(Thanks to Dr. Karen O’Hanlon of Probio Carbon for answering some of my questions about biochar).

Biochar is a product which has been advertised as a beneficial component of bonsai soil over recent years. So what exactly is it?

Biochar is basically charcoal which has been “produced from organic waste using pyrolysis technology under temperatures ranging from 400C to 700C where oxygen is either absent or depleted”.ref Pyrolosis means decomposing carbon-based materials through the application of heat.ref So a feedstock (source material) is acquired and heated in the absence of oxygen for a given period of time to create what you would probably recognise as charcoal. The structure of biochar is shown in the image – as you can see, it has many, many holes in it.

Scanning Electron Microscope image of biochar

So why would you add biochar to your bonsai soil? There are a few good reasons. It has been proven to improve water availabilityref, act as a fertiliser reducing the need for chemical fertilisersref and increase microbial biomassref (ie. it attracts beneficial microbes).

An experiment conducted in Colchester, UK by the Bartlett Tree Research & Diagnostic Laboratory amazingly found that ash trees treated with biochar did *not* get infected by ash dieback disease over a period of 4 years even when the disease was present in adjacent trees on the same site. They believed the reason for this was that the biochar enhanced the trees’ immune system and improved root growth.ref

The microbe aspect of biochar is really interesting – in one study it was found that microbes living in it were able to ‘mine’ the biochar pores for phosphorus. So it appears to have synergy between its composition (with nutrients for plants) and its attractiveness to microbes which can help get those nutrients into plants.

One of the key physical properties of biochar is that it has a massive surface area, relative to its size – in one study on malt spent rootlets (a residue from brewing) it was 340 m2 per gram!!ref That’s larger than the size of a tennis court for every gram of biochar.ref This increased surface area along with the physical structure of biochar having lots of tiny pores, results in greater water retention in the soil.ref

Biochar can be made from basically any organic material, from forestry to food production to agricultural by-products and this source material is the main determinant of its chemical properties.ref So when choosing a biochar for your bonsai soil, you want to know what it has been made from, and what this means in terms of its properties. Some of the properties which vary significantly include pH, surface area and cation exchange capability/electrical conductivity. For bonsai I would say you want high surface area & pore volume (to assist with water availability) and high microbial mass. The fertiliser aspects are probably a nice-to-have. Looking at the table below this means probably biochar made from a wood-based source material is best.

There is quite a bit of research out there on different biochar properties, which I will summarise here for you to read through. Unfortunately I haven’t found any research which looks at volume of microbes for each feedstock, but I would expect this to be positively associated with surface area.

Biochar FeedstockProperties
WoodHighest surface area (leading to better water retention) and highest pore volume (a factor of 10 higher than manure)
Lowest cation-exchange capability
Largest amount of C
Contain less plant-available nutrients
More electrical conductivity
Lowest ash content (associated with lower pH)
Micro-nutrient content mixed (see table here)
Total bioavailable nutrients mixed (see table here)
Crops & grassesHighest average particle size
Highest K content
Lowest calcium carbonate equivalents
Micro-nutrient content mixed (see table here)
Total bioavailable nutrients mixed (see table here)
ManureLowest surface area and lowest pore volume
Highest cation-exchange capability
Highest calcium carbonate equivalents
Lowest average particle size
Highest ash content (associated with higher pH)
Greatest N, S, P, Ca, and Mg concentrations
Highest micro-nutrient content (Fe, Cu, Zn, B, Mn, Mo, Co, Cl)
Total bioavailable nutrients mixed (see table here)

The temperature at which the biochar is created makes a difference too. Increasing pyrolisis temperature leads to “increased biochar C, P, K, Ca, ash content, pH, specific surface area (SSA), and decreased N, H, and O content”ref

Like many things in life though, you can have too much of a good thing. In some studies, too much or the wrong biochar in soil has led to phytotoxicityref You might also be wondering why it doesn’t just remove all the nutrients in the soil like activated carbon, which is used in aquariums and drink bottles to remove metals, chlorine and contaminants. When asked this question Dr. Karen O’Hanlon at Probio Carbon said it was because biochar is not ‘activated’ to the same degree as activated carbon. Reading more about this, the absorbent properties of biochar are “1/6th to 1/12th that of high quality activated carbons”.ref Activation forces more pores and surface area into the charcoal, this is done by varying the temperature and pyrolysis process. So whilst there probably is some nutrient absorption, it’s not going to be at the same level as activated carbon and can be compensated for by the nutrients within the biochar themselves and the increased microbial activity.