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. 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; 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 choice.

This article is written as an introduction to Bonsai soils, it does not discuss every soil ingredient or mix that is available or possible, nor does it tell which soil mix is the 'best'. The individual enthusiast can only answer that question after experimenting over time with his own trees and care routines.
 

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 that is used, directly affects the health and vigour of the tree. It is my experience that unhealthy trees that lack vigour are very often also 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 liable 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, also lacks aeration and drainage and this can lead to ill-health in the roots and tree and root rot.

Varying Soil Mixtures To Suit Different Tree Species

Though all Bonsai require free-draining, water-retentive soils, different species vary in their requirements for water and nutrients and 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 water-retaining material to drainage materials is varied according to the tree that it is intended for. Very often grit is used to provide additional drainage to a bonsai soil.

By increasing the ratio of grit to the mix, the soil becomes increasingly free-draining; by increasing the amount of water-retentive material, the greater its water-holding capacity becomes.
 

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 are more difficult to locate than organic materials, but can be found in garden centres, bonsai nurseries, and in the case of some fired clays, supermarkets and hardware stores.
 

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 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, in my opinion it is no better than the cheaper and more easily obtainable fired-clay soils that are produced in the West.

Furthermore, Akadama can break down into a solid mush within 1 or 2 years. This old soil must therefore be washed out of the roots every one to two years. For this reason it is not recommended for species that will not tolerate regular bare-rooting (Pines for instance).

Seramis/Turface/Oil-Dri are fired clays are readily available in the UK and US compared to Akadama and much cheaper. Fired clays are also stronger than Akadama and thus will not break down over time.

As with Akadama, fired 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 vicinity for the names and availability of different baked and fired clays that you can source locally.

Catlitter (also jokingly known as 'Kittydama') or Diatomaceous Earth or 'Diatomite' For anyone living in the UK, I would wholeheartedly recommend 'Tescos Premium Lightweight Cat litter'. This fired clay has excellent properties as a bonsai soil.

I have been using this 'soil' for all of my trees for a number of years now and would not switch to anything else. It is very cheap and given that almost everyone in the UK lives within 5 miles of a Tesco Supermarket, it is easily obtainable.

For more information about inorganic soils, sorces for these soils around the world, please see this article Cat Litter as Bonsai Soil

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, 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 many 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.

Though there are of course knowledgeable outlets selling good quality soil products, these appear to be in the great minority. Because you bought some ready-mixed bonsai soil from a nursery does not necessarily make it suitable for the health of your bonsai.

Similarily, 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.