Using the right soil mixture for your Bonsai trees is crucial. Soil is important to supply your trees with nutrients, but it also needs to drain properly, provide enough aeration and retain water. Though most (online) Bonsai shops sell ready-mixed soils, doing this yourself will save money and enables you to adjust mixtures per tree-species.
The quality of soil used directly affects the health and vigor of your tree. It is our experience that unhealthy trees, that lack vigor, are very often planted in a poor (often organic) Bonsai soil. Or worse; planted in normal garden soil. Such soil easily hardens when it gets dry, which give no advantage to the growth of Bonsai, in fact, it is very harmful to the tree.
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. Too much water-retention will also cause the roots to rot, killing the tree.
- Good aeration
The particles used in a Bonsai mix should be of sufficient size to allow tiny gaps or air pockets between each particle. Beside the need of oxygen for the roots, it is also important to let the good bacteria and mycorrhizae intact, so the processing of food will take place before being absorbed by the root-hairs and send to the leaves for photosynthesis.
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.
Bonsai soil movie
Organic or Inorganic Soils
Soil mixes are described as being either organic or inorganic. Dead plant matters such as peat or leaf-litter or bark are described as being organic soil components. The (potential) problem with organic soil components is that over time organic matter will break down and harm drainage. Most potting composts, once completely dry, absorb water very poorly. This is one of the biggest problems for cheap indoor Bonsai trees purchased at garden centers; you think you watered the tree but in fact the water runs past the soil into the bottom of the pot!
Inorganic soil components contain little to no organic matter such, for example volcanic lava, calcite (baked) or fired clays. They absorb less nutrients and water than organic soils, but are great for drainage and aeration.
The most important components for Bonsai soil mixtures are Akadama, Pumice, Lava rock, organic potting compost and fine gravel (grit).
From left to right; organic potting compost, Akadama, Pumice and lava rock.
Akadama is hard-baked Japanese clay, specifically produced for Bonsai purposes and available at all (online) Bonsai shops. It needs to be sifted before use. Keep in mind that after about two years akadama starts to break down, reducing aeration to a certain extent. This means that regular repotting is required, or that Akadama should be used in a mix with well-draining soil components. Akadama is rather expensive and is therefore sometimes substituted with similar fired/baked clays that are easily available in any garden center. Even cat-litter can be used as a substitute, check our Bonsai forum which brand is recommended in your country.
Pumice is a soft volcanic product, which can absorb water and nutrients quite well. When used in a Bonsai soil mix, it helps to retain water and it aids the roots to ramify very well.
Lava rock retains water and adds good structure when part of a Bonsai substrate. Roots can't grow into Lava rock.
Organic potting compost includes peat moss, perlite and sand. It has several disadvantages (it retains much water and doesn't aerate/drain very well), but as part of a mixture it can be used perfectly well.
Fine gravel / grit helps to create a well draining and aerated Bonsai soil. It is also used as a bottom layer in Bonsai pots to enhance drainage a bit further. Most experts don't use this anymore, and stick to a mix of Akadama, Pumice and Lava rock.
Recommended Bonsai soil mixtures
Different tree-species demand different soil-mixtures, so make sure to check our tree-species Bonsai guide to find the optimum mixture for individual trees. We can however describe two main mixtures, one for deciduous trees and one for coniferous trees. Both mixtures consist of Akadama (the water retention component), Pumice (good for structure of the substrate) and Lava rock (to provide the mixture with aeration and drainage).
Note that both mixtures can, and should, be adapted to your local circumstances. If you do not have time to check on your trees twice a day, add more Akadama (or even add organic potting compost) to your mix, to increase its water retention qualities. If you live in a wet climate, add more lava rock (or even grit) to enhance the draining qualities of your mixture.