Bonsai soil & substrate

Choosing bonsai soil

Bonsai are grown in a granular substrate rather than garden soil or ordinary potting mix. Potting mix is made for deep pots: its fine, mostly organic particles hold water for days and settle as they break down, and in a shallow bonsai pot that leaves the roots wet and short of air. Roots in a shallow pot need air as much as water: hard, evenly sized particles leave pore space between the grains, so water drains through in seconds, oxygen reaches the roots, and enough moisture clings to each particle to carry the tree to the next watering. Fine dust is what destroys that structure, which is why every bag here is sifted to the size on the label and washed before it is packed.

The components

Akadama is a baked Japanese clay and the traditional base of most bonsai mixes. It holds moisture and nutrients while staying open, and the hard triple-line grade breaks down slowly enough to last several years in the pot.

Sakadama is an akadama substitute with a harder structure and similar water retention, so it holds its particle size longer between repots. It comes in a medium 3-5mm and a large 5-8mm grade and is used in the same ratios as akadama.

Japanese pumice holds air and some water, does not break down in the pot, and adds drainage to any mix. Use it as the inorganic partner to akadama or Sakadama, or straight for cuttings and trees in recovery.

Kanuma is a soft, acidic Japanese pumice and the standard medium for azaleas and satsuki, which need an acidic, free-draining root zone.

Kiryuzuna is a hard, iron-rich Japanese sand blended into conifer mixes. It keeps its shape in the pot and runs the root zone drier, which pines and junipers prefer.

Zeolite is a porous mineral that absorbs water into the particle and releases it slowly, which evens out how quickly a pot dries between waterings without waterlogging it.

Pine bark (Orchiata) is aged New Zealand Pinus radiata bark, the organic part of a mix. It adds structure and helps hold nutrients between feeds.

Bonsai soil mixes by tree

• Deciduous trees such as Japanese maples and elms: akadama or Sakadama straight, or half akadama with a quarter pumice and a quarter pine bark.
• Pines and junipers: equal parts akadama, pumice and kiryuzuna. Conifers prefer a drier root zone than deciduous trees.
• Figs and mixed collections: equal parts Sakadama, pumice and pine bark.
• Azaleas and satsuki: kanuma on its own.
• Cuttings and trees recovering from root damage: pumice on its own.
• Seeds: the seed raising and cutting mix.

Shift the ratios toward pumice if a tree stays wet for days after watering, or toward akadama if it dries out within hours in summer.

If you would rather not blend your own, Keith's Mix is the house blend of akadama substitute, pumice and pine bark, sifted and ready to use straight from the bag.

Particle size

Smaller particles hold more water; larger particles drain faster. 3-6mm is the general-purpose size for most trees and pots, 3-5mm suits smaller pots and finer root systems, and 5-8mm suits large trees, deep pots and the drainage layer laid across the base of the pot.

How much you need

Measured as pot length × width × depth, approximate fill volumes are:

• 15 × 11 × 5 cm pot: under 1 L
• 20 × 15 × 6 cm pot: about 2 L
• 30 × 22 × 8 cm pot: about 5 L
• 40 × 30 × 9 cm pot: about 10 L

Allow extra for a drainage layer in deeper pots. Each soil product page repeats this table next to the bag size.