An Introduction to Bonsai Soils

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Organic Soil Mixes and Components

In past decades, Western bonsai enthusiasts tended to use organic soil mixes, using a large proportion of peat, bark and leaf-litter mixed with grit to aid with drainage.

As time passed, our knowledge and understanding of bonsai in the West increased, it is now acknowledged by most enthusiasts that organic soil components such as peat are not conducive to the good health and vigour of a tree.

Peat and other organic soil components have many disadvantages; they can be too water retentive, leading to the soil being continually sodden, particularly during periods of rain in Autumn, Winter and Spring.

Conversely, during periods of high temperatures, dry peat can be difficult to thoroughly water, leaving dry spots inside the rootball of the bonsai.

Possibly the most serious problem with organic soils is that though they may consist of appropriate sized particles when the bonsai is first planted, they continue to break down in a bonsai pot and become compacted. As the soil compacts it becomes airless and drains poorly. Such waterlogged and airless soils soon suffocate the roots and can lead to rotting roots and ill-health in a bonsai.

The only organic component that I would still recommend using as part of a bonsai soil mix is composted bark, sifted to remove any very small particles (fines) . While bark will break down slowly, it still holds its structure for a long time and until then, will not impede the air circulation or the drainage of a bonsai soil
 

Inorganic Soil Mixes and Components

The advantage of inorganic materials is that they hold their open structure for a long time without breaking down into mush. Inorganic materials retain a certain quantity of water and any excess is immediately flushed through the bottom of the pot; it is difficult to 'overwater' a bonsai planted in a good inorganic bonsai soil mix.

akadama bonsai soil

A bonsai planted in Akadama. The new Akadama around the edges of the image is still holding its structure, however the remainder has very quickly turned into a muddy, airless mess.

Akadama is Japanese baked clay, specifically produced for bonsai and imported into the West; it is normally only available from bonsai nurseries and therefore can be difficult to locate in person, however it is widely available online. There are a number of grades of Akadama available including 'Double Redline' that is more costly but is of premium quality and less likely to break down.

Akadama has some very useful qualities as part of its chemical make-up and can be useful as part of a soil mix. However, Akadama can break down into a solid mush within 1 or 2 years. For this reason it is not a good idea to use a high-proportion of it in soil-mixes intended for species that will not tolerate regular bare-rooting (Pines and Juniper for instance). In general it should be mixed with other soil components that will not break down and will ensure that the soil structure keeps its intergrity.

bonsai soils

Catlitter or 'Diatomite' fired-clay on the left, and right, a blended soil consisting of a variety of different aggregates such as Molar-clay, Lapillo and Pumice volcanic rock.

Seramis/Turface/Oil-Dri/Molar are fired clays are readily available (online). Fired clays are stronger than Akadama and thus will not break down over time, ensuring that the soil retains its structure.

Fired clays, including certain brands of catlitter, are best mixed with 25%-30% bark as an organic component that will still retaining good drainage properties.

A wide number of fired clays are available; I would recommend contacting other enthusiasts in your vicinity for the names and availability of different baked and fired clays that you can source locally.

bonsai soil

Catlitter (aka 'Kittydama') or Diatomaceous Earth or 'Diatomite'

Catlitter (also jokingly known as 'Kittydama') or Diatomaceous Earth or 'Diatomite' These fired clays have excellent properties as a bonsai soil, particularly for deciduous trees. I have used this 'soil' for all of my trees for many years.

Sifting out 'Fines'

Large amounts of dust that remain in the soil mixture can clog the open structure of the soil and disrupts the drainage of excess water.
 For a good soil structure that drains well, where necessary, soils are sifted to remove dust and very small particles.

Switching From Organic to Inorganic Soils

Almost all deciduous varieties will tolerate the transition from organic to inorganic soils immediately and the switch can be made during repotting; coniferous species, in particular Pines, benefit from the retention of some of their old soil which will contain mycorrhizae fungi necessary for health.


The Best Soil Mix for Bonsai

There is no single soil mix that is best for cultivating bonsai; variables such as local climate and rainfall, personal watering regimes and individual tree species all contribute to variations in enthusiasts' soil mixes.

Ultimately, experience of using different soil types and ingredients will shape your own particular preferences. It is recommended that in the first instance, find out the soil-mix that local enthusiasts are using and take it from there. I would however always recommend that an inorganic soil be always used for the health and ease of cultivation of your bonsai

'Bonsai Soils' bought from Nurseries and Garden Centres.

Though it saddens me to say this, the vast majority of products packaged and sold as 'bonsai soils' at plant nurseries, garden centres and even some bonsai nurseries are next to USELESS for bonsai. Often these are simply peat/compost based soils mixed with some sand or grit and (as described previously) have a soil structure that is too water retentive, airless and generally bad for the health of your bonsai.

Related Article: Soil Mixes for Weak and Newly Collected Trees and Yamadori Aftercare