How to Choose Bonsai Soil and Soil Mix Guide | Using Akadama, Kanuma, and Kiryu Sand Properly

The foundation supporting healthy bonsai growth is "bonsai soil." In the limited space of a bonsai pot, selecting and mixing soil appropriate to the tree type and environment is essential to properly supply the roots with necessary moisture, nutrients, and oxygen. Rather than using commercial potting soil as-is, mixing soil tailored to your tree type and conditions can significantly improve your bonsai's growth. This article explains the basics of bonsai soil.

Key Characteristics of Main Bonsai Soils

Here are the representative bonsai soils and their characteristics.

Akadama Soil

• Reddish-brown granular soil extracted from the Kanto loam layer
• The most common and basic soil for bonsai use
• Offers both adequate water retention and drainage
• Grain quality matters—choose hard-grade soil as softer particles break down and reduce drainage
• Grain sizes: large (used as pot bottom stone), medium (standard bonsai soil), small (for shohin/small bonsai)

Kanuma Soil

• Yellowish-white, pumice-like soil extracted around Kanuma City, Tochigi Prefecture
• Offers higher drainage than akadama and is slightly acidic
• Suited to tree types that prefer acidic conditions, such as azaleas and satsuki
• Commonly mixed with akadama soil
• Turns whitish when dry, making watering timing easy to identify

Kiryu Sand

• Volcanic sand and gravel extracted around Kiryu City, Gunma Prefecture
• Extremely high drainage with minimal water retention
• Suited to tree types that prioritize drainage, such as conifers
• Hard particles resist breakdown, maintaining structure long-term
• Contains almost no nutrients on its own—supplementing with fertilizer is necessary

Fuji Sand

• Volcanic sand and gravel from around Mount Fuji
• Like kiryu sand, offers high drainage
• Contains iron, giving it a dark color
• Its weight helps stabilize the pot

Keto Soil

• Peat-rich soil from wetland areas
• High adhesiveness; used to fix roots in literati-style and root-over-rock bonsai
• Extremely poor drainage if used alone, so always mix with other soils

Soil Mixing Ratios by Tree Type

Here are guidelines for soil mixing suited to different tree types.

Conifers (Pine, Chinese Juniper, Cypress, etc.)

• Akadama 6 : Kiryu sand 3 : Fuji sand 1
• A mix emphasizing drainage. Pines dislike excess moisture, so well-draining soil is essential
• For larger pots, you may increase the kiryu sand ratio further

Deciduous Broadleaf Trees (Maple, Zelkova, Beech, etc.)

• Akadama 7 : Kiryu sand 2 : Kanuma 1
• A mix emphasizing water retention. Maples prefer moisture, so a higher akadama ratio is recommended
• If concerned about drying out in summer, increase the akadama ratio further

Flowering and Fruiting Types (Plum, Cherry, Firethorn, etc.)

• Akadama 6 : Kanuma 2 : Kiryu sand 2
• A well-balanced mix. Flowering and fruiting requires adequate water retention and nutrients
• This mix assumes consistent, proper fertilizing

Satsuki and Azaleas

• Kanuma 8 : Akadama 2 (or kanuma alone)
• These species prefer acidic soil, so kanuma is the primary component
• Adding akadama slightly increases water retention

Balancing Drainage and Water Retention

The core of soil mixing is balancing drainage and water retention.

Why drainage matters

• Roots submerged constantly rot
• Oxygen cannot reach roots, inhibiting respiration
• Poor drainage allows harmful waste to accumulate in the soil

Why water retention matters

• Bonsai pots are small, causing water to evaporate quickly
• In summer especially, watering may be needed multiple times daily
• Insufficient water retention prevents roots from absorbing adequate moisture

How to achieve the right balance

• If you can water frequently, prioritize drainage
• If you are away often and watering is limited, increase water retention slightly
• Pot size: smaller pots dry faster, so increase water retention slightly
• Location: well-ventilated spots dry faster, so consider water retention
• Season: summer dries soil quickly, so adjust your mix proportions when repotting with the season in mind

Preparing and Sieving Bonsai Soil

Sieving your soil before use is essential for creating a high-quality soil environment.

Sieving steps

1. After purchasing soil, always sieve it before use
2. Remove particles that are too large and fine powder (dust)
3. Use only medium-grade particles suited to your bonsai's size
4. Take care to remove all dust, as it reduces drainage

Grain size guidelines

• Large bonsai (pot diameter 30cm or more): medium to large particles
• Medium bonsai (pot diameter 15–30cm): small to medium particles
• Small bonsai (pot diameter 15cm or less): very small to small particles
• Pot bottom: large-grade akadama or pumice stone

Common Beginner Mistakes and Solutions

Mistake 1: Using soil without sieving Soil straight from the bag contains fine powder that clogs the pot bottom. Simply sieving out dust before use makes a dramatic difference in drainage.

Mistake 2: Using only fine-grain soil to increase water retention While this reduces drying, it also prevents air from reaching roots, damaging them. Match grain size to your tree type and pot depth, placing large particles at the bottom and medium particles as the main layer.

Mistake 3: Using the same akadama for years Akadama particles break down into mud within a few years. Even if it looks unchanged, drainage is lost, so repotting every 2–3 years with fresh soil is essential.

Find Your Bonsai on GrowerDirect

GrowerDirect features many bonsai of different types for sale. When purchasing, we recommend asking the grower about the ideal soil mix for that particular bonsai. A bonsai's health begins with its soil. With the right soil choice, you can bring out your bonsai's full potential.