Biochar: Best for soil vs. Last 1000 years

This article is suggesting that biochar burned at a higher temperature, like 700 degrees, will last longer, about 1000 years.  Therefore it's great for carbon sequestration.  However, they said that biochar burned at a lower temperature, like 300-500 degrees, is more productive for soil fertility.

John S
PDX OR

https://biochartoday.com/news/high-temperature-biochar-offers-thousand-year-stability-while-low-temperature-varieties-maximize-soil-benefits/

I don't measure the temperature of my biochar when it's burning, so I guess I don't know which of these scenarios is more like what I make.
John S
PDX OR

This makes sense.

I know when I have coals that go out on their own, they are much softer, whereas if I quench them in water they tend to be very hard. The ones quenched in water have been burnt at a higher temperature, for they’d have burnt to ash if not for being quenched.

It seems to me a mix of both may be most useful.

I've read this before and it's interesting if you're producing at scale in a controlled environment, with uniform inputs. You can adjust for whatever your particular situation and goals require.

For us folks doing it at the backyard or farm level, I suspect we're getting a bit of the "best of both worlds" with varying feed stocks, sizes, and temps, we're getting some "now" biochar and some "later" biochar, maybe with a little mineral-rich ash mixed in. The imperfect process may just make the perfect biochar mix.

Here's another study showing it "ages" better at 350C than at 750 C. The electric flow seems to work better at the lower temperature:
John S
PDX OR

https://link.springer.com/article/10.1007/s42773-025-00546-z

There are two main attributes of biochar that have a lot of influence on its performance in soil. One is surface area, and this increases as the temperature of pyrolysis goes up. Pretty intuitive, since what's happening is that all the hydrocarbons are getting cooked out of the microscopic pore structure of the biomass and empty space is what gets left behind. More surface area means greater sponge effects for holding moisture and helping with aeration, and also provides more protected internal space for the soil life to hang out and multiply.

The other important property is referred to as functionality. This is an electrochemical quality that relates to the molecular attachment points on the biochar surface where oxygen and mineral ions are attracted. These in turn help bind nutrients to make them available to microbial and plant life. Functionality is highest in low-termperature biochars because there are still lots of hydrogen and oxygen atoms hanging around. High-temperature biochars will gain functionality after some time in healthy soil or compost.

The ideal biochar for most soil applications would have high surface area and high functionality, but it's sort of an either/or proposition. There are ways to increase the functionality, either by mixing with active compost, or by adding things like clay or metallic oxides (like rust) to the feedstock that form crystalline structures on the surface.

Great information, Phil.  That makes sense to me. I don't quite understand the physics and chemistry at your level, so it's nice to have someone who really gets it and can explain what's going on.
John S
PDX OR

Yes, this is all good information to know - and to know that some of us can tweak our systems to lean one way or the other.

From my perspective, in my heavy clay soil, anything that lightens it a little, that I can produce here on my own land, is worth it, regardless of whether it's perfect or not.

If it holds water effectively, that's a major bonus. Yes, clay holds water also, but it can be hydrophobic when it gets really dry and the water will run off rather than soaking in. If the biochar gives the rain avenues to soak in, that's really useful to me.

Does the article say how long the ~300 degree biochar lasts? One year - 50 years - 300 years? Because I consider it a renewable resource on my land, if the low temp stuff I can make easily and that helps my soil, can last even 10 years (and I suspect most of it lasts longer than that), I think I'm still ahead so long as I keep making it.

I admit that most of mine goes into the compost. Sometimes it goes into soil mixes wet, but not charged with helpful nutrients, but my mix usually had enough good stuff there that the plants do well. I'm just not organized well enough to reliably do the compost tea thing.

Good question, as it is a matter of cyclical feedback processes. I would figure how the Amazonians did it worked pretty well.

Phil,
I have a lot of clay soil where I live. Would it be helpful to add some clay soil to the wood stock before I start the biochar burn?  The functionality sounds like a good way to improve soil fertility.
John S
PDX OR

Jay Angler wrote:

Does the article say how long the ~300 degree biochar lasts? One year - 50 years - 300 years? Because I consider it a renewable resource on my land, if the low temp stuff I can make easily and that helps my soil, can last even 10 years (and I suspect most of it lasts longer than that), I think I'm still ahead so long as I keep making it.

Lower-temperature biochars oxidise and degrade more quickly than high-temperature ones. But it's like a long ramp, not falling off a cliff. Let's say we had two batches of biochar made from the same type of wood, one produced at 300 degrees and the other at 700, and put them into soil.  The total carbon stored starts off at 90% for both types, and if you sample at 100 years the 300-degree batch might be down to 60% and the 700-degree batch is 85%. At 200 years they're at 50% and 80%, respectively, and at 1000 years the numbers might be 20% and 60%.

Aside from giving up some of its carbon, the pore structure of biochar more than a few decades old gets plugged, so if you've got plenty of feedstock and like the results, there's a good incentive to keep topping it up. The terra preta soils probably developed over centuries at least, and deposits of pyrogenic carbon in ecosystems with recurring fires often go back tens of thousands of years. Really old biochar is called inertinite and is found in coal seams where it's been preserved for as long as 300 million years.

John Suavecito wrote:Phil,
I have a lot of clay soil where I live. Would it be helpful to add some clay soil to the wood stock before I start the biochar burn?  The functionality sounds like a good way to improve soil fertility.
John S
PDX OR

Yes, this is a great idea. If you're ambitious, try making a clay slurry and soaking a batch, then letting it dry out prior to a burn.

A couple of data charts I like sharing on what is happening with the peak temperature used in forming biochar:



Both charts indicate that 500C (plus or minus) may be optimal for use in the soil.  As a reference to how hot that is, that is the temperature where you'll first start to see objects begin to glow red.

The electron donor and receiver groups are both important for aiding soil bacteria to carry out their metabolism at a lower energy cost, helping them aid plants in putting on more growth and yield.

I can see the chart clearly.  I like stuff like this because I don't quite really understand it, but if I keep some of the ideas in my head, I can stretch over time to understand more of this complicated sciency stuff over time.  It's like free college education.

John S
PDX OR

How much biochar should be used per square foot in a 18" tall raised bed?

I don't have data on that.  I dig it in as a dripline around the trees and bushes, from 1" to 6" deep.
John S
PDX OR

Yeah for most of us making it in a burn barrel or rocket stove it's probably a mix of temperatures anyway so we're getting a bit of both. Not sure that's a bad thing. Immediate soil benefit plus some longer term carbon. I'll take it.