I suspect that many people don’t yet realize it, but acidic soils are a growing problem (so to speak!) around the world, already impacting more than 50% of soils used or suitable for agriculture. Humans are both the cause and the casualty of soil acidification, though other biota are also impacted. We humans have come up with various solutions to combat low soil pH each with their own pros and cons some of which are outlined in a recent paper on this topic by Dai et all (see my summary infographic above).
In attempting to parse the biochar qualities that are relevant to increasing soil pH, the authors highlight feedstock and production temperature as being critical to designing or selecting the right biochar for this particular task. They note that higher temps tend to increase fixed and total C, pH, ash, total exchangeable and soluable base cations and surface area. At the same time however higher temps tend to lower yield, volatile matter, total O and CEC. Generally speaking though if a ‘soil toiler’ wants to increase pH using biochar, so far the best advice points to using relatively high temperature chars (e.g. ~600C) from manure biomass or using a blend of biochars made from different feedstocks.
The discussion on mechanisms or how biochar reduces acidity, gets deep into the weeds of agronomy, but these are the general takeaways from my perspective:
Soil acidity: alkaline biochars help acid soils but not alkaline soils – pretty logical!
Certain biochars reduce Aluminum (Al) bioavailability & thereby its toxicity to plants; high surface area chars may provide more adsorption sites for Al and other metals
Nutrient Availability – research is all over the map at this stage with one of the only consistent findings being that manure chars have more nutrients than chars from plant or woody feedstock. This is widely acknowledged in the literature.
Soil nitrification impacts – also seemingly inconclusive at this stage with much more research recommended.
Lots more work to be done on this subject, especially on long-term impacts of biochar and other soil amendments on soil pH, but overall it was a hopeful read!