Kanuma acidic bonsai soil - 2/5 mm grain - 17 lt bag

What it is and where it comes from

Kanuma is a Japanese clay soil of pumice origin, quarried in the area it takes its name from. It comes as pale, lightweight granules with a low kanuma pH that makes it the reference substrate for acid-loving plants. Here it is graded at 2/5 mm and packed in the original 17-litre bag.

How it behaves in the pot

The granule is porous and absorbs more water than akadama, while still letting the excess drain away without compacting the root ball. This twofold behaviour is why azaleas thrive in it: they stay moist between waterings without the fine roots suffocating.

The structure stays open and airy. That said, kanuma is soft: over time, especially after repeated frosts, the granule tends to break down and compact, which is why many growers prefer not to use it neat and pair it with a more stable material. When wet it turns darker, which makes it easy to read the moisture level of the substrate at a glance.

Features

• Japanese clay soil for bonsai
• Grain size 2/5 mm
• Original 17-litre bag
• Low pH, suited to acid-loving plants
• High water absorption, greater than akadama
• Good drainage

Which grain and for which plants

The 2/5 mm is the most versatile size in the range: it covers shohin and small pots, but also works well on medium-sized specimens, filling the spaces between roots without leaving gaps. It is used throughout the substrate and, if a uniform surface is needed, as a top dressing too. The 17-litre bag is a handy quantity for a repotting session covering several plants.

Satsuki azaleas first and foremost, then camellias, rhododendrons, kalmias and heathers: anything that suffers in alkaline substrates. If you are looking for a general-purpose bonsai soil, akadama is the better choice; kanuma is a specific material, and on a conifer or an elm it makes little sense. A common question is whether kanuma can be mixed into a bougainvillea bonsai soil: bougainvillea is not an acid-loving plant, so a small share of kanuma will do no harm in a well-draining blend, but it should never be the main component.

How to use it when repotting

First of all, sieve it. The dust created in transit must be removed, otherwise it settles at the bottom and blocks drainage. Put a layer of the coarser grain recovered from sieving at the bottom of the pot, then carry on with the rest.

Work it between the roots with a chopstick, without pressing: the granule is soft and crumbles under pressure. After repotting, water thoroughly, several times, until the water runs clear from the drainage holes. That first generous watering also helps settle the material around the nebari.

Frequently asked questions

Can kanuma be used on its own?

Mixing it with other soils is recommended. On its own it holds a great deal of water and, being a soft granule, it tends to break down over the years: lightening it with a more stable, free-draining material extends the life of the substrate between repottings. A simple kanuma bonsai soil mix recipe for azaleas is roughly two parts kanuma to one part pumice or lava grit.

How does it differ from akadama?

Kanuma has a low pH and absorbs more water than akadama. Akadama is a good all-round substrate for most species; kanuma is specific to acid-loving plants, where the acidity of the material is part of the result and not a minor detail.

Is the 2/5 mm grain right for my pots?

Yes, if you work with shohin, shallow pots and small to medium plants. On large specimens in deep pots, many growers prefer a coarser grain throughout the substrate and keep the 2/5 mm for the surface.

What is included

• Japanese kanuma, grain size 2/5 mm
• Original 17-litre bag

If you prefer to measure out the components yourself, you can build your own blend on the custom soil mix page.