In bonsai there are three big questions that get debated and argued on a regular basis, and which bonsai soil to use is one of these questions. Very often you will get lots of differing opinions, and while some solutions may work for others, there is no true single answer.
We can suggest a bonsai soil mix, and indeed we do so later in this article. But you may still need to undergo your own research to find the best bonsai soil mix for your trees. Factors that influence the mix that you eventually settle on will include tree genus, location, exposure to rain and sun, and the regularity of watering.
The environment within the bonsai pot should support healthy roots, which in turn support a healthy tree.
There are of course a wider range of factors that impact the ability of a tree to not only grow, but to thrive in a potted environment. These will be the subject of another article and here we will look at the basic qualities that contribute towards that healthy environment.
Drainage – The ability of water to pass through the soil may be viewed as the most important factor when choosing soil. This is because it has an impact on all the other factors as well.
Drainage is essentially the ability of water to pass freely through the soil depth and out of the pot.
If your soil does not allow excess water to drain through and away, the tree will sit in too much water. This makes it hard for the tree to acquire nutrients and oxygen, leading to decline and death of the roots and tree.
Water retention – While free draining soil is desirable, it also needs to retain sufficient water for the tree to take up and use. We achieve this through the type and composition of the soil components.
The soil needs to be able to retain sufficient water to supply moisture to the Bonsai between each watering.
Aeration – As well as water, the roots take up nutrients and oxygen. The oxygen is held in the gaps between the substrate for absorption by the roots. This is then refreshed by water running through the soil and air filling the gaps as the water drains away.
Nutrition – All of the previous factors combine to allow a healthy root system to absorb nutrients as well as water and oxygen. The choice of soil substrates can impact the way in which the soil either retains nutrients or breaks down to provide nutrition.
Organic soil components are the plant based materials such as moss, leaf mould, compost, chopped bark and garden soil. Over time, these decay and release nutrients into the soil, meaning that it retains nutrients for longer. However they can also reduce drainage in the pot at the same time.
Organic components can be used as part of a soil mixture with good outcomes, particularly for fruiting tree varieties.
Chopped bark is great for opening up a soil structure, giving better aeration and water retention. It is a long living organic component and ideal for use with organic fertilisers.
Sphagnum moss has incredible water retention properties and also has the ability to help plants and trees recover root health. It is best chopped and used alongside kanuma for Satsuki Azalea and other acid loving trees. It is commonly used for air layering and it is also good for orchids.
Inorganic soil components are generally clay, rock or volcanic rock materials. These increase drainage and aeration, but have low water and nutrient retention properties.
Akadama is a form of hard-baked Japanese clay, that has been used for centuries in Japan for bonsai. It is generally used in conjunction with a grit or rock based component to compensate for its propensity to break down.
Crushed lava rock while being a different colour, is very similar to pumice (see below) and provides the same benefits. It does however require sieving before use to remove fine dust.
Kanuma is highly suited to trees that prefer acidic soil conditions. This volcanic medium also has higher water retention properties for trees that like such conditions.
Kiryu has a slightly acidic pH combined with an essential ability to drain excess water from the soil and retain an open soil structure. It is therefore ideal for any tree that prefers ericaceous conditions.
Kurodama is a very lightweight fired clay component which provides great water retention and aeration to the mix. Kurodama will not break down and works particularly well along with organic fertilisers.
Kyodama is a grit based product,often used to maintain an open structure alongside Akadama, Kanuma or Kiryu.
Pumice is a soft volcanic rock with an ability to absorb both water and nutrients. When used as part of a soil mix it will add structure and can be sieved to be re-used.
Recommended soil mix
The following are our basic recommendations for soil mixes for different types of tree. You should take into account all of the factors mentioned at the start of this article.
Individual species may have further requirements which are not reflected below:
Deciduous trees – 60% Akadama, 40% Lava rock
Fruiting Trees – 60% Akadama, 40% Lava rock
Satsuki Azalea – 80% Kanuma, 20% Chopped sphagnum moss
Juniper and Pine trees – 33% Akadama, 33% Pumice, 33% Lava rock