A Discussion On Bonsai Soil
Bonsai trees can grow in a range of soils. These soils are generally mixtures of various organic and inorganic mediums such as leaf mould, peat, sand, gravel and clay granules.
These soil mixtures create certain conditions which the trees roots will grow in. Changing the mixture will alter various qualities in the soil medium. These qualities include;
How much water the medium will hold – its porosity. Porosity is also a factor in what level of nutrients the medium will hold. There for two kinds of porosity – Spacial Porosity (How much space there is between particles in a medium) and Effective Porosity (How easily water / nutrients pass into the particles)
How far the soil will compact – Compact soil has limited room for air to circulate ( there are not many small spaces in it for the air to flow through.) Compact soil is also more difficult to water.
How long the soil will keep its structure for – Since your bonsai will be in the same soil for some time, many years in some occasions, it needs longevity and not to break down over time. The qualities of the soil may change over time.
What level of nutrients are in the soil initially – This affects how we feed our bonsai initially and in the longer term.
The PH level of the soil – How acidic or alkaline the soil is. Plants have a range of PH in which they will grow happily. Azaleas for example prefer an more acidic soil PH 4.5 to 6. Neutral PH is 7.0
Kanuma bonsai soil is ideal for acid-loving species
Lets look at some common soil mediums / components and evaluate the relative advantages and disadvantages of each.
Pure Peat and Compost
A medium only comprising of general compost, the kind of multipurpose compost available from garden centres and supermarkets.
Most bonsai beginners start by using this as its straightforward and easily available. Multipurpose compost tends to have lots of nutrients in it which means that little or no feeding is required for a few months. When the soil is fresh it holds water well.
It has a number of drawbacks however. The nutrients inherent in the compost get used up quite quickly and after that the tree will need feeding. The company has few air spaces, remember that bonsai need good air circulation below in the soil level to ensure good root growth. The biggest problems is that the soil tends to compact over time, making watering difficult. This is especially true if the soil has been allowed to dry out completely. Water may simply run off the surface of the soil without soaking in, meaning that you may think you’ve watered the bonsai when the roots are in fact totally dry.
Horticultural grit sand is found in many soil mixtures. Never used building sand as the grains are too fine and its frequently full of salt.
Easily available from garden centre retailers, coarse grain sand promotes good aeration and drainage. Reducing the chances of wet roots rotting. Sand is inorganic and does not break down over time. It is also fairly inexpensive.
Sand is not not very effectively porous, the grains don’t soak up with water. Sand can also be compacted.
Gravel of the right size, no more than a few millimetres from garden centres.
Gravel provided excellent draining and promotes good aeration in the soil, making for healthy roots.
Gravel does not however hold much water in its particles, is isn’t very effectively porous. It will result in a soil with dries out quickly. It therefore cannot be used on its own and should be part of a mixture.
Kyodama is a better option, its porous and holds some water, while being solid and never breaking down.
Akadama is a clay sub-soil from Japan. It is a hard soil which breaks down slowly. Its granular structure drains well but holds lots of water. Akadama is the best known and is the authenic Japanese bonsai soil medium. .
Being granular, it has excellent draining properties similar to gravel. What makes it better is that it is very effectivily porous – each granule holds water and releases it slowly. it takes a while to break down too, keeping those all important air gaps in the soil.
Akadama can be used on its own. Having been imported from Japan, its quite expensive though so you may wish to keep it for your best tree’s.
Akadama is granular and porous
So which is best?
As it most things in bonsai, there isn’t any hard and fast answer. Some species of tree have specific requirements from their soil. Very briefly;
Pine & Junipers – very free draining soil
Azaleas – Ericaceous (acidic) soil
Willows & Wisteria – very moist soil
For most of the others, a general soil mixture can be created and used. We’ll look at this now.
General Bonsai Soil Mixture
Many knowledgeable bonsai artists (Peter Adams, Harry Tomlinson, Dan Barton ) suggest that a mixture of Grit, Peat and Loam will provide an excellent growth medium for most species.
Whats Loam I hear you say? Wikipedia says loam is a mixture of sand, silt and clay. http://en.wikipedia.org/wiki/Loam
I’m currently using equal amounts of peat, perlite and akadama, increasing the amount of akadama when potting my best trees. When I’m working on starter bonsai or projects and part-finished bonsai, I won’t use any akadama at all but will add gravel and maybe sand.
If working on pines or junipers, I also use Kiryu to make to mixture even more free-draining.