Biochar looks like ordinary charcoal, but it behaves very differently once it goes into soil. It is one of the few materials that can improve farmland and remove carbon from the atmosphere at the same time. Here is what it is, how it is made, and how to tell good biochar from bad.
A stable form of carbon
Biochar is made by heating biomass, such as wood, crop residue or invasive weeds, to between roughly 350 and 700°C in an environment with little or no oxygen. The process is called pyrolysis. Without oxygen the biomass cannot burn to ash; instead, its volatile compounds are driven off as gases and oils, and what remains is a solid that is mostly carbon.
That carbon is very different from the carbon in fresh plant material. During pyrolysis it is rearranged into stable, ring-shaped structures that soil microbes find very hard to break down. Compost and crop residues are mostly consumed by soil life within months or a few years, returning their carbon to the air as carbon dioxide. Biochar persists: a meta-analysis of decomposition studies estimated its average residence time in soil at several hundred years, with the most stable fraction lasting far longer.
An old idea with new science
The modern interest in biochar began in the Amazon basin. Scattered through a region of famously poor, weathered soils are patches of extraordinarily fertile dark earth, terra preta, created by indigenous communities hundreds to thousands of years ago. Researchers found that these soils contain large amounts of charcoal, mixed with pottery, bone and organic waste. Centuries later, they still hold more nutrients and organic matter and support better crops than the soils around them.
Traditional farmers in many parts of the world, including India, have long mixed charred material and ash into their fields. What is new is the ability to make biochar consistently, measure its properties and understand how it works in soil.
Millions of tiny pores
Under a microscope, biochar retains the cell structure of the plant it was made from: a honeycomb of channels and pores of many sizes. A single gram can have an internal surface area of tens to hundreds of square metres. That structure gives biochar the three abilities that tired soils most often lack.
It holds water, in pores small enough to keep it from draining away. It holds nutrients, on surfaces that develop a negative charge as biochar ages in soil and attract positively charged nutrients such as potassium, calcium and ammonium. And it gives microbes a home, sheltered spaces where bacteria and fungi can colonise, protected from drying out and from grazing by larger soil organisms.
Why it is different from ash and charcoal
Ash is what is left when biomass burns completely in oxygen: a small amount of minerals with almost no carbon and no structure. It can supply some potassium and raise soil pH, but it is quickly washed away. Biochar keeps its carbon and its structure. That is the difference between a spent fuel and a soil-building material.
Charcoal and biochar are made in a similar way, but they are made for different purposes. Charcoal is made to burn and is judged on its heat value. Biochar is made to go into soil, so it is judged on its stability, its purity and its effect on plants. Traditional charcoal kilns also release a lot of smoke and methane; modern biochar plants burn those gases to heat the process.
What makes good biochar
Not every char is good for soil. The feedstock and the temperature both matter. Higher temperatures generally produce biochar with more stable carbon and more surface area, while lower temperatures keep more of the original nutrients. Woody feedstocks give low-ash, carbon-rich biochar; manures and crop residues give biochar richer in minerals.
International standards set out what to test. The International Biochar Initiative (IBI) standards and the European Biochar Certificate (EBC) both look at carbon content, the ratio of hydrogen to organic carbon (a measure of stability), ash, pH, and limits on contaminants such as heavy metals and polycyclic aromatic hydrocarbons. Buyers should ask for a certificate of analysis for every batch, covering at least fixed carbon, ash, moisture and bulk density.
How Sainya Krishi makes biochar
Our feedstock is mainly Prosopis Juliflora, an invasive tree that has spread across dry lands in Tamil Nadu and elsewhere in India, drawing down groundwater and crowding out native vegetation. Clearing it is a public priority; turning it into biochar gives the cleared wood a valuable use. We supplement it with agricultural residue and wood waste.
The biomass is dried and processed in a continuous rotating-drum reactor at 350–550°C. The gases released are burned to heat the dryer and the reactor. The biochar is then cooled, screened into consistent particle grades and tested batch by batch to IBI and EBC parameters.
How biochar is used
Most biochar goes into soil, ideally after it has been charged with nutrients by mixing it into compost or soaking it in liquid manure or fertilizer. It is also used in potting media, as a compost additive, in animal feed and litter, in water and odour filtration, and increasingly in construction materials. Because the carbon in biochar stays locked away for centuries, every ton applied also counts as durable carbon removal, which can be verified and sold as carbon credits.
Three common misunderstandings
Biochar is not a fertilizer. It contains little nitrogen and only modest amounts of other nutrients that plants can use straight away. Its value lies in how it changes the soil: holding water and nutrients, hosting microbes and raising pH in acidic soils. That is why it is usually applied together with compost or fertilizer, not instead of them.
More is not always better. Very heavy doses of fresh, uncharged biochar can hold back nitrogen and, in alkaline soils, push the pH too high. Moderate, charged applications built up over several seasons are safer and usually more economical.
Not every black, burnt material is biochar. Open-burnt residue, kiln fines of unknown origin and ash all look similar but behave very differently, and some can carry contaminants. Buy biochar from a producer who can tell you the feedstock and the production temperature, and who tests every batch.
How to read a biochar lab report
A certificate of analysis tells you what you are buying. Fixed carbon is the stable carbon left after volatile matter is driven off; higher values, typically above 60% for wood-based biochar, mean more durable carbon per ton. Ash is the mineral fraction: a little brings useful potassium and calcium and raises pH, but high ash means less carbon. Moisture tells you how much of the weight you are paying for is water. Bulk density tells you how much space a ton takes up, which matters for transport and for calculating application rates by volume. More detailed reports add pH, the hydrogen-to-organic-carbon ratio, surface area and contaminant tests for heavy metals and polycyclic aromatic hydrocarbons.
Getting started
If you are new to biochar, start small. Try a ready-charged product such as biochar-enriched compost, or charge a few bags of biochar in your own compost heap, and compare a treated plot with an untreated one over two or three seasons. Measure what matters to you: yield, water use, fertilizer use and soil organic carbon. The results on your own soil are the best guide to how far to scale up.
Sources and further reading
Lehmann, J. & Joseph, S. (eds.) (2015). Biochar for Environmental Management: Science, Technology and Implementation, 2nd edition. Routledge.
Glaser, B., Lehmann, J. & Zech, W. (2002). Ameliorating physical and chemical properties of highly weathered soils in the tropics with charcoal: a review. Biology and Fertility of Soils 35, 219–230.
Wang, J., Xiong, Z. & Kuzyakov, Y. (2016). Biochar stability in soil: meta-analysis of decomposition and priming effects. GCB Bioenergy 8, 512–523.
Woolf, D., Amonette, J. E., Street-Perrott, F. A., Lehmann, J. & Joseph, S. (2010). Sustainable biochar to mitigate global climate change. Nature Communications 1, 56.
International Biochar Initiative: Standardized Product Definition and Product Testing Guidelines for Biochar That Is Used in Soil (IBI Biochar Standards).
European Biochar Certificate (EBC): Guidelines for a sustainable production of biochar. Carbon Standards International.
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