Tag Archives: Growing Medium



(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)
Source: https://link.springer.com/article/10.1007/s42773-020-00067-x/tables/1

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.

Bonsai Tree Growth Stages

Most bonsai trees progress through stages of development, each with a different objective. In general the progression is thicken trunk -> achieve branch & root structure -> achieve branch, foliage & root ramification -> reduce leaf size -> evolve as branches grow/fall off. The faster we can move through the first few development stages, the faster we will have beautiful, well-proportioned bonsai – harnessing the tree’s natural growth is a way to speed this up. We also want to avoid doing things which slow down a tree’s growth during these phases, as this will mean it takes longer to get the tree we want. Read about how trees grow before starting at #1 below. Also consider what do old trees look like?

1. Trunk

Some bonsai enthusiasts collect mature trees for bonsai specifically so they can start with a thick trunk, following a collection process which minimises damage to the tree. The alternative is growing your tree’s trunk. Once a tree has its roots and foliage reduced in size in a bonsai pot, it won’t generate the energy needed to make significant sapwood additions and its girth will only increase by small increments every year. So you really need to be happy with the trunk size first before you stick it in a tiny pot. But – how big should a bonsai tree’s trunk be?

2A. Branch Structure & Overall Shape

Arranging the branches is what gives you the canopy and overall foliage shape that you’re after and the first step in this process is growing (or developing) the branches you want in the positions they are needed. Growing a branch starts with a new bud, which, unless it’s a flower bud, becomes an extending shoot and eventually a new branch. So firstly you need to work out where new buds will grow on your tree and then deal with the extending shoots as needed to get the required internode length.

You may need to remove some buds and shoots if they don’t help achieve the shape you are looking for – this should be done as soon as possible to avoid wasting the tree’s finite energy reserves. You have a trade-off to make here because leaving more foliage on the tree will provide more energy overall which contributes to its health and ability to recover from interference. However, growing areas of the tree which won’t be part of the future design is a waste of energy. You don’t want to remove so much of the tree’s foliage that it struggles to stay alive or develop the areas that you do want to grow out.

When you are creating your branch structure, often you will need to reposition branches – this is done with a wide range of different tools and techniques. A more advanced technique for adding new branch structure is grafting.

Sometimes the trunk itself or larger branches need a rework, to make them more interesting or to make them look more like old trees – for example adding deadwood or hollowing out the trunk. Usually this is achieved through carving.

2B. Creating a Strong Root System

The trunk thickening and branch structure phases both work best when the tree has lots of energy and so letting it grow in the ground or in a decent sized pot during these phases will get you there quickest. This also allows the roots to keep growing, but you want to understand about the role of roots, and root structure & architecture even if you still have your bonsai in a training pot. Particularly in this case, knowing about how to foster the the rhizosphere will help your tree stay vigorous. To maximise the roots’ exposure to nutrients and water you want to encourage Ramification of Roots (lateral root development).

Eventually it’s time to move the tree into a bonsai pot. This requires cutting back the roots, but as long as the roots are balanced with the foliage in terms of biomass, the tree should be OK. Root growth is usually prioritised outside of times of stem/foliage growth, and above 6-9 degrees C. So repotting might be best conducted at times that meet this criteria. Your growing substrate/medium is an important consideration.

3. Ramifying Branches & Foliage

Ramification is when branches subdivide and branch, giving the impression of age and a full canopy – and a well-ramified tree is a bonsai enthusiast’s goal. There are some techniques for increasing the ramification of branches and foliage. But not as many as there are for root ramification.

This stage also involves ongoing branch selection and reshaping (see 2A above). Another consideration is whether to keep or remove flower buds.

4. Reducing Leaf Size

An end stage in the journey to bonsai perfection is leaf size reduction. In nature, leaf sizes reduce relative to the biomass of the tree as it ages but since bonsai are small this effect doesn’t translate since the biomass never gets large enough. The tried and tested method for reducing deciduous tree leaf size is actually to practice one of the various methods of defoliation. A couple of others are covered here in reducing leaf size.

When to conduct these various activities depends on when the tree can best recover from them – which is a function of the Tree Phenology (or Seasonal Cycles).

5. Evolving Branches

Trees are not static organisms – they obviously continue to grow which is what we harness in the above steps. Part of this is that eventually branches may become too large for the design, or they may fall off (Peter Warren notes that Mulberry are known for this). As bonsai artists we want to have this in mind so that branches are being developed which can take their place in the future. This is an ongoing version of step 2A.

bonsai substrate

Bonsai growing medium

Now here’s a topic to generate some internet debate! This is really a subject that every bonsai enthusiast has an opinion about – whether akadama is worth the money, whether cat litter is a legitimate medium, whether to add organic material, there is a ton of disagreement on this subject. So how might we take a scientific approach?

Well the starting point is that the growing medium needs to enable the supply of everything that the tree via its roots requires – specifically water, oxygen (for respiration) and nutrientsref. Now, you may add nutrients via fertiliser, but the medium needs to catch those nutrients so that the roots (or symbiotic bacteria) can absorb them, similarly with water – so one important characteristic is that the medium must hold water in a form which is accessible to roots.

Another super-important attribute of the medium should be that it helps establish and nourish a thriving rhizospere. This means providing a home for beneficial bacteria and fungi, enabling the roots to come into contact and to interact with them and for the roots to generate their exudates. The medium needs to hold and release the substances which are important to these microorganisms, and it needs to allow them to breathe.

We also want to have a medium in which roots grow freely, and ramify, to better support the tree in the pot and provide more surface area for nutrient and water absorption.

Wouldn’t it be amazing if there was such a medium out there? Oh, well actually there is – soil! The world over, the nutrient, rhizosphere and root growth requirements of trees are supplied by soil. According to the Royal Society, “‘well-structured soil’ will have a continuous network of pore spaces to allow drainage of water, free movement of air and unrestricted growth of roots…typically, a ‘good’ agricultural soil is thought to consist of around 50% solids, 25% air and 25% water,”ref


They also say that “bacterial diversity is affected by soil particle size, with a higher percentage of larger sand particles (ie coarser soil) causing a significant increase in bacterial species richness” and “the ability of soil structure to hold moisture is linked to a high microbial diversity and more robust populations of soil mesofauna and macrofauna”ref

This study found that bacterial and fungal abundance was positively associated with high phosphate, high pH, a lower Carbon:Nitrogen ratio, sandiness of soil texture and soil moisture. It was negatively associated with the presence of Chromium, Zinc, silt, a high Carbon:Nitrogen ratio or clay soils.ref

So what can we conclude from all of this? In terms of structure we want the right ratios of soil/water/air (50% soil particles, 25% water, 25% air) and the soil to have a higher percentage of larger, sandy particles (not clay or silt). The question for bonsai comes down to water retention since a pot with a hole is much more draining than soil. Options for water retaining elements in bonsai medium include bark, compost, biochar, perlite or vermiculite. Clay also has high water retention but perhaps too much, as it can cause anaerobic conditions which results in nasty gases being produced by bacteria. Different components such as akadama, lava rock, pumice and so on can provide the structural part of the mix which create air spaces.

Some media have so-called pores – tiny holes which hold water which is accessible by roots. “The higher the large pore (macropore) density, the more the soil can be exploitable by plant roots… the presence of continuous macropores significantly benefits root growth.”ref An example would be biochar which has a huge surface area thanks to many tiny tubes and pores throughout its structure.

What you want to avoid in your bonsai medium is anything which is too acidic (except if you have an acid-preferring tree) as this would reduce the microbes, or anything with anti-fungal or anti-bacterial properties (such as – ahem – cat litter or diatomaceous earth). You also want to avoid (per the above) anything which reduces the roots’ access to air & water by getting overly compacted or wet, or having overly draining components which don’t hold water.

Bonsai wisdom says that adding ‘organic’ components such as compost or leaf litter is bad for various reasons – they break down and reduce drainage, they run out of nutrients too quickly, they aren’t controllable. But personally I think adding organic matter of some kind is a good thing, as it mimics the natural world, has all sorts of beneficial compounds (such as those included in some non-nutrient additives) and provides some small particle sizes as part of an overall mix.

As it happens, I finally found a bonsai-specific research study! These are extremely rare. In the Journal of American Bonsai Society this article showed the results of an experiment measuring the water retention of different bonsai soil components. See below:

Based on this, if you were using the 25% air 25% water rule of thumb, most of these would be fine as bonsai soil with just a bit of added water retention. Interesting that pine bark is actually quite similar to akadama – I have recently been wondering whether you could grow trees entirely in bark if it was the right size. Maybe it’s time to try!

Another study looked at particle size, finding that “media components that differ significantly in particle size have lower total porosity, water-holding capacity and air-filled porosity than media composed of similar particle sizes.”ref

One final word on different mediums for different trees. Obviously, different trees come from different habitats and happily grow on soils native to that habitat. I have a tiny olive in a tiny pot with extremely coarse medium that dries out easily and it’s thriving (albeit, I live in London). Angiosperms transpire more than gymnosperms so in theory need more a more moisture-retaining medium. A tree with a very high foliage ratio relative to the size of the tree will also need a lot of moisture. So think about the ‘natural’ habitat of your tree and what the soil conditions likely are, and try to adjust accordingly.

The nice thing about the scientific method is that it’s not all theory – observation and experiment is an integral part. If you start with a general medium, you can adjust it to be more water-retaining by adding compost or bark, or less by adding more akadama/pumice or increasing the particle size. See how things go and adjust when you repot.