An Introduction to Bonsai Soils

One of the most widely debated subjects for most bonsai enthusiasts is soil composition. Ready-mixed soils can be bought from bonsai nurseries and garden centres but these tend to be relatively expensive and dubious quality. Faced with repotting more than 3 or 4 trees in the spring, most enthusiasts learn to mix their own soils. There are a large number of soil ingredients that can be used when mixing your own soil or “substrate”; different mixes are used by different enthusiasts with varying degrees of success.

For the beginner, choosing which soil mix to use can be a daunting task. This article is written as an introduction to Bonsai soils, it does not discuss every soil ingredient or mix that is available or feasible, nor does it tell which soil mix is the ‘best’. The individual enthusiast can only answer that question after experimenting over time with their own trees and care routines.

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The Basic Requirements Of Bonsai Soils

A bonsai is confined to a relatively small quantity of soil throughout the year on which its very existence depends. Through the soil in the pot, the tree must be able to obtain water, nutrients and gases in order to grow.
For this reason, a bonsai must be planted in a good quality bonsai soil, the quality of the soil used directly affects the health and vigour of the tree. In my experience, unhealthy trees that lack vigour have often been planted in a poor (often organic) bonsai soil.

There are a number of qualities that are required in a good soil mix:
Good water-retention. The soil needs to be able to hold and retain sufficient quantities of water to supply moisture to the bonsai between each watering.
Good drainage. Excess water must be able to drain immediately from the pot. Soils lacking good drainage are too water retentive, lack aeration and are susceptible to a build up of salts.
Good aeration. The particles used in a bonsai mix should be of sufficient size to allow tiny gaps or air pockets between each particle. It is important to the health of the roots that they have access to oxygen. A particle-based, well-structured inorganic soil allows fast drainage of water and allows fresh air to continually enter the soil. A compacted organic soil that lacks any structure will also lack aeration and drainage, this leads to unhealthy roots and trees.

Pumice is a very useful soil component that can be mixed with other soil components such as bark, akadama or lapillo and will create a very vigorous root system.

Varying Soil Mixtures To Suit Different Tree Species

Although all Bonsai require free-draining, water-retentive soils, different species vary in their requirements for water and nutrients, this should be reflected in their soil composition.
Pines and Junipers for instance require less water than most other species; this in turn means that they require a less water retentive soil mix. Alternatively, flowering and fruiting species have increased water requirements and tend to be planted in soil mixes with relatively high water retaining capacities.
When mixing your own soil, the ratio of waterretaining material to drainage materials is varied according to the tree that it is intended for. Very often the particle size of the soil granules is increased to provide additional drainage to a bonsai soil.
Alternatively, by increasing the amount of water-retentive material and its particle size, the greater its waterholding capacity becomes.

Lapillo or lava, and pumice are an extraordinarily useful soil mix for Junipers and Pines, particularly if a little akadama is added.

Organic or Inorganic Soils

Soil mixes are described as being either organic or inorganic. Dead plant matter such as peat or leaf-litter or bark are described as being organic soil components. Inorganic soil mixes contain little to no organic matter; instead, they are made up of specially-formulated soils such as volcanic lava, calcined (baked) or fired clays. These materials can be more difficult to locate than organic materials, but have become readily available online

Chopped bark can be added to a soil mix to increase water retention which can be very useful, particularly for deciduous species.

Organic Soil Mixes and Components

In past decades, Western bonsai enthusiasts tended to use organic soil mixes, a large proportion of peat, bark and leaf-litter mixed with grit to aid with drainage.
But as time has 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 although they may consist of appropriate sized particles when the bonsai is first planted, they continue to break down in the 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 particles less than 2mm. 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 is Japanese baked clay, specifically produced for bonsai and imported into the West; it is normally only available from bonsai nurseries and therefore difficult to locate. 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 is the soil of choice for many Japanese bonsai Masters and enthusiasts.

This is partially due to its relatively low price in Japan where it is also easily obtainable. However, while Akadama might be considered a good quality soil component, it can break down into a solid mush within 1 or 2 years if used without volcanic soil components such as pumice or lava to keep the soil structure open.. Because soils made using just akadama breakdown so badly, they must therefore be washed out of the roots every two to three years.
For this reason it is strongly not recommended to be used on its own for species that will not tolerate regular bare-rooting (Pines for instance). Akadama should be mixed with volcanic or clay granules such as pumice that will ensure the soil stays open for a good length of time. Fired clays are also stronger than Akadama and thus will not break down over time. As with Akadama, fired or volcanic clays can be used on their own, mixed with grit for faster draining soil or mixed with 10%-20% bark if an organic component is required for greater water retention, while still retaining good drainage properties. A wide number of fired clays are available; I would recommend contacting other enthusiasts in your country and investigating online as to the best soil components and pre-mixed soils available to you.

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; coniferous species, in particular Pines and Junipers, require the retention of some of their old soil that will contain the mycorrhizae fungi necessary for their survival.

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 you 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 used to improve 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.

There are of course knowledgeable outlets selling good quality soil products. However, just because you bought some ready-mixed bonsai soil from a nursery does not necessarily make it suitable for the health of your bonsai. Similarly, if you have bought a bonsai from anywhere other than a well-respected specialist bonsai nursery that will care about the quality of the trees it is selling, do not assume that the tree is planted in a good soil.

Feeding Trees Growing In Inorganic Soils

If there is one thing that seems to worry enthusiasts about switching to an inorganic soil is the lack of ‘proper’ organic and a ‘lack of nutrients’. Firstly, I and many (most) experienced enthusiasts have switched to largely inorganic soils (such as lava, pumice, Akadama etc) and have been using these products for many years. The reason for switching to inorganics is purely for the increased health, vigour and strength that it provides a bonsai (or any plant for that matter). Be confident that switching to inorganic soils is widely considered ‘best practice’.
Bonsai grown in an inorganic soil do not need any special fertilising regime or special ingredients to keep them healthy. There are of course some differences between compost-based organic mixes and inorganic mixes. Inorganic soils contain little or no nutrients, however, compost/ peat based organic soils just as equally only provide very limited nutrients to a bonsai and these are quickly depleted within a bonsai pot.
Organics are able to retain more nutrients better than inorganic’s, after feeding, but this is not necessarily a good thing. Trees growing in inorganic soil need more fertiliser than those growing in an organic soil. Many enthusiasts see this as being advantageous though as it allows the enthusiast to feed their bonsai heavily to encourage better growth, health and faster development. That an organic bonsai soil can hold nutrients for a little longer than an inorganic bonsai soil is hardly advantageous, as bonsai growing in organic soils still need to be fed regularly!