Any way to calculate the pH effect of biochar?

I'm interested in replacing some of my lime requirements on pasture with biochar.  I find plenty of info saying it raises pH, I find no info regarding how much.  Or does char just vary too much (feedstock/temp/etc) to make an accurate recommendation?

I thought if it was really good biochar (no ash) then it didn't affect the pH?

All woody feedstock has minerals in it. The proportions may vary, but high-quality biochar typically has a carbon content of around 80-90%. The rest is the mineral fraction, which is bound up in the material instead of being reduced to physical ash. This is what gives biochar its high initial pH, but over time it the soluble part leaches away, the insoluble substances react with acidic soils and rainwater, and the nutrients get used by microbes.

Since one of the minerals present is calcium carbonate, there is literally some lime in most biochar. The IBI biochar classification standard has a set of liming classes that are used to define how well a particular biochar can substitute for lime when applied to soil.

I would start with a glass of water test the PH,   then add biochar, then test the restult.


There is difference in biochar based upon what you started with.      I found out that chicken manure once chared  still has it's nitrogen levels.   ( not that I would want the smell of chicken manure being chared ;-)

Nancy Reading wrote:I thought if it was really good biochar (no ash) then it didn't affect the pH?

I ain't good.  More like quick and dirty...

Mart Hale wrote:I would start with a glass of water test the PH,   then add biochar, then test the restult.


There is difference in biochar based upon what you started with.      I found out that chicken manure once chared  still has it's nitrogen levels.   ( not that I would want the smell of chicken manure being chared ;-)

Then how to convert to lbs per acre?

Phil Stevens wrote: The IBI biochar classification standard has a set of liming classes that are used to define how well a particular biochar can substitute for lime when applied to soil.

Very interesting.  I will have to spend some time with this to see if I can come up with something useful.

Gray Henon wrote:

Mart Hale wrote:I would start with a glass of water test the PH,   then add biochar, then test the restult.


There is difference in biochar based upon what you started with.      I found out that chicken manure once chared  still has it's nitrogen levels.   ( not that I would want the smell of chicken manure being chared ;-)

Then how to convert to lbs per acre?

Now that is a different question from the PH of biochar.

From one of my books I have read "The Intelligent Gardener"   by Steve Solomon,    he recommends that the first step is a soil test,   you can't really know what your soil has until you have done the soil test.     After that is done then you can move forward to add what the soil needs.


I recommend getting a soil test,  then make changes based on those test results.

Mart Hale wrote:

Gray Henon wrote:

Mart Hale wrote:I would start with a glass of water test the PH,   then add biochar, then test the restult.


There is difference in biochar based upon what you started with.      I found out that chicken manure once chared  still has it's nitrogen levels.   ( not that I would want the smell of chicken manure being chared ;-)

Then how to convert to lbs per acre?

Now that is a different question from the PH of biochar.

From one of my books I have read "The Intelligent Gardener"   by Steve Solomon,    he recommends that the first step is a soil test,   you can't really know what your soil has until you have done the soil test.     After that is done then you can move forward to add what the soil needs.


I recommend getting a soil test,  then make changes based on those test results.

I got a soil test.  It says I need 1.5 tons per acre of lime.  I just need to figure out how much biochar it would take to meet this requirement.

Alright, fresh charcoal produced in an open burn is going to consist of 2 things: the char itself, and a certain amount of wood ash. I am going to assume that the mineral content within the char is bound, because charcoal always produces ash when burnt - so I will assume that is the only way those elements can be separated from the carbon. So the carbon fraction should not have an effect on soil pH, and so any increase in the soil pH will be coming from the wood ash. Wood ash composition and quantity will vary based on the feedstock used. Also, this assumes the char is fresh, because as soon as you start adding water, the soluble alkaline compounds are going to leach out. A search about the lime value of wood ash turned up this:

" Have the soil tested and spread dry wood ashes at the same rate as recommended for agricultural lime.

Although wood ashes usually have a lower percentage of lime (calcium carbonate equivalent—see table), the alkaline compounds present are more reactive than agricultural lime. Therefore, pound for pound, the two are about equal in raising soil pH."
https://www.aces.edu/blog/topics/vegetables-lawn-garden/wood-ashes-for-lime-and-potash/

So basically, you would need to spread 1.5 tons of wood ashes per acre to achieve the same thing. So if you could estimate the ash content of fresh charcoal you could get a pretty good estimate. Wood contains a few percent ash, so if a quarter of the wood you put in the pit to make the charcoal burned up (this is likely entirely too large a percentage, but bear with me) you would only have a few quarters of a percent ash. Lets make things simple, and say the char contains 1% ash. So each ton of biochar would contain 20lbs of ash. To get 3000lbs/acre you would need 150 tons of biochar. If my math is right, that would be a little over a foot of coverage.

I suspect biochar might be changing the soil in other ways, as I know that increasing soil carbon tends to buffer soil, and lead to better pH values without actually neutralizing the acidity. Since you have a soil test to serve as a baseline, why not try a heavy addition of biochar in one area, and then have the soil tested again in a year or so to see how the soil reacted.

Gray Henon wrote:I got a soil test.  It says I need 1.5 tons per acre of lime.  I need to figure out how much biochar it would take to meet this requirement.

Ask the lab to characterize your biochar for calcium carbonate equivalent (w/w percent). If 10% CCE, then need 15 tons biochar per acre to achieve the same pH raising effect as using 1.5 tons ag lime.

One of the fascinating aspects of biochar CCE levels is that the calcium, potassium, and magnesium oxides and hydroxides in freshly made ash have high CCE, like freshly kilned lime does. Over time the oxides and hydroxides convert to carbonates (like limestone, pH 8.3, no joy for making soap). For biochar, this natural conversion means losing CCE as it seasons. Freshly kilned ash (like burnt lime, pH 12, caustic enough to use to make soap) can have a CCE of 110%, but ash left exposed to nature (seasoned) will lose CCE.

If [kilned] lime is left exposed to the atmosphere, it will, over time, revert back to calcium carbonate by absorbing carbon dioxide from the air:

(IV) Ca(OH)2(s) + CO2(g) <-----> CaCO3(s) + H2O(l)

In other words, the soluble material, slaked lime, if left exposed to the air, converts to the insoluble material limestone. What we have here is the first cement! Modern cements are more sophisticated and "cure" more quickly than slaked lime, but lime continues to be a major component of modern cement. When quicklime is painted on wood, it forms a rock-hard white coating called whitewash.

Source: Caveman Chemistry

Another fascinating aspect of biochar is that the carbon component of biochar may provide a small but measurable alkalinizing effect. This effect was observed in a carbon-filtered municipal water supply study, where an alkalinizing effect long that could not be accounted for by the mineral (non-carbon) content within the carbon filtration material. I don't have the study with me, but I recollect that alkalinizing potential was small but persistent over a long period of time and the effect did not diminish with time. Add this to the very long what-we-know-we-don't-know list for biochar. I suspect we-don't-know-what-we-don't know list is long for biochar.