Soil is one of the most discussed topics among Bonsai enthusiasts and with good reason. It plays a vital role in the health of our beloved trees.
For Bonsai to survive, it needs the right amount of air, water, and nutrients. Soil, which is made up of minerals, organic matter, water, and air, is the medium that fulfils these needs.
All Bonsai enthusiasts have their own soil mix that suits their collection. But no matter what soil mix you use, they all should fulfil the following requirements:
- Retains as much water the tree requires in the form of vapour
- Supplies nutrients
- Provides good drainage to prevent the roots from rotting
- Allows air space to permit the roots so they can breath
Most Bonsai mixes are made from a combination of inorganic and organic soil to balance the Bonsai’s nutritional requirements, such as minerals (from inorganic soil) and living and dead organisms (from organic soil). Inorganic compounds tend not to breakdown, allowing air circulation and good drainage for the roots, while organic matter tends to breakdown, retaining a higher volume of moisture.
To choose the right mix, you should also consider the following:
- Specimen: Some species require a drier environment or higher acidity than others.
- Exposure: Will the tree be exposed to full sun, semi-shade or full shade? The level of exposure will influence the moisture levels in the pot. For example, a tree in full sun will need higher moisture in the soil because it will tend to dry faster.
Once the right mix been chosen, the moisture level can be easily amended by the soil grade. For instance, a coarse soil will retain less water and have better drainage while a fine-grade soil will stay moist for longer periods.
Here some inorganic and organic matters commonly used for Bonsai purposes:
- Akadama: A granular, clay-like mineral often used on its own because it has great drainage and moisture-retention properties
- Fuji Grit: a volcanic soil great for improving aeration
- Pumice: a light and porous volcanic rock ideal for creating air space
- Kiryu: a neutral, PH-heavy Japanese pumice resistant to frost
- Kanuma: an acidic pumice mainly used for azaleas
- Peat Moss: a fine organic material with high water retention
- Bark Mulch: promotes beneficial fungi and has high water retention
- Compost: a decomposed matter rich in nutrients with high water retention