Bonsai Soil Mix & Repotting Guide — Using Akadama, Kiryu Sand & Kanuma Soil with Species-Specific Recipes

Bonsai Soil Composition and Repotting Guide | How to Use Akadama, Kiryu Sand, and Kanuma Soil with Tree-Species Specific Mixing Recipes

The health of a bonsai is determined by the "condition of its roots." And the condition of roots is directly linked to "the quality of the soil." Bonsai soil is fundamentally different from general gardening soil, with carefully designed balance in drainage, aeration, and water retention. This article provides detailed explanations of basic bonsai soil materials, species-specific mixing recipes, and practical repotting procedures.

Basic Principles of Bonsai Soil

The requirements for bonsai soil differ from those of general plant soil.

Three Essential Characteristics of Bonsai Soil:

1. Drainage (Water Permeability): Prevents root rot from excessive moisture. Small pots cause water to accumulate easily
2. Aeration (Air Circulation): Roots need oxygen to breathe
3. Water Retention (Appropriate Moisture Holding): Maintains adequate moisture to prevent roots from drying out excessively

This three-way balance changes depending on tree species, environment, and season, making it important to adjust the composition accordingly.

Why Leaf Mold and Vermiculite Are Not Used: Leaf mold and vermiculite, commonly used in general gardening, are not suitable for bonsai. Their water retention is too high, making root rot likely, and they break down over time, causing loss of aeration.

Primary Bonsai Soil Materials

Akadama (Red Ball Clay)

Characteristics: Volcanic ash soil from the Kanto loam layer. Granular with good balance of drainage and water retention.

Size Types: Small grain (3-6mm) • Medium grain (6-12mm) • Large grain (12mm+)

Use in Bonsai: - The most versatile material. Forms the base of most bonsai soil mixes - Hard-quality akadama is difficult to break down and has high durability (choose hard-quality if possible) - Breaks down over time and loses aeration, so use fresh soil with each repotting

Kiryu Sand

Characteristics: Sand-based soil from the Kiryu region of Gunma Prefecture. Excellent drainage and aeration.

Features: - Difficult to break down and highly durable - Low water retention (suitable for drought-loving tree species) - Also used as decorative surface sand

Kanuma Soil

Characteristics: Lightweight volcanic stone-like soil from Kanuma, Tochigi Prefecture. Strongly acidic (pH ~5.5)

Features: - Ideal for species that prefer weakly acidic to acidic soil (azaleas, enkianthus, blueberries, etc.) - High drainage and aeration - Dries faster than akadama

River Sand

Characteristics: Uniformly-sized sand with excellent drainage.

Uses: - Added to increase drainage in soil mixes - Also used as decorative surface sand for appearance

Zeolite (Natural Mineral)

Prevents root rot, adsorbs and retains nutrient components. Effective when mixed in small amounts (5-10% of total).

Bark

Fermented and dried tree bark. Contains trace nutrients as an organic material.

Uses: Added in small amounts to improve aeration. However, since it breaks down easily, avoid using in large quantities.

Species-Specific Mixing Recipes

Conifers (Pine, Cypress, Hinoki Cypress, Cryptomeria, Japanese Cedar)

Prefer dryness and are sensitive to excessive moisture. Composition prioritizes drainage.

Basic Mix: - Akadama (small grain): 60% - Kiryu sand: 30% - Kanuma soil: 10%

Special Mix for Pine (Black Pine, Red Pine): - Akadama: 40% - Kiryu sand: 50% - River sand: 10% (Mix with particularly high drainage)

Deciduous Trees (Zelkova, Maple, Japanese Beech, Japanese Hornbeam, Mountain Maple)

Require appropriate water retention. Standard composition works well.

Basic Mix: - Akadama (small grain): 70% - Kiryu sand: 20% - Bark (leaf mold alternative): 10%

Flowering and Fruiting Trees (Plum, Cherry, Apple, Mountain Camellia)

Require nutrients and appropriate water retention.

Basic Mix: - Akadama (small grain): 50-60% - Kiryu sand: 20-30% - Bark: 10-20%

Azaleas and Satsuki

Prefer strongly acidic soil. Composition based primarily on Kanuma soil.

Basic Mix: - Kanuma soil (small grain): 70-80% - Akadama: 10-20% - River sand: 10%

Wildflower Bonsai and Grass Bonsai

Composition with enhanced water retention. Fine roots require smaller grain size.

Basic Mix: - Akadama (extra-small to small grain): 50% - Kanuma soil (extra-small grain): 30% - River sand: 20%

Practical Repotting Guide

Signs Repotting Is Needed

• Roots protruding from pot drainage holes
• Water absorption into soil is slow even after watering
• Branches and leaves lack vigor (sign of root-bound condition)

Repotting Frequency Guidelines: | Tree Type | Frequency | |-----------|-----------| | Young trees / fast-growing species | Every 1-2 years | | Mature conifers | Every 3-5 years | | Azaleas / Satsuki | Every 2-3 years | | Deciduous trees | Every 2-3 years |

Best Time for Repotting

Optimal Timing by Species: - Pine (Black Pine, Red Pine): Mid-February to March (just before new buds break) - Maple, Zelkova: March-April (slightly before budbreak) - Azaleas, Satsuki: Right after flowering (around June) - Plum: Right after flowering (March-April) - Hinoki Cypress, Cryptomeria: March-April

Times to Avoid: Midsummer (July-August) and midwinter (December-January) cause significant stress.

Repotting Steps

Materials Needed: - Fresh soil mix (pre-blended) - Root pruning shears (clean) - Root rake (tool for loosening soil) - Mesh screen and wire (for covering drainage holes) - Bamboo pick (for packing soil between roots)

Procedure: 1. Remove the tree from the pot (if roots are tightly packed, use a spatula carefully) 2. Gently remove old soil with a root rake without damaging roots 3. Inspect roots and cut away dead, rotted, or severely tangled roots 4. Reduce overall root mass by cutting 1/3 to 1/2 to keep it compact 5. Place mesh over drain holes and spread a thin layer of drainage stone 6. Spread fresh soil, temporarily position the tree, and adjust height and position 7. Fill with soil while tamping with bamboo pick to pack soil between roots 8. Water thoroughly immediately after repotting

Post-Repotting Care: - Keep in bright shade for 1-2 weeks away from direct sunlight - Avoid fertilizing until roots stabilize (no fertilizer for 1 month after repotting)

Summary

The basic principle of bonsai soil is adjusting the composition to suit tree species, environment, and pot size. By mastering the basic pattern—using akadama as the base, adding Kiryu sand for drainage, and using more Kanuma soil for azaleas—you can accommodate most species.

Repotting is one of the most important tasks in bonsai care. By repotting at the appropriate time with correctly composed soil, roots develop healthily and maintain a beautiful tree form above ground. Deepening your understanding of soil is the first step toward improving bonsai techniques.