Ever since the discovery of the incredibly fertile ‘terra preta’ soils in the Amazon in the 1980’s biochar has been prodded and probed by science to find how and why it is so good for long term soil fertility. The consensus so far, as so often in the world of scientific research, reveals there is a simple answer but when we delve more deeply it gets more complicated. Lets start with the simple concept – surface area.
Biochar can be made out of pretty much any organic material (bones, chip, coconut husks, bamboo etc) but in the UK it is typically make out of wood so we will use that as an example. When wood is subjected to pyrolysis (i.e. the heating, burning, and then shutting off the oxygen before it is completely burned) it leaves the wood pretty much physically intact but now consisting of mainly of carbon. This means that all the intricate channels and vessels that transported water and nutrients to the tree when it was alive are also still intact – effectively creating a vast network of pores and channels which results in a huge surface area (see photo below). It is this microscopic labyrinth of holes and tunnels that provides a home to a vast array of micro-organisms, fungi, and nutrients in the soil which helps to create more fertile and healthy soil. It also serves as a ‘water buffer’ soaking up water in wet periods and releasing it in dry periods thus making the soil (and therefore plants) more flood and drought resilient
Biochar under a microscope
All biochars are not the same
When we delve a bit deeper into the science it seems that the right size and structure of the microscopic pores is key to creating the right ‘home’ for the micro-organisms. Biochar derived from wood tends to keep is structure and create the correct pore sizes, whereas biochar derives from materials such as straw or coconut shells may not be as good. It seems that the correct size of pore allows microbes to live there and also be protected from predators. At the same time the walls of the microscopic pores chemically ‘soak up’ nutrients and allow the microbes to feed on the these nutrients – this process is on going and is thought to be highly beneficial to microbe populations and therefore the soil health. Whats more, biochar is not broken down in the soil like most organic matter so its stays in the soil for decades, possibly hundreds of years.