Akadama, Pumice and Lava: What Bonsai Soil Actually Does

Why bonsai soil contains no soil, what each component actually contributes, and how to mix a substrate that suits your species, climate and watering habits.

The first thing that surprises people about bonsai soil is that it contains no soil. A good bonsai substrate is a graded mineral aggregate — hard particles, sieved to a consistent size, with almost no organic matter and almost no fine dust. It looks more like aquarium gravel than anything you would find in a garden.

That is not tradition for its own sake. It is a direct response to the problem a bonsai pot creates.

The problem the substrate solves

A bonsai pot is shallow and small. In shallow containers, water does not drain the way intuition suggests — a phenomenon called the perched water table. Below a certain depth, capillary action holds water in place regardless of how many drainage holes you drill. The shallower the pot, the larger the proportion of the soil volume that stays saturated.

Garden soil or standard potting compost in a bonsai pot therefore stays wet at the bottom more or less permanently. Roots there get no air, and roots without air die and rot. This is why the overwhelming majority of bonsai lost by beginners are lost to root problems rather than to anything happening above the soil line.

A coarse mineral substrate solves this by having large air gaps between particles. Water drains through, air follows it in, and each particle holds a small reservoir of moisture on and within its own structure. You get water retention and aeration at the same time, which ordinary soil in a shallow pot cannot deliver.

The three main components

Akadama

Akadama is a naturally occurring baked clay granulate from Japan. It is the classic bonsai component and the most argued-about.

• What it does well: holds water and nutrients within the particle while remaining structurally open. Roots ramify beautifully in it. It also visibly changes colour as it dries, which makes judging watering far easier for beginners.
• The catch: it breaks down. Over two to three years akadama softens and collapses into fine particles, which clog the mix and undo the aeration you bought it for. In climates with hard freezes this happens faster, sometimes in a single winter.
• Quality varies enormously. Cheap akadama turns to mud in months. Higher-fired grades last considerably longer and are worth the difference.

Pumice

Pumice is a volcanic glass, extremely light, full of internal pores.

• What it does well: excellent aeration, holds a useful amount of water in its pores, and — critically — does not break down. A pumice-heavy mix keeps its structure for many years.
• The catch: low nutrient-holding capacity compared to akadama, and its lightness can make large pots unstable in wind.

Lava rock

Crushed volcanic lava, usually red or black, harder and heavier than pumice.

• What it does well: structural stability, sharp drainage, good root anchorage, and it never degrades. The rough surface gives roots something to grip.
• The catch: holds less water than either of the others, and the sharp edges can be tough on very fine roots.

Mixing for your conditions, not for a recipe

The commonly cited starting point is equal parts akadama, pumice and lava. It is a reasonable default, but the useful skill is knowing which way to shift it.

Shift toward more akadama (more water retention) if:

• You grow deciduous species — maples, elms, hornbeam — which are thirstier.
• You cannot water daily in summer.
• Your climate is hot and dry.

Shift toward more pumice and lava (more drainage) if:

• You grow pines and junipers, which resent wet feet.
• Your climate is wet, humid, or has cold wet winters.
• You have a habit of overwatering — and most people do.

Adjust particle size for pot depth: finer particles for shallow pots and small trees, coarser for deep pots and large trees. Sieve out dust before use regardless. Fine dust is the enemy; it migrates down, fills the air gaps and recreates exactly the drainage problem you were trying to avoid.

Species that break the rules

• Pines do well with a leaner, sharper mix and benefit from mycorrhizae; do not sterilise a pine mix.
• Azaleas want an acidic substrate — kanuma rather than akadama — and will chlorose in an alkaline mix.
• Tropicals kept indoors tolerate a slightly more moisture-retentive mix, since indoor conditions dry pots more slowly.
• Collected yamadori in recovery are usually best in pure pumice, which maximises aeration while new roots establish.

What about organic material?

Some growers add a small proportion of bark, coir or composted material for nutrient retention, typically ten to twenty percent. It is a legitimate approach, particularly for deciduous trees or where feeding is irregular.

The risk is that organic matter breaks down faster than the mineral components, compacts, and reduces aeration over time. If you use it, keep the proportion low and shorten your repotting interval. A fully inorganic mix demands that you feed consistently, because the substrate itself supplies nothing — see our feeding guide.

The honest cost picture

Proper bonsai substrate is expensive relative to potting compost, and the temptation to economise is strong. Two things are worth knowing:

• Shipping dominates the cost. These are heavy, dense materials. A bag that seems reasonably priced often doubles once freight is added, so buying several bags at once is substantially cheaper per bag than buying one at a time.
• Cheap akadama is a false economy. Low-fired akadama that collapses in one season costs you the material, the repotting labour, and potentially the tree's root health.

If you are buying, our shop carries graded substrate components and sieves. If you are starting out and the full three-part mix is not practical, a pumice-heavy mix with a modest akadama fraction is more forgiving of both watering mistakes and budget constraints than anything organic.

When to change the substrate

Substrate is renewed at repotting, not on its own schedule — see the repotting guide for timing by species. The signs your mix has degraded:

• Water sits on the surface rather than draining straight through.
• The mix looks and feels muddy when wet, or sets hard when dry.
• The tree's vigour has declined over a season or two with no other explanation.

A tree in collapsed substrate will decline slowly and confusingly, showing symptoms that look like a dozen other problems. If your mix is three or more years old and something is quietly wrong, the soil is a strong suspect.

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