Biochar, A Soil Biology Perspective

By Sara Gaucher, PhD

Biochar and Soil Microbes
Photo courtesy of International Biochar Initiative

You may have heard about biochar as a component in a “magically” fertile soil, terra preta.  Or maybe you’re hearing about it for the first time and want to understand what people are so excited about.

Biochar, plus soil microbes, creates a catch and release mechanism for plant nutrient cycling and water molecules.

Biochar’s not magic, but it is a tool that’s capable of many functions – a Swiss army knife of soil amendments.  Each attachment, used in the right context, can be really useful.  And like a Swiss army knife, you’re probably not going to have all the attachments open at once, because that would be, um, kind of awkward.  SymSoil focuses on using biochar to improve soil health.  Biochar is an integral part of SymSoil’s products, because it’s a great match with the soil microbe biome: biochar brings the chemistry, compost brings the biology, and the soil microbe biome flourishes.  The perfect recipe for health soil.

A soil amendment to increase soil health

Biochar is the dark black, dry, lightweight material produced when an organic material like wood is heated in the absence of oxygen.  Bottom line, it’s a type of charcoal specifically produced for use as a soil amendment.  It has a special sponge-like structure that holds on to nutrients and water to retain them in the upper layers of soil where plants and microbes need them.  With all its nooks and crannies, it even provides a comfortable home for the microscopic members of the soil microbe biome – the hardworking organisms that partner with plants and help them grow.

What’s old is new again

Biochar is a new incarnation of a technology that’s been around for a thousand years or more.  The most famous example is the so-called terra preta:  These stretches of man-made, fertile, dark earth in the Amazon stand in stark contrast to the natural, red, nutrient-poor soil in that area.  When scientists closely examined terra preta, they discovered the presence of charcoal, a material that isn’t likely to have arisen there naturally.  Later, more such charcoal residues were found in Ecuador, Peru, Benin and Liberia in West Africa, and these soils, too, have high levels of fertility.  So the correlation between the presence of charcoal and such highly fertile soils was tantalizing.  Did the charcoal play a role in developing and maintaining these soils? Scientists think yes.

It appears that biochar is like a skeleton made up mostly of carbon that’s linked up in three dimensional structures that organisms have a hard time breaking down as food.  But it makes great housing for these organisms.  What’s more, after it’s aged a short time, it becomes coated with a “skin” that’s like a kind of molecular Velcro.  This “Velcro” can hold on to important substances plants need like water, nitrate, ammonium, phosphate and more.  Being Velcro, the substances are stuck in place and can’t just wash away – but when the plant is ready to use them, they’re right there for the taking.  And the benefits are still going strong after hundreds of years.

These discoveries are relevant today because it suggests that this biochar material can persist in soil for hundreds of years and continue to function as a soil amendment.  That long residence time makes biochar different from compost, organic matter that, while longer lasting than undigested organic material, still needs to be reapplied annually.

Organizations like the International Biochar Initiative  and the United States Biochar Initiative  are dedicated to learning more about how to make and use this unique substance, biochar.

Transforming carbon from consumable to catalyst

So how can Biochar stick around so long, when in comparison, compost is gone in a fraction of the time?  Because we’ve learned how to transform organic material like woodchips or walnut shells from something microbes can eat (a consumable) to a material that remains in soil and mediates chemical reactions without getting used up (a catalyst).

To get a bit more technical, biochar is the result of a chemical process called “pyrolysis” – the transformation that occurs when an organic material is heated at temperatures from 450-750 °C (roughly 850-1400 °F) but without oxygen. Most of us are probably familiar with “combustion” – the result of heating organic material like wood in the presence of oxygen (in air).  During combustion, also called burning, the carbon in the burned material combines with oxygen to produce gases like carbon monoxide and carbon dioxide that go up the chimney.  At the end, all that’s left is gray “ash” – mostly made up of the elements that won’t be released as gas when combined with oxygen.  Pyrolysis is different – since the only oxygen available is what was in the organic material originally, most of the carbon stays put as a solid.  Not only that, the structure of the remaining carbon atoms undergoes a transformation where they start linking up in rings, and these rings start linking up with each other to form an irregular three-dimensional lattice.

The resulting material, which we call biochar, is now porous like a sponge – all those nooks and crannies mean there’s a lot of surface area to catalyze some pretty useful chemistry, as we’ll see.  Also, it’s resistant to further chemical or biological degradation, so it’ll stick around to do its job for many years.

There’s one more step to condition this long-lived biochar “skeleton” and make it suitable for agricultural applications – it needs to get a “skin”.  Just out of the cooker, the surface of biochar is hydrophobic, meaning, it doesn’t play well with water.  But when the biochar is aged, its surfaces change: It acquires a coating of organic molecules that turn biochar into an amazing hub of chemical activity.  This is the “Velcro” part of the biochar – the catch and release mechanism for food and water molecules.  Luckily, we can speed up this aging process by co-composting  biochar.  Now, the conditioned biochar has the physical and chemical properties we desire to positively impact nutrient cycles, water retention, and microorganism populations.

There’s one property that can be “tuned” when creating biochar during the pyrolysis step.  Scientists have found that if the organic material is heated just a bit hotter – above 750 °C – the carbon “skeleton” starts to get more regular and orderly.  And it turns out that the regular pattern of connections, atom to atom, makes a material that can rapidly transport electrons , as though the material were a wire.  Why that’s useful is probably not immediately obvious, so hold that thought until the next section when we discuss the value of high temperature biochar.  Spoiler: microbes need electrons to live.

Still curious?  Check out Kelpie Wilson’s description of biochar as an electric carbon sponge for more details.

Biochar, Swiss Army Knife

Biochar functions in the rhizosphere  as a central banking system between plant roots and the soil microbe biome.  The currency is food and water.  All of the nooks and crannies in biochar mean there’s a lot of surface area.  And all those surfaces are covered with a “skin” that functions like Velcro: water and nutrients deposited by, say, a dying bacterium stick to the biochar instead of flowing away.  Then, this “deposit” can be “withdrawn” by a hungry or thirsty plant root.  The plant root can make its own deposits that other living microbes can withdraw as well.  So goes the rhizosphere economy, optimized by the biochar bank.

There’s another banking function that’s unique to biochar.  With just a few tweaks, biochar can serve as a banker within a microbe community itself – microbe to microbe.  Here the currency is electrons.  For living organisms to keep on living, they need to metabolize food and turn it into energy.  A part of the chemical process involves moving electrons from one molecule to another.  Or, in the case of some microbial communities, moving electrons from microbe to microbe.  In the absence of a banking structure, a microbe with an electron to donate has to throw a “nanowire” to a microbe looking to accept an electron.  This process is slow and expensive for the microbes – they have to spend precious resources building protein structures that act as the nanowires.  But if a particular type of biochar, called high temperature biochar, is added to the soil, it acts as a central repository for electrons.  The difference is that this biochar is made (pyrolyzed) at temperatures above 750 °C.  Here a shift in the structure of the final biochar starts to occur.  As the heat is turned up, the carbon skeleton with its three-dimensional structure of interconnected carbon atoms becomes more and more ordered.  And it becomes ordered in such a way that electrons can now move easily onto and off of the structure.  Now microbes plugged in to this structure can freely carry out their daily metabolism, without needing to find a partner microbe.  The result is a supercharged microbial population.

Before moving on to talk about how SymSoil uses biochar in synergy with compost, it seems only right to include a note about biochar feedstocks – the organic material used to create the biochar.  While the primary focus at SymSoil is to create tools to improve soil health, we look at the bigger picture, too.  It’s great for the soil microbe biome to have the long-term support from biochar – whether for optimizing the food and water economy or the electron economy.  But it can be great for the planet, too, to retain carbon in the soil instead of releasing it into the atmosphere. (Click here for Dr. David Johnson’s comments on carbon sequestration and biochar & fungi.) An important point is that this applies in cases where biochar was created from feedstock that would have otherwise been fully degraded and all carbon released to the atmosphere.  Cutting down a tree that would have continued to grow and sequester carbon from the atmosphere for many years, to instead pyrolyze it to make biochar – not a net benefit.  Creating biochar from wood waste, sequesters at least some of that carbon in solid form for a longer time, compared to the amount released to the atmosphere if the wood had been allowed to rot.

The Compost Connection

SymSoil uses biochar both to create our products and to perform as the product.  Now that’s versatile!  How does it work?  It starts with the type of biochar used – Rogue biochar from Oregon Biochar Solutions and high temperature biochar from All Power Labs/Local Carbon Network.  This biochar has been produced such that the bulk material has enough electron transfer capacity to boost the composting process (“creating the product”) and enough pore space and surface area to catalyze nutrient exchange between plants and microbes once it’s in the soil (“perform as the product”).

Take SymSoil® RC (Robust Compost): it is approximately 90% compost, but the 5% to 7% biochar is critical for creating compost with a rich (you might say, robust) array of micro-organisms.  The result is a product targeted toward building up the soil microbe biome, with plenty of beneficial living organisms that are delivered to the soil from the compost.  The added benefit, unique in the world of compost, is the biochar component that persists in the soil and continues to provide support for the organisms in the form of housing and nutrient exchange.

On the other hand, where the soil microbe biome is already going strong, SymSoil® FIB (Fungal Infused Biochar) kicks the microbe support up a notch and conditions the soil.  At 90% biochar, 10% compost, now the compost is playing the supporting role – ensuring the biochar has developed the rich organic coating critical for the biochar to function as the central banking system for food and water.

Need a product to function somewhere in between?  Especially farming in arid areas, which need biology, organic matter AND soil conditioning.  That’s where SymSoil® V50comes in – a one-to-one blend of RC and FIB.

About SymSoil Inc.

A California B (Benefit)-Corporation, SymSoil is a leader in development of biological soil amendments for agriculture that restores the microbes that provide the right food to the plant roots, improving plant health, and making food more nutrient dense and flavorful, the way nature intended. SymSoil has products and services for growers using regenerative agriculture methodologies which improve a grower’s profitability. For more information, visit our website SymSoil.com

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