Differences between biochar and charcoal

I thought this was a good explanation, showing the differences between the two:
John S
PDX OR

https://www.wastex.io/post/biochar-vs-charcoal

What caught my eye was this:

"Biochar's pH can vary more widely than charcoal, typically ranging from slightly acidic (around 4.6) to moderately alkaline (around 9.3). This variation depends on the feedstock used and the production temperature. "

Creating a low pH (acidic) biochar would be extremely useful. But what feedstock? What temperatures?

I have read that what you put in matters a lot.  Coniferous wood makes a very different end product than hardwoods.  I remember them saying that it was preferable in some places to make biochar out of conifers. I think it was because the conifer soil is normally very acidic, so it can put your soil out of whack.  I guess if you have alkaline soil and you don't want it to get more that way, conifers are probably a good idea.  We have very acidic soil here.

John S
PD XOR

Yes, all the literature seems focused on "how to fix acidic soil with biochar."

My searches on "how to reduce biochar acidity / pH" basically boil down to "add elemental sulfur." Which helps me not.

I wonder how the source above came up with their information. Some more details (or links) would be very helpful for those of us trying to reduce pH in our soil.

EDIT: I mistyped above -- my goal is to increase biochar acidity / reduce pH. Apologies for being unclear.

Biochar pH is going to be a function of two main variables: Feedstock mineral content, and highest treatment temperature. Hardwoods tend to have more minerals than coniferous species, so the biochar will have more embodied ash and a correspondingly higher pH. Softwoods like pine and fir don't produce nearly as much ash and yield a more neutral or even acidic biochar.

At lower temperatures, the reactions of pyrolysis form various hydrocarbons that bond to the carbon molecules of the char substrate. These are reactive, often acidic, and tend to lower the pH of the biochar. Higher process temperatures destroy most of these substances and leave behind more pure nonreactive carbon along with the minerals, so the resulting biochar will be more alkaline.

I'm a little surprised at that cited article's claim that charcoal is higher in pH. I would expect products made at low temperatures (to retain more of the hydrocarbons for easy lighting and greater heat value) to be more acidic in the same way biochar is. Also, the realistic range for biochar production is more like 350-750 C. Below that the pores are plugged, and once you get around 800 degrees the structure turns to graphene sheets and porosity decreases due to the flattening effect.

If you're trying to fix acidic soils, go for hardwoods and high temperatures. If your soils are alkaline (or you're growing acid-loving plants) then stick to conifers and keep the temps down. Post-treatment like composting or washing with rainwater will also lower the pH of the finished product.

Excellent answer, Phil.

John S pdx or