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Types of Bonsai Soils
Use
Mainly used for pre-bonsai and bonsai; various types of substrate (soil) are used either independently or mixed in a ratio best suited to the type of tree that will be planted in it.
Akadama
Akadama is a type of natural, granular clay soil from Japan that’s widely used in bonsai cultivation.Here’s a clear explanation:
- 🌋 Origin: Akadama (赤玉土) means “red ball earth.” It’s mined from volcanic clay deposits in Japan, then dried and sometimes lightly baked to increase hardness.
- ⚙️ Structure: It has a porous, granular structure that holds moisture while still allowing excellent airflow and drainage—vital for healthy bonsai roots.
- 💧 Function: Akadama absorbs water and nutrients, slowly releasing them to the tree’s roots. It also changes color (from reddish-brown to darker brown) when wet, helping growers gauge soil moisture.
Lava Rock
Lava rock is a popular inorganic component used in bonsai soil mixes. It’s formed from cooled volcanic magma and has a porous, rough texture that provides several important benefits for bonsai cultivation.
Here’s a clear explanation of its properties and uses:
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What Lava Rock Does in Bonsai Soil
- Excellent Drainage:
Lava rock prevents water from pooling around the roots, reducing the risk of root rot. Its irregular shape creates air pockets, allowing water to flow freely through the soil. - Aeration:
The rough, porous structure helps maintain oxygen flow around roots — essential for healthy root development and microbial activity. - Water Retention (Moderate):
Although it drains well, the pores in lava rock can still hold small amounts of moisture, keeping the soil slightly damp between waterings — ideal for most bonsai species. - Root Grip:
Its texture allows fine roots to cling tightly, promoting strong and compact root growth that is easy to manage during repotting. - Long-Lasting and Stable:
Lava rock doesn’t break down easily over time, meaning it helps maintain soil structure for years without compacting or decomposing like organic matter does.
Pumice
Pumice is a lightweight, porous volcanic rock. Its natural porosity allows it to hold water and air simultaneously, making it excellent for bonsai soil.
Uses of Pumice in Bonsai
- Drainage – Prevents waterlogging while retaining enough moisture for roots.
- Aeration – The porous structure lets air reach the roots, promoting healthy root growth.
- Soil Structure – Helps keep the soil loose, preventing compaction.
- Root Development – Encourages fine root growth by providing a stable but airy medium.
- Nutrient Retention – Can hold small amounts of nutrients for gradual release to roots.
Calcined Clay
Calcined clay in bonsai is a type of clay that has been heated to high temperatures in a kiln, causing it to harden and become porous. Unlike regular clay, which can retain too much water and compact easily, calcined clay is stable, lightweight, and doesn’t break down over time.
Properties:
- Porosity: Holds some water while allowing excess to drain, preventing root rot.
- Aeration: Provides air pockets in the soil, essential for healthy root growth.
- Stability: Doesn’t decompose like organic matter, keeping soil structure intact over years.
- Mineral content: Can supply small amounts of nutrients.
Pine Shreds (or other organic materials)
Pine bark is the outer layer of pine trees that has been collected, aged, and often shredded or chunked for horticultural use. It is organic, lightweight, and porous. In bonsai, it is typically used in small chunks (1/8”–1/4”) to mix with other soil components. Pine bark is a common component in bonsai soil mixes, especially for species that prefer slightly acidic, well-draining soils.
Uses in Bonsai
- Improves Soil Aeration – Pine bark creates air pockets in the soil, which allow roots to breathe and prevents compaction.
- Enhances Drainage – Its porous structure helps excess water drain away, reducing the risk of root rot.
- Retains Moisture – While it drains well, pine bark also holds some moisture, providing a balance for bonsai roots.
- Provides Organic Nutrients – As it slowly decomposes, pine bark releases nutrients into the soil, benefiting the tree over time.
- Improves Soil Structure – Mixing pine bark with inorganic elements like akadama, pumice, or lava rock gives the soil a mix of stability, aeration, and water retention.
- Acidifies Soil Slightly – Pine bark can slightly lower soil pH, which is beneficial for acid-loving species like azaleas or pines.