What Even IS Biochar? A Gardener’s Perspective.

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