Biochar: Does It Work?

Biochar sales copy leans on the Amazon, where old patches of dark, fertile terra preta soil were built up with charred wood and plant waste. It adds a second pitch: biochar is made by heating wood or plant waste with little oxygen (pyrolysis), so its carbon resists decomposition for centuries and spreading it is "carbon-negative gardening". The carbon claim holds up. The fertility and water claims are disputed, and the evidence is strongest on sandy, low-fertility soils, whereas about two-thirds of Scotland's land is podzol, peat or gley.

What biochar is

Biochar is organic material (wood, crop residue, manure) carbonised at 300 to 1000°C in the near-absence of oxygen. Pyrolysis drives off bio-oils and gases and leaves a solid residue that is at least 80% carbon. Feedstock and temperature change the pH, porosity and nutrient content, so "biochar" on a bag does not tell you much on its own, and a study that found a benefit used a specific biochar, made a specific way, at a specific rate, on a specific soil.

Follow the application rate on the bag, since it is tied to how that biochar was made, and treat a first bag as a trial on one bed, not a purchase for the whole plot. The European Biochar Certificate is an industry quality standard worth looking for on a bag. It is not a government or scientific body, but it is more than a supplier's own claims.

Grading the four claims

Fertility: disputed, and soil-dependent

Some studies show no plant effects from biochar on its own, and some show harm. The field data points in different directions depending on soil:

Utah State's trials, on loams, silt loams and a loamy fine sand, concluded there was "no consistent benefit of biochar to yield, crop quality, or available soil water", and at about $3,580 an acre wood biochar was "not an economically feasible option for growers without financial incentives or assistance". The systematic review's sandy-soil gain is the strongest positive figure, but the review itself cites earlier work saying sandy-textured soils are more likely to benefit in the long run than other textures, so heavier soils may respond less.

That split is why fertility gets a disputed verdict. The trials were on food crops, and nothing here suggests risk near vegetables; the pH caution below concerns acid-loving ornamentals and fruit.

Water retention: disputed

The marketing case is that biochar's porous structure holds water for roots. A grower in a US advisory article reported irrigating beds with 10% biochar significantly less than beds with compost alone. But the Utah State trial measured soil water directly across three years and several crops in an irrigation-dependent region, where an effect should show most clearly, and found no consistent benefit. A single grower's account and a multi-year field measurement disagree, so the claim stays disputed.

Carbon sequestration: true

This is the part of the pitch with the least dispute. Biochar's carbon is chemically stable in a way fresh organic matter is not, and the UK Biochar Research Centre at the University of Edinburgh describes biochar as a leading carbon dioxide removal technology. Whether that adds up to a meaningful climate benefit in one garden is a separate question from whether the mechanism is real. It also depends on the feedstock being waste, sustainably sourced and traceable, with FSC certification one marker to check. Biochar made from wood that would otherwise have grown as a tree has a much weaker carbon case.

Charging biochar first: true

Raw biochar can tie up nitrogen as microbes start working on it, so straight-from-the-bag biochar may temporarily hold back yield. Mixing it with compost, manure or nutrients and letting it sit for a few weeks before use is the standard fix, and plants are worth watching for signs of nitrogen deficiency over the first few months. Pre-charged, nutrient-enriched products exist to skip that step. Uncharged biochar has no nutrient value of its own.

Acid-loving plants, whatever your soil

Biochar can raise soil pH, especially a high-ash biochar, and the RHS warns that adding alkaline biochars will be detrimental to acid-loving plants such as rhododendrons. Rhododendrons, azaleas, blueberries and heathers are common in Scottish gardens, so check what a bed is growing before adding biochar near it.

Two more checks before buying. Feedstock from treated or painted timber, or biochar made at poorly controlled temperatures, can carry contaminants, so a supplier who cannot say what feedstock and temperature they used is one to be cautious of. Some biochar is also high in salts. Do not use ash from a barbecue or charcoal grill, which may contain chemical residue from lighter fluid, and do not treat barbecue briquettes as a substitute. If you make your own from manure or straw, persistent herbicide residues in that material can survive and damage sensitive crops such as tomatoes, potatoes and beans.

The Scotland angle

Peat covers about 22.5% of Scotland's land area and gleys about 20.6%, concentrated in the Highlands and Southern Uplands, while the Central Valley is mostly mineral soil. Peat soils are generally acid and not very fertile, and clay-heavy ground generally needs substantially more lime than sandy or loam soil to shift pH by the same amount. Most biochar trial data comes from sandy or loam soils, the texture that gave the clearest yield gain in the review, which is not what most Scottish land is.

One argument cuts the other way: because biochar raises pH much as lime does, it could in principle do some of the work a lime application already does on acid ground. A three-year (2011 to 2013) experiment tested biochar in pots filled with sandy clay, sandy silt loam and clay loam taken from a James Hutton Institute farm near Dundee, measuring plant biomass, carbon cycling and soil biodiversity. It is a pot experiment, not a garden or field trial, and its published dataset does not summarise findings anywhere a gardener can easily check. Whether biochar's pH effect helps Scottish clay and peat in practice is an open question.

On water, Scotland is wetter than most places biochar is marketed to, and the one direct field measurement, from an arid US region, found no consistent benefit. A claim that is unproven where dry conditions should make it easiest to detect has less reason to hold up here.

The UK Biochar Research Centre is based in Edinburgh, but there is no institutional Scottish supply chain a home gardener could currently point to.

The verdict for a home gardener

A raised bed filled with bought-in growing medium sidesteps the native clay or peat question somewhat, but no trial here covers biochar in a raised bed specifically, so treat that as an extension of the same logic and not an exemption.

For fertility, biochar competes with cheaper, better-established options such as garden compost and well-rotted manure, which most Scottish soils respond to reliably. The carbon-storage case holds whatever the soil, but it will not show as a visible difference in your beds. If you try it: charge it in compost for a few weeks first, follow the manufacturer's rate, buy traceable, sustainably sourced biochar, and keep it away from rhododendrons, blueberries, heathers and other acid-loving plants.

Common questions

Weighing up other soil additives too? See does rock dust work? and do mycorrhizal fungi products work?

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