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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.
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.
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.