Substrate is, together with watering, the factor that most determines the health of a bonsai or a cactus. Unlike a conventional potted plant, these species need mineral substrates that combine good drainage with controlled moisture retention.
What a good substrate should provide
- Fast drainage: prevents waterlogging and root rot, the main risk for cacti and bonsai.
- Aeration: roots need oxygen; a compacted substrate suffocates them.
- Structural stability: it must hold its shape through repeated watering, without breaking down the way organic soil does.
Common mineral substrates
Materials such as crushed expanded clay (Akadama-type), volcanic gravel or pumice are used precisely because they combine these three properties. For cacti and succulents, the proportion of coarse aggregate is usually increased to maximise drainage; for bonsai species that tolerate more moisture, it is balanced with somewhat more moisture-retentive materials.
A different substrate depending on species and climate
There is no universal mix: an indoor bonsai does not have the same needs as an outdoor one in a humid climate, and a cactus from an arid region requires more drainage than one from a tropical forest. As a general rule, the warmer and more humid the environment, the higher the proportion of coarse aggregate that should be used.
You can see the full range in our collection of decorative substrates and in Bonsai, Cacti and Aquariums.