Biochar is a fancy name for charcoal, coined because of the specific use of charcoal as a soil amendment rather than as a fuel. Like charcoal, biochar is the solid material left behind when organic material is burnt in low levels of oxygen. This can happen naturally such as when belowground peat deposits burn but more commonly is engineered by humans in special combustion chambers through a process called pyrolysis.

What happens in pyrolysis is that rather than combust completely to ash, as when we burn wood on an open fire, there is not enough oxygen for all the organic material to burn. Instead the organic material forms a resistant black substance that is almost complete carbon, the char, and liquid gas products.water

When biomass smolders for a long-time at low temperatures it generates mostly char and when the temperature of pyrolysis is higher and the process faster more gas is produced.

The actual structure of the char is like a honeycomb with air pockets between a fine matrix of carbon atoms.

How to make biochar

Making biochar is simple enough in principle. You need a supply of organic material that can be almost anything from dead plants to animal waste. It can be wet or dry so long as there is sufficient organic matter to support combustion.

Then some device is required in which to burn the organic matter in low oxygen that can be as simple as a trench in the ground covered with leaves, or a kiln on the back of truck, all the way to a full-scale industrial facility.

In the industrial version organic material, that the engineers would call the feedstock, is dried and fed into a kiln and burnt in low to no oxygen at temperatures between 400 and 700 Celsius. More char is produced at the lower temperatures.

The challenge in manufacture is to obtain a consistent supply of feedstock to make it easier to maintain the desirable conditions with the combustion chamber and maintain production.

Biochars have been made from

wood and wood chips
food waste
poultry manure
sewage slurry
bagasse (sugarcane waste)
Each type of organic material requires certain conditions for effective biochar pyrolysis. The wetter the material the more difficult it is to maintain the combustion. As a rule of thumb around 40% by weight of the organic material into ends up as char.

Industrial production can be achieved where the feedstock (biomass materials) can be transported to a central facility and processed. These facilities usually operate to generate gas and char, where the gas is further combusted to generate electricity.

Alternative approach is to use vehicle mounted kilns that can move between biomass sources, such as from one farm to the next. Current equipment can process up to 450 kg of biomass per hour.

Why is biochar useful?

When a plant grows it uses water and nutrients from the soil. Fine roots grow into the tiny spaces between the soil particles and nutrients flow across from the soil water into the cells in the roots. It is a world we cannot see that is dependent on microbial activity in those small soil spaces.

Biochar is carbon with a microscopic honeycomb structure with the actual size of the pore spaces dependent on the type of biomass used and the method of pyrolysis. This honeycomb structure is ideal for both holding water, providing micro-habitat for microbial activity and enhancing nutrient exchange between the soil and plant roots.

The carbon in char is stable and resists further degradation or decomposition even when it has been in the soil for hundreds of years.

These properties make biochar an excellent soil amendment that can help improve poor or degraded soils.

What we call poor soil is soil that has low levels of plant nutrients, or has nutrients locked away in clods that roots cannot penetrate, has low oxygen levels and is either dry or waterlogged.

All these conditions are hard for plants to prosper and this, in turn, limits soil biological activity as there are fewer organic inputs to sustain soil animals and micro-organisms.

Healthy soil that supports plant production is

aerated
has stable physical structure
biologically active
high in carbon and
retains moisture
These properties promote efficient nutrient exchange and strong plant growth.

Applying soil amendments can help improve soil health and biochar is one of the better amendments to apply because it

adds carbon
helps retain water
improve soil water quality
reduces nutrient losses (leaching)
helps lower acidity
Together these benefits can also reduce the need for fertilizers and irrigation. The good thing is that these benefits can apply to the soil in your garden as well as an arable field.

The biochar opportunity

In the next 30 years global demand for food and fibre to support a growing and increasingly affluent human population can only be met from a doubling of current agricultural production. Whilst we have doubled production before this has been through clearing land for production, the use of artificial fertilizers and improved crop varieties.

These options are now far more difficult as most of the useable arable land is already in use, fertilizer use is becoming less effective [and more costly], and genomics has already made its biggest gains.

Making matters worse is that a significant amount of agricultural land is degrading after many generations of crop production. In many areas soil quality is declining with loss of carbon and nutrients and, at the extreme, is being lost to wind or water erosion.

The biochar opportunity is to achieve agricultural production gains through a focus on soil quality by adding carbon to soil.

Biochar is ideal for this purpose. It is an inert form of carbon that would immediately increase the carbon content of soil. The honeycomb structure of biochar increases the surface area of soil providing vital micro-sites for microbial growth and nutrient exchange. The honeycomb also helps retain moisture that, in turn, promotes biological activity.