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What Even IS Biochar? A Gardener’s Perspective.

Three thick arrows are arranged in a recycling sign on a grainy black background. In the middle, surrounded by the arrows, "BIOCHAR" is written. Within each arrow is an illustration. For the arrow on the top, a cow is illustrated with multiple methane molecules as well as some piles of garbage with banana peels. The bottom right arrow shows a thermometer. In the background, orange flames are drawn with a title reading, "Pyrolysis". The final arrow on the bottom left depicts a young girl sitting on a grass field with a yellow straw hat. Beside her is a straw basket with some carrots. The field is decorated with many pink flowers.

Written by Jem Barrett
Illustrated by Maggie Huang

Biochar is an idea both new and ancient. It’s a charcoal-like product intended for use as a soil additive and has a wide range of effects on both the soil and the plants grown in it.¹ The addition of biochar to soil tends to increase plant growth and crop yield, especially in coarse, rapidly draining, or acidic soils.² Biochar increases crop yields via improved soil water- and nutrient-retention abilities and the liming effect, meaning that it makes soil less acidic.²

Biochar has a vast number of interesting applications. In 2013, the International Biochar Initiative highlighted biochar’s potential as an additive to increase soil fertility, improve nutrient- and water-use efficiency, mitigate environmental pollution, sequester carbon, and reduce greenhouse gas emissions.³ It can also be used as a food supplement for livestock, an additive to compost and even to concrete, thus reducing the amount of concrete needed and the overall greenhouse gases emitted.⁴ However, today we will be focusing on what a curious gardener might want to know about biochar without painstakingly sifting through the scientific literature.

How is biochar produced?

Biochar is created by heating sustainably sourced organic material to temperatures of 350˚ C to 700˚ C⁵ in the presence of little to no oxygen.⁴  This process is known as pyrolysis and it transforms the organic material into a solid, black, carbon-rich product. Feedstock is what we call biomass, which typically consists of agricultural or forestry waste products, municipal waste⁶, food waste, and more that is then pyrolyzed to produce biochar.⁷

Using different feedstock results in biochars with different physical and chemical properties. Typically, wood-based biochar has more carbon content and lower plant available nutrients, while manure-based biochar exhibits lower carbon content but higher plant available nutrients. Grass-based biochar falls somewhere in the middle.⁷ Another potential feedstock that’s being explored is the use of invasive plant tissue such as kudzu that’s been removed to promote native plant growth in ecosystems.⁸ When biochar is produced using waste material or renewable, sustainably sourced biomass, it highlights the potential of creating a circular economy of biochar production and usage. This production can happen at large, high-tech facilities such as Stockholm⁹ or in open-cavity steel kilns¹⁰ demonstrating the accessibility of biochar at a range of economic realities.

Is biochar the same as charcoal?

It is incredibly similar to charcoal! There’s a significant amount of overlap between charcoal, biochar, and activated charcoal, all of which are produced by heating up some carbon-rich material known as feedstock in a process known as pyrolysis.⁴ The feedstock can vary from coal and peat to wood and waste materials.⁴ Biochar specifically is produced using sustainably sourced biomass,⁴ which can include the range of feedstocks listed in the previous section. In terms of chemical differences, activated charcoal undergoes processes of activation after pyrolysis, which often involves heating the carbonized product at high temperatures, above 700˚ C, to remove tars and other products of incomplete combustion.¹¹

Charcoal production, on the other hand, ends once carbonization occurs, so some products of that incomplete combustion stick around in the charcoal’s pores. Furthermore, charcoal is typically produced slowly, at lower temperatures.¹² This means that charcoal has less surface area than activated charcoal and biochar, which reduces its ability to adsorb various molecules. Adsorption refers to the ability of a solid substance such as biochar to attract gas or liquid molecules to its surface.¹³ Adsorption differs primarily from absorption in that adsorption occurs on the surface of the solid substance while during absorption the molecules enter the solid.¹¹ The high surface area of biochar makes it extremely effective at adsorbing nutrients, making them readily available for plants to access.

One of the other principal differences lies in the intended use of your end product. Charcoal is typically used for fuel and grilling, activated charcoal is often utilized for water filtration, and biochar is used as a soil amendment or animal feed additive, although it can be used for any of the purposes listed. If biochar were to be burned for fuel, it would be classified as charcoal,⁴ albeit with higher surface area than most charcoals! Because of the intended purpose of charcoal, charcoal briquettes can have ignition accelerants added to it, which could be toxic to your garden.¹⁴ It’s best to use biochar that is explicitly intended for use as a soil amendment – that way it’s been created with your garden in mind!

What are some beneficial effects biochar has on soil?

First are the physical benefits. The physical honeycomb structure of biochar improves soil aeration as well as both water and nutrient holding capacity.⁵ It also helps prevent soil from becoming compacted which prevents roots from being able to access everything they need to grow! Biochar alone doesn’t provide all the nutrients that your soil needs, but it improves your soil’s ability to retain nutrients, making them easier for your plant to access. Thus, biochar should be considered, first and foremost, a soil conditioner rather than a fertilizer.⁵ Next come the biological benefits. The porous structure of biochar provides a micro-habitat for beneficial microbes within the soil, such as bacteria and fungi which helps defend roots and fights off pests. Finally, the chemical side of things. Biochar is extremely high in carbon, which facilitates a process known as cation exchange. This basically means that more nutrients are retained, making them even more accessible for your plants!¹⁵

Is biochar a brand-new idea?

The idea of using charred organic matter as a soil conditioner is an astonishingly ancient idea. In South America, in the basin of the Brazilian Amazon, there are areas known as Terra Preta soils. Terra Preta literally translates to black soil and refers to the extremely fertile soil that was likely created by the indigenous people of the Amazon basin by adding charcoal and organic matter such as fish bones and food waste to their soil.¹⁶ Radiocarbon dating has allowed us to estimate that a large portion of these rich soils were formed around 1000 AD.¹⁷ Other studies have dated some of these soils back 7000 years.¹⁸ These pockets of high-quality soil are typically around 20 hectares large and contain significant amounts of charcoal which were key in transforming what were once poor quality, acidic soil into productive agricultural grounds capable of sustaining larger populations.¹⁶ What makes it so much more fertile? These Terra Preta soils have been found to have more organic matter, better nutrient- and water-holding capacity, more nutrients such as nitrogen, phosphorus, calcium, and potassium.¹⁹ In fact, the Terra Preta soils were found to contain three times as much nitrogen and phosphorus as the surrounding soils, and 18 times as much soil organic content.¹⁸ They are also less acidic and allow for higher productivity, according to local farmers.¹⁶ The existence of these ancient ‘biochar’ amended soils provides proof as to the extremely long-term stability of biochar in soil.⁵

On the North American side of things, as early as the mid-1800s farmers were aware of the benefits that could arise from adding charcoal to their soil.

“For two years past I have used some fifty loads each session of refuse charcoal, and being fully convinced that it pays, I wish to recommend it to my brother farmers. I have tried it on grass, corn and potatoes – have tried it alone and in the compost heap, and in all situations it has proved faithful to its trust. As a top dressing for grass, it gives a green colour and luxuriant growth. Applied to half an acre of early potatoes the last summer, the yield was 75 bushels of as fine healthy potatoes as could be desired, that sold readily for one dollar per bushel, and yielded the best profit of anything raised on the farm.” (The New Jersey Farmer, 1856)²⁰

A brief interlude into chemistry!

 On the chemistry side of things, biochar has a polycyclic aromatic structure, meaning that it is both chemically and microbially stable and capable of persisting in the soil for centuries.¹⁶ Microbial stability means that the addition of biochar to soil helps it maintain a healthy and resilient microbial community within the soil, which is great for your plants! Biochar is an extremely porous material which is why it can increase the water-retention ability of soil.²¹ Biochar stores water in a manner that is accessible to plants experiencing drought conditions and prevents water from simply flowing through the soil every time we experience a heavy rainfall!

Biochar’s surface is negatively charged which improves its ability to adsorb positively charged molecules such as hydrogen ions and nutrients in the soil.²² This binding of hydrogen ions is what increases the pH of the soil, making it less acidic. The binding of the nutrients means that they will no longer leach out of the soil when it rains, which also means less fertilizer runoff into our river ecosystems! The binding ability of biochar also allows it to bind tightly to lead, cadmium, and other heavy metals contaminants in urban soil, preventing them from ending up in organisms.²² Biochar’s extremely high surface area means that it has lots of room for adsorption to occur! The surface area of biochar increases with pyrolysis temperature up to 500˚ C, but it then begins to decrease at temps above 600˚ C, demonstrating once again how feedstock and pyrolysis can affect the properties of biochar.⁵

 What should gardeners know about biochar?

 Biochars are not all made equal! Biochar’s physical and chemical properties vary according to the feedstock used to produce it and the temperature it was pyrolyzed at.³ It’s important to select a biochar suitable for your garden’s soil conditions. Biochar offers the most benefits when applied to soils that are degraded, have poor water retention capabilities, or are overly acidic.³ The pH scale ranges from 0 to 14. A pH of 7 is neutral; results above 7 are alkaline while those that fall below 7 are acidic. Biochar pH can range from roughly 6 to 11.²³ The addition of biochar typically makes soil more alkaline, but it’s important to keep your plants’ individual needs in mind! There are some plants that love an acidic environment such as rhododendrons, azaleas, blueberries and hydrangeas, and they might not thank you for adding biochar to their soil.

Some biochars are high in soluble phosphorus, potassium, and calcium, but these are all short-term benefits that depend heavily on the feedstock used to produce the biochar.²⁴ The long-term benefits of increased nutrient and water availability is where biochar really shines.

Biochar is also incredibly stable, remaining intact for centuries! This means that a single application of biochar will affect your garden soil for many, many years. While your garden may still need fertilizer added on a regular basis, the presence of biochar in your soil will make these additional nutrients much more accessible, as well as reducing the overall amount of fertilizer needed. And the other benefits to your soil such as increased water-holding abilities, reduced acidity, increased soil aeration, increased microbial activity, and reduced soil compacting will be available for many years to come!

Finally, is there anything we gardeners should watch out for with biochar?

It is important to know that biochar generally has low nitrogen content. It also adsorbs ammonium, which further reduces the nitrogen available in the soil.¹ This issue can be addressed by using biochar as a soil additive alongside fertilizers with high nitrogen content²⁵ or even good ol’compost.²⁶ Compost made with green and moist material such as grass clippings, plant cuttings, and fruit and veggie scraps tend to be higher in nitrogen.²⁷ Another concern that some studies have discovered is that earthworms may avoid soil that’s been amended with high concentrations of biochar.²⁸ Researchers have hypothesized that this was due to the changes in soil moisture levels, and that this effect could be avoided by wetting your biochar either before or immediately after applying it to your soil.²⁹

As I’ve mentioned, biochar is most beneficial in acidic or neutral soil because of its tendency to increase soil pH. Be careful that you aren’t adding biochar to already highly alkaline soil, or around plants that prefer an acidic environment! Finally, feedstock matters and should be sustainably sourced. It’s important to know where your biochar comes from and what’s in it!

If all this biochar talk has piqued your interest, I encourage you to look into locally based biochar companies that prioritize transparency in their production methods. As interest grows, so too will supply and demand. In fact, as of 2024, construction on Canada’s largest biochar production plant is underway in Port-Cartier, Québec, where they expect to produce more than 10,000 tons per year.³⁰


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