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Biochar chemistry and biology

Chunks of biocharI've posted previously about the benefits of biochar, but how does biochar actually cause those increases in crop yield?  It's probably easiest for the average gardener to understand that the increase in pH is caused by the ash that inevitably comes along for the ride --- using wood ashes to increase the alkalinity of (or "sweeten") soil is common wisdom among farmers.  And we can visualize the water retention properties too since a closeup look at biochar makes it look like a tiny sponge.

The complexities come in when you start thinking of the chemistry involved.  Since biochar is negatively charged, it naturally attracts anything positively charged in the soil, and that includes important plant nutrients like calcium, nitrates, phosphorus, and silicates.  This chemical attraction is why biochar in your soil prevents nutrient leaching --- the minerals are kept in the root zone rather than washing into the subsoil during heavy rains.

All of these factors help your plants grow, but probably the greatest benefit of biochar is its ability to increase the populations of beneficial microorganisms in the soil.  In fact, one study of Amazonian Dark Earth (aka terra preta) compared to nearby, natural soil showed that the Dark Earth contained a completely different set of microorganisms than the natural soil.

Mycelium and biocharBiochar helps many bacteria and fungi grow because it provides tiny nooks and crannies just big enough for these microorganisms but not large enough for predator microorganisms.  In their protected dens, the bacteria and fungi grow like crazy, especially if you make your biochar at a relatively low temperature so that the insides are coated with energy-rich tars for them to eat.  Scientists have found that arbuscular mycorrhizae (the fungi that attach to your plants' roots and help them get hard-to-find nutrients) are especially aided by biochar in the soil.

Nitrogen-fixing bacteria also love biochar, but for a different reason.  The biochar pores tend to create low oxygen conditions where these anaerobic bacteria thrive.  As a result, legumes create more nitrogen-fixing nodules when biochar is in the soil and free-living nitrogen-fixing bacteria also multiply.  That means more nitrogen for your plants without any work on your part.



This post is part of our Biochar in the Backyard lunchtime series.  Read all of the entries:





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If you're going to make biochar, go for a retort kiln instead of a charcoal mound. You'll need less fuel and create much less pollution.

So, is biochar, wood ash and urine the the holy trinity of gardening? :-)

Comment by Roland_Smith Tue Feb 15 13:48:54 2011

Right now, I'm content to just use the charcoal I screen out of the wood stove. If my trials this summer turn out well and I want to expand, we'll definitely try to build something rather than just making charcoal in the ground. But I figure it's smart to start small so that I can tweak things.

I'm not sure I'd consider biochar, wood ash, and urine the holy trinity of gardening. Maybe compost, manure, and cover crops?

Comment by anna Tue Feb 15 16:55:29 2011

Good Day,

I have been looking for information on biochar just about everywhere. Do you know chemically why the biochar is negatively charged? Is it much like humus in that it has hydroxyl groups?

Does adding organic acids (humus) to biochar increase it's CEC ? Change it's structure (ie breaking double bonds and adding hydroxyl groups?)

Also, isn't the lye (ie KOH) extremely water soluble? You should be able to remove the lye and thus remove the alkalinity. This would also mean that the acid controlling properties of biochar are temporary as the KOH leaches through the soil profile.

Any technical documents you find or good references i would absolutely love. I understand chemistry very well and i want to understand how biochar works on the same level so i can explain this to people.

Cheers!!!

Comment by Miguel Amat y Leon Wed May 1 01:13:43 2013
Miguel --- I'd assume the negative charge on charcoal is similar to that in organic matter, but I honestly don't know for sure. I also agree with you that the alkaline effect would likely be short-lived until the ash washes away. Not sure about the CEC question. This post is pretty much all I know about biochar's chemistry. :-) Maybe you'll hunt down some answers and report back?
Comment by anna Thu May 2 08:33:22 2013

Sounds Good.

I have some new recent published papers that might shed some light into it. They are a couple 100 page documents so it might be over the weekend :P Also I might contact my UW professors too - it's a good question: Seems everybody is stumped, which is nuts cause the molecular composition of a material tells you pretty much everything you need to know about it :D
While the KOH in the ash is rather water soluble, the ASH also contains copious amounts of Calcium Carbonate (CaCO3). CaCO3 <-> Ca + CO3(2-). CO3(2-) + H+ <->> HCO3(1-). HCO3(1-) is super soluble in water, but calcium carbonate is not, so the overall reaction is slow. HCO3(1-) + H+ can also result into H2Co2, which can then break up into Water and CO2. The overall point here is that the ASH in terms of Calcium Carbonate can result in the removal of acids (H) permanently albeit slowly. Also, because calcium carbonate is not very soluble in water, i don't think there is a way to eliminate it if you already have alkalinic soils. The question here is how much CaCO3 is in the actual biochar vs just the ash. I think if you simply just separate the biochar from the ash, you remove most of the Calcium Carbonate. Then you can run water through the biochar to remove the KOH .

It's part of the puzzle anyway. NOTE: The previous is party based on informed speculation and is not 100% factual. Still working on the 100% :)

Comment by Miguel Amat y Leon Fri May 3 02:23:39 2013

The reason Biochar is negative : It's functional groups.

Biochar may contain numerous functional groups, such as hydroxyl (OH), keton (O-R), ester (C=O), aldehyde (C-O), amino (NH2), nitro (NO2), and carboxyl [(C=O)OH].

-O and -OH bonds are very "magnetic" to positive charged nutrients (K+, NH4+, CA+, Mg+2). This is what Cation Exchange Capacity is all about.

I have to dig in a little more - but it seems that it also has really good Anion Exchange Capacity - equally as nice to have.

Essentially, as you said, it has a ton of similarities to humus. Only slightly burned so the carbon structure does't decay.

But...

Freshly produced biochar is hydrophobic and contains few polar, function groups at the surface, but after exposure to water and oxygen in the soil the biochar surfaces oxidizes and forms more carboxylic [(C=O)OH] and phenolic group(O-R). (Gheng, et al, 2008b)

My thought is that even better than putting it on air, compost speeds up the oxydation of biochar. THAT is it's true potential - after it's been fully oxidized. What's worse, if you put it in your soil without composting it, it can actually lead to nitrogen immobilization from the bacteria that are eating the remaining oils in the biochar. This goes away after a couple months or if you add nitrogen to compensate. My case is for composting - allow the material to fully oxidize and then load it up with nutrients from the compost. Perfect.

Comment by Miguel Amat y Leon Sat May 4 02:42:39 2013

The reason Biochar is negative : It's functional groups.

Biochar may contain numerous functional groups, such as hydroxyl (OH), keton (O-R), ester (C=O), aldehyde (C-O), amino (NH2), nitro (NO2), and carboxyl [(C=O)OH].

-O and -OH bonds are very "magnetic" to positive charged nutrients (K+, NH4+, CA+, Mg+2). This is what Cation Exchange Capacity is all about.

I have to dig in a little more - but it seems that it also has really good Anion Exchange Capacity - equally as nice to have.

Essentially, as you said, it has a ton of similarities to humus. Only slightly burned so the carbon structure does't decay.

But...

Freshly produced biochar is hydrophobic and contains few polar, function groups at the surface, but after exposure to water and oxygen in the soil the biochar surfaces oxidizes and forms more carboxylic [(C=O)OH] and phenolic group(O-R). (Gheng, et al, 2008b)

My thought is that even better than putting it on air, compost speeds up the oxydation of biochar. THAT is it's true potential - after it's been fully oxidized. What's worse, if you put it in your soil without composting it, it can actually lead to nitrogen immobilization from the bacteria that are eating the remaining oils in the biochar. This goes away after a couple months or if you add nitrogen to compensate. My case is for composting - allow the material to fully oxidize and then load it up with nutrients from the compost. Perfect.

Comment by Miguel Amat y Leon Sat May 4 12:25:36 2013

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