial biochars tested in a 2017 comparison, pH varied by more than two full points and electrical conductivity varied nearly eightfold, all four products marketed for identical use.

A separate 2019 study by Chrysargyris, Prasad, Kavanagh, and Tzortzakis tested four more commercial biochars, this time on cabbage seedlings, and found the same pattern from a different angle: two of the four reduced seed emergence when added at 20% by volume, while the others maintained or improved growth at lower rates of 5 to 10%. Same nominal ingredient, same inclusion rate in some comparisons, different outcome, because the biochars themselves weren't the same material.

Does More Biochar Always Mean a Better Mix?

More biochar is not automatically better biochar. A study on Viola production tested hardwood biochar at 10%, 25%, and 50% by weight against a peat-based mix. Without added fertilizer, the 10% and 25% rates improved biomass, root length, and flowering. The 50% rate did the opposite: reduced growth and flowering. Once fertilizer was applied to compensate for the nutrient drawdown biochar causes at higher rates, the differences between rates disappeared, growth held steady regardless of biochar percentage.

The exact tipping point shifts by species and by the rest of your mix. What doesn't shift is the pattern: moderate rates help, heavy substitution needs correction to avoid net harm. A related study on Syngonium podophyllum in an outdoor environment found that a 20% by volume biochar-compost amendment to a peat-based substrate improved growth, with bulk density, air space, water-holding capacity, and total porosity all increasing measurably at that rate.

Myth Check: A product listing that says "safe up to 20%" is not the same claim as "more within that range is better." Across the research, growth benefits typically peak well below the stated ceiling, and pushing toward it without adjusting fertilizer is where the studies above saw harm show up.

Why Has Biochar Research Exploded Since 2020, and What Does That Actually Mean?

A 2026 scientometric analysis mapped 973 published studies on biochar in plant growing media between 2011 and 2024, using bibliometric tools to track authors, institutions, and thematic trends across the field. The growth curve is steep: a strong positive trend from 2020 to 2024, peaking at 169 publications in 2024 alone. China led output with 297 articles, followed by the United States with 122. The most frequent terms across the whole body of research were biochar, soil, compost, and growth, in that order.

It's tempting to read a graph like that as validation, the same way a thousand five-star cheap Amazon grow light reviews reads as proof a product works. That reading gets the shape of the evidence backwards. Publication volume climbing this fast doesn't mean researchers converged on one validated recipe and kept confirming it. It means the combinatorial space is still being mapped: different feedstocks, different pyrolysis temperatures, different inclusion rates, different crop species, tested against each other because the last combination's results didn't reliably transfer to the next one. More studies existing is evidence that the standardization problem is still unresolved, not evidence that it's been solved.

FYI: The 973 studies mapped in the 2026 scientometric review span 2011 through 2024. Growth was essentially flat before 2018 and became exponential only after 2020, which tracks with peat-alternative research accelerating industry-wide, not with biochar itself becoming more consistent.

So What Should You Actually Check Before Adding Biochar to Your Mix?

Start with what the marketing and label tells you, and treat silence as information too.

• Feedstock disclosure. Clean woody feedstocks tend to run milder in pH and electrical conductivity than manure-, litter-, or high-ash-derived biochars. If the listing doesn't say what was pyrolyzed, you're buying an unknown.
• A stated pH or EC value. Very few consumer listings include one. If yours doesn't, assume you're on the higher end of the range shown above until you've tested a small batch yourself.
• A moderate starting rate. The research consistently supports something in the 10% to 20% by volume range as a starting point, not a floor to build up from.
• Your existing mix's particle size and porosity. Biochar changes both. Run a new inclusion rate through the Soilless Porosity Calculator before committing a full repotting session to it.
• What you're actually hoping it does. If the goal is long-term nutrient holding indoors, recalibrate that expectation against the charcoal breakdown first. If the goal is structure and a manageable, disclosed baseline chemistry, that's a fair reason to use it, at a moderate rate, from a feedstock you can identify.

Pro Tip: Before adding more biochar than your last repotting used, check the listing for a stated pH or EC value. If neither is disclosed, treat the bag as an unknown quantity and start at the low end of 10%, not the high end of whatever range the seller advertises.

That single check, feedstock and a stated pH or EC, does more to predict how a bag will behave in your mix than any percentage on the packaging.

Nerd Corner: Pyrolysis temperature does more than raise porosity, it changes the carbon itself. As temperature increases, biochar's hydrogen-to-carbon ratio drops and the remaining carbon becomes more aromatic, meaning more of it locks into stable ring structures that resist microbial breakdown. That's good news for carbon stability and long-term structure, but it's a separate axis from pH and salinity, which track more closely with feedstock mineral content than with temperature alone. A biochar can be highly stable and still carry a heavy salt load, or mildly reactive and still fall apart structurally within a season.

None of this is about whether biochar is worth buying. It's about whether the specific bag in front of you behaves anything like the bag that produced the result you read about (if they were even accurate). Usually, without checking, you can't know. The label rarely tells you what actually determines the outcome. The feedstock and the furnace conditions do, and both of those are usually left off the packaging entirely.

FAQ

Sources and Further Reading

• Biochar in Plant Growing Media: A Scientometric Analysis (2011–2024). Journal of Soil Science and Plant Nutrition, 2026. https://doi.org/10.1007/s42729-026-03345-y
• Chemical characterization of biochar and assessment of the nutrient dynamics by means of preliminary plant growth tests. Journal of Environmental Management, 2017. https://www.sciencedirect.com/science/article/abs/pii/S0301479717303614
• Chrysargyris, A., Prasad, M., Kavanagh, A., & Tzortzakis, N. Biochar Type and Ratio as a Peat Additive/Partial Peat Replacement in Growing Media for Cabbage Seedling Production, 2019. https://agris.fao.org/search/en/records/67599375c7a957febdfce2f6
• The Negative Effects of High Rates of Biochar on Violas Can Be Counteracted with Fertilizer. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8875082/
• Biochar or Biochar-Compost Amendment to a Peat-Based Substrate Improves Growth of Syngonium podophyllum. Agronomy, 2019. https://doi.org/10.3390/agronomy9080460
• UG: Charcoal in your Soilless Mix?
• UG: Your Soilless Grow Mix
• UG: The Worm Castings Myth
• UG: Plant Roots & the Root Zone
• UG: Indoor Overwatering Myths: Oxygen & Porosity
• UG Tool: Soilless Porosity Calculator