Bonsai Soil Mix &
Drainage Ratio Calculator
Containerized root systems fail when potted in dense organic garden soil. Use this botanical calculator to configure volumetric ratios of Akadama, Pumice, and Scoria Lava based on species morphology, container dimensions, and climate humidity.
Select your cultivar parameters and pot dimensions to compute exact component volumes and aeration properties.
The Physics of Containerized Root Aeration
Why does garden topsoil kill a bonsai? In ground cultivation, water drains downward through soil columns via gravity. In a shallow ceramic bonsai pot, however, water physics changes entirely due to capillary action. Water forms a continuous saturation zone at the base of the container called the perched water table.
When standard potting compost or peat moss is placed in shallow pots, the microscopic particle size creates tiny capillary pores that pull water upward, trapping it and excluding air. Roots drown not from excess water, but from the absence of dissolved oxygen.
By transitioning to uniform granular aggregates (2mm to 6mm), we eliminate fine capillary entrapment. Macropores between particles allow excess gravitational water to drain instantly, while micropores inside the stones hold moisture without suffocating delicate feeder roots. This prevents sudden dieback seen in our ficus bonsai troubleshooting guide.
Mineral Breakdown: The Core Aggregate Trio
Modern bonsai cultivation relies on three primary inorganic components. Balancing these three substrates forms the foundation of container health:
1. Hard Akadama (赤玉土)
A volcanic clay mined exclusively in Japan, dried and heat-treated. Akadama provides high Cation Exchange Capacity (CEC), meaning its porous surface chemically binds positively charged nutrient ions (Potassium, Calcium, Magnesium, Ammonium) from fertilizers and slowly releases them to roots. While exceptional for root branching, Akadama naturally breaks down over 2 to 3 years, necessitating scheduled repotting.
2. Hyuga Pumice (軽石)
Pumice is a lightweight, porous volcanic silicate. It does not break down over time, providing permanent structural channels for gas exchange. Its neutral pH prevents substrate acidification. Learn more about aggregate alternatives in our perlite vs. pumice comparison.
3. Scoria Lava Rock (溶岩石)
Crushed volcanic basalt rock with sharp, irregular edges. As fine root tips grow and collide with the hard jagged edges of lava particles, the apical root tip splits, stimulating dense radial root ramification. Lava also adds container weight, stabilizing top-heavy trees against heavy winds.
4. Kanuma (鹿沼土) for Acidophiles
A soft, acidic pumice mined in the Tochigi prefecture of Japan. With a naturally low pH of 4.5 to 5.5, Kanuma is mandatory for ericaceous cultivars like Satsuki Azaleas and Camellias that suffer chlorosis in neutral Akadama.
Species-Specific Baseline Formulas
Adjust your aggregate blend to match tree respiration habits and water demand:
| Cultivar Classification | Akadama | Pumice | Lava Rock | Target Retention |
|---|---|---|---|---|
| Conifers & Pines (Juniper, Black Pine) | 33% | 33% | 34% | Low (Max Aeration) |
| Deciduous Trees (Trident Maple, Elm) | 50% | 25% | 25% | Moderate Retention |
| Tropical Varieties (Ficus Ginseng, Jade) | 60% | 20% | 20% | High Moisture Buffer |
| Acidophiles (Satsuki Azalea) | 20% | 10% | 70% Kanuma | Acidic Buffer (pH 5.0) |
Avoid common misconceptions regarding soil recycling. Learn why old clay cannot be safely reused in our breakdown on reusing bonsai soil, or review the risks of organic amendments in our guide on vermiculite in bonsai soil.