Can heavy applications of biochar result in combustible soil?

I was torching some fireants near a fruit tree and noticed a little bit of my biochar smoldering.  Got me thinking about some of the carbon sequestration plans that include tilling in multiple tons of biochar per acre.  Any chance this could result in combustible soil?  I'm imagining something similar to underground coal fires that are nearly impossible to extinguish and can smolder on for decades.  Any thoughts?

Hm! Hadn't thought of that. It seems a small risk. But forest fires from smouldering tree roots are certainly a well known problem.

On the one hand, all soil (organic material) is combustible if it's very dry. And char has a low ignition point compared to wood.

On the other hand, one of the reputed effects of char is to help soil retain moisture. I wonder if that balances things out?

Sounds like you are ready for an experiment.

Mix biochar with soil in various ratios. Find out what ration, by weight, is needed for the soil to become flammable.  

I suspect based on intuition alone that it would need to be above 50% char by weight. Dry soil pretty good at smothering fires.

Then work out what biochar application that would equate to per acre if broadcast and tilled. I suspect that the numbers will be a long long way apart.

I have no solid facts to base this on, but my gut feeling is that, if that is even possible, the amount of biochar would be far in excess of that needed to help the soil.  It's an interesting question though.

Ask Bryant Red Hawk.

Charred, previously burned areas tend to be what stop the worst wildfires. Some charred debris ignites, but it usually doesn’t have the network of ladder fuels to spread.

Oxygen.

I'm thinking there isn't enough oxygen in the soil to allow combustion.  There would only be enough oxygen on the surface.  If the surface did burn, the non-organic mineral and clay would mostly remain (I'm assuming that doesn't go aerosol) and would form a lid, preventing oxygen from getting to lower layers.

Underground coal fires smolder on because there are fissures - and mine shafts - that provide just enough air exchange for them to breath.

I'm thinking if it did happen, it would be more like a peat fire.  From Wiki:  "Peat has a high carbon content and can burn under low moisture conditions. Once ignited by the presence of a heat source (e.g., a wildfire penetrating the subsurface), it smoulders. These smouldering fires can burn undetected for very long periods of time (months, years, and even centuries) propagating in a creeping fashion through the underground peat layer... In the summer of 2010, an unusually high heat wave of up to 40 °C (104 °F) ignited large deposits of peat in Central Russia, burning thousands of houses and covering the capital of Moscow with a toxic smoke blanket. The situation remained critical until the end of August 2010."  Very bad indeed if biochar could do the same.

Interesting counterpoint from Trace ...

a thought ... peat is the result of plant material growing and collapsing onto itself, creating a mat of almost entirely organic matter (am I right on that?). Its akin to a bunch of fallen logs, suspended in water.  When the water goes away, that organic matter can burn (slowly).

I'm still betting that the mineral content of soil would inhibit fire spread in soil.

Coal and peat are fuels.  Biochar has been burned to the point that it is no longer fuel. It is just carbon. That's the difference between char for biochar and chacoal, which is also fuel.  I don't think it is a serious problem.  It wouldn't make sense. My two cents.

John S
PDX OR

John Suavecito wrote:Coal and peat are fuels.  Biochar has been burned to the point that it is no longer fuel. It is just carbon. That's the difference between char for biochar and chacoal, which is also fuel.  I don't think it is a serious problem.  It wouldn't make sense. My two cents.

John S
PDX OR

I'm with John on this.  If the biochar was properly made then it no longer is able to generate flammable gases that could smolder.  Also, if there is soil moisture then the biochar will contain water that would keep the carbon from burning until the water vaporized off.  I would be shocked if this was much of a problem.  

I'm curious what the smoldering was that you saw Gray.  Any chance you were seeing steam or else was your biochar not fully charred and still had some wood inside it?  My only other thought was that the biochar was well saturated with organics/life and that that was smoldering?  Some of my soil surface biochar in moist gardens has moss growing on it that would probably smolder nicely if it dried out.

John Suavecito wrote:Coal and peat are fuels.  Biochar has been burned to the point that it is no longer fuel. It is just carbon. That's the difference between char for biochar and chacoal, which is also fuel.  I don't think it is a serious problem.  It wouldn't make sense. My two cents.

John S
PDX OR

I'm not following the thought that biochar has been burned to the point that it is no longer fuel. Biochar is just charcoal that has been inoculated.  Biochar hasn't somehow been burned longer than charcoal. Biochar and charcoal are exactly the same thing before the biochar is inoculated.

Greg Martin wrote:

Also, if there is soil moisture then the biochar will contain water that would keep the carbon from burning until the water vaporized off.  I would be shocked if this was much of a problem.

I think this is a key part. I would think that the biochar would hold enough moisture to keep it from burning.

I think it's potentially an issue where we're talking about heavily amended soils that are then permitted to dry out. Even a heavy amendment to soil occupied by a thriving system shouldn't be able to combust, for all the reasons mentioned. It should be too wet and lacking oxygen to smoulder, even if ignited by a sufficiently hot source.

This might be a consideration in drought- and wildfire-prone areas, at least to the point that, as we all know we should, hydrology must be considered first. Also, arid climates might not be suitable for broad-acre biochar amendment, unless part of a reforestation program that includes increasing the average ambient humidity.

But I would suspect that as long as there's water enough to host life, and barring intentional aeration of the soil layers in arid conditions immediately prior to a lightning strike or forest fire, the conditions will be unsuitable to propagate fire.

-CK

Chris Kott wrote:I think it's potentially an issue where we're talking about heavily amended soils that are then permitted to dry out. Even a heavy amendment to soil occupied by a thriving system shouldn't be able to combust, for all the reasons mentioned. It should be too wet and lacking oxygen to smoulder, even if ignited by a sufficiently hot source.

This might be a consideration in drought- and wildfire-prone areas, at least to the point that, as we all know we should, hydrology must be considered first. Also, arid climates might not be suitable for broad-acre biochar amendment, unless part of a reforestation program that includes increasing the average ambient humidity.

But I would suspect that as long as there's water enough to host life, and barring intentional aeration of the soil layers in arid conditions immediately prior to a lightning strike or forest fire, the conditions will be unsuitable to propagate fire.

-CK

I think that is exactly right. Evidence also seems to suggest that biochar works best to help save nutrients in areas of heavy rain, and as such, it may not be worthwhile in very dry areas anyway.

Trace Oswald wrote:

John Suavecito wrote:Coal and peat are fuels.  Biochar has been burned to the point that it is no longer fuel. It is just carbon. That's the difference between char for biochar and chacoal, which is also fuel.  I don't think it is a serious problem.  It wouldn't make sense. My two cents.

John S
PDX OR

I'm not following the thought that biochar has been burned to the point that it is no longer fuel. Biochar is just charcoal that has been inoculated.  Biochar hasn't somehow been burned longer than charcoal. Biochar and charcoal are exactly the same thing before the biochar is inoculated.

Sorry John, but Trace is correct. Pure-ish carbon (char or coal) burns just fine, given the right conditions -- low moisture level and high oxygen flow. It does take sustained heat to start combustion though.