New to Climate Change?


Biochar is a type of processed plant matter that is very rich in carbon. Lightweight, black and very porous, it looks and feels very much like charcoal. But where charcoal is used for cooking and heat, biochar is used in soils to help grow crops.

It can also help address climate change. Biochar is one of several “carbon removal” techniques that target carbon dioxide (CO2), the most important climate-warming greenhouse gas humans have been adding to the atmosphere.

Biochar and the carbon cycle

As plants grow, they breathe in CO2 from the air, using the carbon they absorb to build their tissues. Then they die and rot or decompose, releasing CO2 into the air again.
But if they are turned into biochar, the carbon is instead converted into a solid, which can stay locked in soil for many years. In this way, plants become a sort of carbon removal engine, drawing climate-warming CO2 out of the air and storing it in the ground.

Biochar can be made from all kinds of plant waste material: wood, shells, crop waste, the waste from paper mills, sawmills or breweries, and more. This waste material is loaded in a special stove-like device called a “torrifier,” “pyrolyzer” or “gasifier,” or a low-tech version like a kiln. Inside the device, the raw materials are deprived of oxygen as they’re heated to between 200° and 700° C (or “pyrolyzed”).
Without oxygen, the plant matter can’t catch fire, and its carbon does not turn into CO2 and escape into the air. Instead, it is converted into biochar.
Depending on the operating temperature, the process also yields liquid (called tar) or gas (called syngas). These can be burned to produce the heat needed to maintain the operation of the pyrolyzer. In this way, a pyrolyzer can power itself, or make some extra fuel or energy for sale.
Once produced, biochar can be added to soil. It can be sprinkled on top, buried in a layer or in holes, or mixed in with compost or seeds. The biochar’s carbon can stay inside the soil, and out of the atmosphere, for an extended period of time. Indigenous people in the Amazon have been adding charcoal to soil for over 2,000 years, producing a dark, fertile earth called terra preta that is still enriched with carbon today.

Biochar for farming

Biochar is only useful as a climate solution if farmers agree to use it. Luckily, there are good reasons to do so.
In the right conditions, biochar can make crops grow larger and faster. Biochar rarely contains many nutrients itself, but it can change the soil in ways that improve “nutrient availability”: the ability of soil to hold onto nutrients like nitrogen, phosphorus, and calcium in forms that plants can easily absorb. It can also help restore soils that have become too acidic, or contaminated with certain heavy metals.
And it can help farmers spend less on other resources. Biochar is porous, helping soil hold onto water, which can save on water usage especially in dry areas. Crops grown with biochar may also need less fertilizer. This is another climate benefit, as the production of fertilizers is itself energy intensive.
The challenge for biochar is that it is not a cheap or one-size-fits-all product. It can be made of many different plant materials, and pyrolyzed at lower or higher temperatures, all with different results. A type of biochar that is great for a specific soil, or a specific crop, may be very little benefit to another. In general, biochar is better for poor, nutrient-depleted or acidic soils than healthy ones. 
All these variables make it harder to mass-produce biochar—which is one reason it tends to be more expensive than other soil amendments like fertilizer or compost. Many biochar systems focus on using local waste products as raw ingredients, which saves on transportation costs but limits how much can be produced and used. If biochar is to grow into an industry large enough to make a real difference for climate change, we will likely need both new innovations to make it more cheaply and efficiently, and a great deal of on-the-ground work to inform farmers of the benefits of biochar and help them use it in the best ways for their land.

Published February 27, 2024.


Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International license (CC BY-NC-SA 4.0).
Photo Credit
Simon Dooley via Flickr