What is biochar?
Unlike ash or charcoal, biochar is highly porous and can last hundreds to thousands of years in the soil. This makes it both a soil enhancer and a way to lock carbon safely out of the atmosphere.
Pyrolysis methods:
How is Biochar made?
There are many ways to make biochar, ranging from simple pits dug in the ground to advanced automated machines.
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Pit or trench – Simple and low-tech, often made where the biomass is.
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Kon-Tiki kilns – Cone-shaped, can produce about 1000 litres in a single afternoon.
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Ring of fire kilns – Portable, made from metal sheets, and designed for clean burns.
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Retorts – Batch systems where vessels are filled, sealed, and heated until complete.
What determines Biochar's quality?
Not all biochar is created equal. Its usefulness depends on three main factors:
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Temperature & time
How hot and how long the biomass is heated (called the highest treatment temperature or HTT) shapes the final biochar:-
Around 450 °C – the minimum for forming strong carbon ring structures that make biochar long-lasting in soil.
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About 600 °C – considered ideal, giving the best balance of stability and porosity.
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Above 750 °C – diminishing returns, as the structure begins to collapse and lose surface area.
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Feedstock preparation
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Woody materials (like forestry slash, sawdust, bark, orchard prunings, or brush) produce the highest quality biochar — high in carbon, porous, and stable.
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Non-woody wastes tend to make weaker biochar, with more ash and less stable carbon. These are often better suited for composting instead.
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The size and shape of the feedstock matters too: continuous pyrolysis systems usually need chipped or shredded material, while batch kilns can handle bigger pieces but take longer.
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Quality control
Using the wrong feedstock can be dangerous:-
Treated or painted wood can release heavy metals and toxins.
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Contaminants like chlorine or fluorine in biomass can form harmful compounds.
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Small amounts (<2% dry weight) of oil or plastics are generally broken down safely by the high heat, but more than that poses risks.
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For commercial production, regular testing is essential. The International Biochar Initiative sets standards to make sure biochar meets safe and effective guidelines before it’s used in soil or compost.
10 ways Biochar improves composting
Adding just 2–10% biochar into compost can transform the process. Here’s how:
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Holds water – Acts like a sponge, helping compost stay moist without going soggy.
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Improves airflow – Its porous structure supports aerobic microbes.
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Locks in nutrients – Reduces nitrogen loss and improves the C:N ratio (the proportion of carbon to nitrogen in organic material).
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Balances pH – Naturally alkaline, biochar helps counter compost acidity.
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Supports microbes – Provides a home for bacteria and fungi that drive decomposition.
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Reduces leaching – Keeps nutrients in the pile instead of washing away.
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Cuts greenhouse gases – Lowers methane and nitrous oxide emissions.
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Stores carbon long-term – Recognised by the IPCC as a key climate solution.
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Adds market value – Biochar-enriched compost can be sold as a premium product.
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Improves air quality – Reduces odours like ammonia and hydrogen sulphide.
Looking to the future + benefits
The benefits of biochar as a co-composting addition to compost inputs are numerous and compelling. Imagine a future when a pyrolysis kiln or machine is a standard part of any commercial composting operation. Compost creation is more environmentally friendly with the carbon sequestered in the end users soil application leaving it permanently improved for future generations as well as being a positive action mitigating climate change.
Thanks to the Biochar Network New Zealand for sharing this article with us.
