Biochar is any bio-mass heated in a limited oxygen environment to drive off the volatile part, leaving a highly porous carbon rich char which is chemically very stable. This process is known as pyrolysis. The bio-mass may be wheat straw, chicken manure, sewerage, woodchip; in fact anything organic. The process of pyrolysis also has many variants, from low temperature long duration to high temperature short duration, and many kiln designs which add further to the spectrum of what biochar can be.
All chars will have certain things in common; porosity and general chemistry; but there will be differences. For example wheat straw will produce a char which will add many nutrients but will have a smaller percentage of ‘fixed’ carbon.
At SA Biochar Works I use hardwood branches as my source, (also known as feedstock) and heat it to about 700 degrees celsius. This produces a char which is highly porous with a high percentage of fixed carbon and makes my biochar especially good at retaining moisture in the soil, and sequestering carbon, because fixed carbon, as the name suggests, is extremely durable.
The extreme porosity of biochar makes it an ideal sponge, holding moisture within the soil until it is used by plants. The spaces also provide habitat for soil microbes, making them more resilient to drought and able to rebound more rapidly when conditions improve. For this reason I recommend leaving chunks as they come rather than crushing the lot to powder.
The surface of the char will attract and hold in a loose bond, cations, which are positively charged ions. These are many of the nutrients we add fertilizer for; phosphorus, potassium, magnesium, manganese etc. Thus fertilizer we add is more efficiently utilized since the amount that is leached from the soil is reduced. Plus, adding biochar is like adding lime to soil with a concurrent raising of soil pH.
Biochar will also reduce the amount of non CO2 greenhouse emissions (CH4, N2O), and improve soil structure.
All in all a powerhouse of horticultural delights, but it doesn’t end there. Because of the inert aromatic structure of the carbon in biochar, it can potentially remain in soil for thousands of years. The terra preta soils are over 2,000 years old and still going strong.
Clearly biochar is a product whose time has come.