Most beginners use a general potting mix for bonsai soil until they get the
hang of things - and I recommend that you do this too - just until you
familiarise yourself with the other processes involved in bonsai. As long as the
mix is open and well-draining you should not have a problem and it wont impact
negatively on the plant. If you can get pre-made bonsai soil instead however,
use it. Bonsai soil is much more free draining than potting soil and doesn't
contain as much fertilizer as normal potting soil which is usually for plants in
large pots that people want to grow really big. It is usually available at
specialist bonsai nurseries, and even some normal nurseries stock it. This is
probably the easiest, cheapest and least time consuming way.
Making your own soil mix isn't as hard as you think. A lot of bonsai growers
make a bit of an unnecessary fuss about it. A basic bonsai soil mix to use - and
one that would apply to almost all species is: one part loam, two parts sphagnum
peat moss, two parts granite grit. All of these ingredients should be easy
enough to obtain.
An opening 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. 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
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.
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 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;
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.
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.
Altering Soil Mixtures to Suit diverse 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
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.
Organic or Inorganic Soils
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.
Basic or Organic Soil Mix
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 become compacted and poor-draining relatively quickly
. 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 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 the surface of the soil can be dressed
with grit or Akadama to hide the colour. It does however retain its open
structure for a long period of time.
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.
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 2 mm, should always be removed. Dust that remains in the
soil mixture clogs the open structure of the soil and disrupts the drainage of
Switching From Organic to Inorganic Soils
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.
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 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.
Popular soil mixtures
- 50% River Gravel
- 35% Turface
- 15% Bark
- 3 parts terragreen
- 2 parts peat
- 1 part soil
Mix C (inorganic soil mix)
- 50% Seramis
- 50% Grit
- Top of of Akadama (more pleasant appearance)
- 50% Sand
- 25% Leaf Mold
- 25% Earth