pH buffering
Biochar can raise soil pH and buffer against future acidification
Biochar commonly shifts soil pH upward (typical amendments raise pH 0.1–0.5 units) and adds buffering capacity against acidification from rainfall, fertiliser, or organic matter cycling.
What the literature says
Biederman & Harpole’s 2013 meta-analysis — pooling 371 individual observations from the biochar literature — found a consistent pH increase from biochar amendment, averaged across feedstocks and soils. The Dai 2017 review catalogued the acidification-amelioration effect across Asian acidic soils with an average pH rise of ~0.4 units.
Mechanism
Two mechanisms at play:
- Ash alkalinity. The ash fraction of biochar is alkaline. Hardwood chars are typically in the pH 8–10 range. When blended with soil, that alkalinity neutralises some acidity.
- Cation exchange. As CEC rises (see the CEC entry), the buffering capacity of the soil increases. The biochar-amended soil resists further pH shifts from rainfall or fertiliser inputs.
What this means for a gardener
- Acidic soils (pH < 6): blueberries and acid-loving plants may not appreciate biochar — it will nudge pH upward. Use biochar selectively or avoid in acid beds.
- Near-neutral soils (pH 6.5–7): biochar’s effect is typically mild and beneficial.
- Alkaline soils (pH > 7.5): the pH shift is small but present; not enough to cause issues, but worth noting.
Important caveats
Ash percentage and feedstock determine the magnitude of the pH effect. Our hardwood biochar has a modest ash fraction — our supplier specifies a maximum of 5 % — and the pH shift is typically in the 0.1–0.3 range at a 10 % by-volume application. An independent ash determination is scheduled for our next lab submission and will be published on the Lab Results page when it lands.