Article written by Harry Harrington, of Bonsai4me.com.

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 more than 3 or 4 trees to repot 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, nor does it tell which soil mix is the 'best'. That question can only be answered by the individual enthusiast after experimenting over time with his own trees and care routines.
 

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 obtain water, nutrients and gases in order to grow. For this reason, the soil that the tree is planted in must be of the correct quality. The quality of the soil that is used directly affects the health and vigour of the tree.

There are two basic qualities that are required in a good soil mix;

1. Good water-retaining and nutrition-absorbing capacity. Often provided by the use of dead organic matter/ Humus, the soil needs to be able to hold and retain sufficient quantities of water and nutrients for the tree to absorb.
2. Good drainage. If excess water is not able to drain immediately from the pot, the rootsystem will be prone to rotting. Drainage is normally provided by the use of grit or small stone which keeps the mixture 'open', allowing excess water to pass through the mix freely. Good drainage also allows air to penetrate the compost, which is required by the plant.
    

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 grit is varied according to the tree that the mix is intended for. By increasing the ratio of grit to the mix, the soil becomes more freely-draining; by increasing the amount of water-retentive material, the greater the water-holding capacity.
 

Soil mixes are described as being either organic or inorganic; both types contain grit to aid drainage but the water retentive ingredient(s) differ. Organic soil mixes are made up of dead plant matter such as peat and the ingredients are easily available at garden centres and DIY stores. Inorganic soil mixes contain no organic matter; instead, specially-formulated soil conditioners ranging from volcanic lava to calcined (baked) clays are used. These materials can be harder to locate, but, are available from good garden centres and bonsai nurseries.
 

The basic soil mix for bonsai has, for many years been grit and peat, mixed to a ratio of around 50:50. The peat holds water and nutrients whilst the grit provides drainage and keeps the soil open. The peat used is moss peat rather than sedge peat, the grit, either flint grit or crushed granite. All ingredients should be sifted to remove particles smaller than 2mm.

There are other organic substitutes for peat; leaf mould and composted bark are sometimes used along with other peat substitutes. Farmyard manure, garden compost or garden soils should never be used as they are of questionable quality and frequently carry soil-borne diseases.

The standard mix for bonsai is 50:50 peat to grit, but when repotting Junipers, Pines and other species that require a free-draining mix, the percentage of grit should be increased to as much as 75:25.
 

In the past decade, there has been a significant move towards inorganic soils. Though organic soils are cheaper and more easily available, they are also of inferior quality. Peat and peat substitutes have many disadvantages; they can be too water retentive, holding large quantities of water for long periods. This can lead to the soil mix being continually sodden, particularly during periods of rain in the Autumn, Winter and Spring. Conversely, during periods of high temperatures, dry peat can be difficult to re-wet properly. Organic soils also continue to break down and relatively quickly become compacted and poor-draining. All these factors can lead to root problems and in particular root-rot.

The advantage of inorganic materials is that they hold their open structure for a long time without breaking down into a 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 over-water a bonsai planted in a good inorganic soil mix.

There are a number of materials that can be used along with grit when mixing an inorganic soil mix, ranging from volcanic lava to baked/fired clay granules;

Akadama is a white Japanese clay, specifically produced for bonsai; it is normally only available from bonsai nurseries so can be difficult to buy. There are a number of grades of Akadama available including 'double redline' that is more costly but is of premium quality. Avoid buying low quality Akadama as it can lose its structure quickly in European and US climates where frequent watering during the Summer is necessary.

Seramis (pictured left) is an orange baked clay, similar to higher-grade Akadama. It is far more easily available in the UK and US where it can be purchased from good garden centres. An advantage of Seramis is that it turns a different shade of orange as it dries allowing an easy indication that it requires watering; the colour is not to everyone's taste but when top-dressed with grit it is not particularly noticeable. It does however retain its open structure for a long period of time.

Fired clays are excellent soil materials but are costly and can be difficult to locate. They do however hold enormous amounts of water whilst remaining very free-draining.

There are also a number of cheaper soil materials available at most garden centres, they are often sold as soil conditioners. These products can be used instead of baked clays or alternatively they can be mixed with clays to bring down the total cost of the soil mix which is often necessary for repotting large numbers of trees!

Perlite (pictured right) is a naturally occurring, volcanic rock that is heated to a high temperature. As with baked clays, it is very well structured but also holds large quantities of water. It is also a very light material that makes it useful for reducing the weight of large pots.

Vermiculite (pictured left) is a naturally occurring, mica mineral similar to Perlite. Again, it has excellent water retention and drainage properties. Its advantage over Perlite is that it is a more pleasant colour though Vermiculite tends to contain quite a lot of small particles; around 1/4 of a typical bag can be lost after sifting out fines.

For a good soil structure that drains well, all soils whether organic or inorganic, must be sifted to remove dust and very small particles. Soil particles smaller than 2mm should always be removed. Dust that remains in the soil mixture clogs the open structure of the soil and disrupts the drainage of excess water.
 

Almost all deciduous varieties will tolerate the transition from organic to inorganic soils as long as they are healthy; coniferous species, in particular Pines, benefit from the retention of some of their old soil which will contain mycorrhiza fungi necessary for growth. If in doubt, make the transition slowly increasing the amount of inorganic material at each repotting.

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 of local enthusiasts who have found a successful mix and take it from there. I would however recommend the use of inorganic soils and not organic as they are nearly always of a better quality.

My personal preference when mixing bonsai soil is as follows; (by volume, not weight)

30% Seramis clay granules, 40% Vermiculite and 30% Grit. For additional water retention I increase the volume of Vermiculite at the expense of grit and Seramis, for a very free draining mix the volume of Grit is increased to 50%. My experience of this mix is that even in the height of summer, watering is necessary no more than once or twice a day though excess water is allowed to run straight through drainage holes of the pot. I have yet to experience any drainage or compaction problems, nor have I had any trees suffer with root-rot. Lastly, all these materials are readily available at all good garden-centres at a reasonable cost.

Copyright © Written by Harry Harrington, of Bonsai4me.com.