Choosing and preparing a workable mix

Give a recipe a job and an address

“What is the best bonsai soil?” leaves out most of the information needed to answer. A useful plan names the tree, its condition and goal, the container, the site, the actual materials, and the care available. It also says how you will notice a mismatch.

Here is our documented teaching starting point, not a report of a real experiment:

Jonas Dupuich describes his use of a roughly equal-part akadama, pumice, and lava mix in a firsthand account. That establishes a documented working approach, not a universal recipe or validation of our invented maple case. Our case uses it as a simple comparison baseline; it does not inherit every additive or technique mentioned in the source. Bonsai Tonight: Bonsai soil.

The 2–5 mm band makes this particular test repeatable. It is not a prescription for seedlings, tiny bonsai, every species, or every container. Even equal volumes of three differently graded products can produce a very different pore network.

Explain the starting choice

Pumice contributes porous particles, lava contributes relatively durable structure, and akadama adds a granular water-retaining component. All three still require scrutiny of the actual grade and source. The ratio is easy to record and compare; it is not mathematically optimal.

Start with one coherent mix rather than adding a separate fine layer and coarse “drainage layer.” Choose sufficient stable root contact without crushing particles or packing out air spaces. Root arrangement and anchoring during an actual repotting are taught in Course 11; preparing ingredients does not authorize disturbing this maple today.

Change the conditions before changing the ratio

Move the same proposed maple to a hot, exposed balcony where wind accelerates water loss and nobody can check at midday. A more water-retentive candidate may be worth comparing, but the care gap cannot be solved by making the entire pot stay saturated. Reduce excessive exposure where appropriate, arrange reliable checking, or choose a container and growing arrangement with a more manageable reserve. If no plan keeps the tree supplied through the unattended period, the site-and-care combination needs changing.

Now imagine a cooler, sheltered site with frequent rain and slow drying. Check actual root-zone moisture and whether rain reaches the pot. More consistently open structure or less of a particularly retentive fraction might help in a future mix, but unnecessary extra watering and obstructed outlets must be corrected first. “Wet climate” is not proof that every pot is wet.

The workbench explores those tendencies. It cannot calculate a perfect recipe or guarantee a watering interval.

Make a plan for each site, then name what evidence would cause you to revise it. A strong answer includes both a material choice and a care response. “Add more akadama” is incomplete unless you explain the intended effect, confirm the batch's properties, and check whether the change provides enough reserve without persistent saturation.

Prepare for consistency, not ritual

1. Identify and label ingredients. Record supplier, product, grade, and batch if available. Read handling information and check for unwanted additives.
2. Control dust. Work in ventilation away from people and pets, pour close to the tray, and avoid making airborne clouds. Use suitable respiratory and eye protection when the product's handling instructions call for it. Dampening a material can reduce dust if that preparation suits it; do not wet-screen a soft material into paste.
3. Screen where it serves a purpose. Screening separates size fractions. Remove excessive dust or unsuitable oversize pieces when useful, and record the screens used. A pregraded product may need little work. “Remove every small grain” is not a universal rule.
4. Measure consistently. Use loose volumes of similarly prepared ingredients; do not measure one compacted and another fluffy. Mix gently and thoroughly so early and late scoops have comparable proportions.
5. Keep a batch record. Your batch record connects the recipe with its actual ingredients and preparation. Retain a small labeled sample if useful, and store remaining material cleanly under suitable conditions.

Kusamura Bonsai Club's firsthand preparation guide shows screening as part of producing a repeatable mix. Its local recipes are examples of practice rather than requirements for this course. Soil basics.

Run a comparison without a tree

Use two equal unplanted containers at the intended depth, with the same open outlets and comparable initial moisture. Label the baseline A and a single changed candidate B. Fill to the same level using the same gentle settling method. Apply water evenly and slowly until the samples wet through and discharge; record the water applied and drainage observations. Let both drain for the same observation period before comparing them.

Record evenness of wetting, runoff pathways, the post-drain weight of each pot, and later changes under the same exposure. Compare each pot's change in weight, rather than interpreting a heavier mineral mix as automatically wetter. Raw weight includes the pot and solids. For a more controlled water-retention comparison, record the separate starting masses and method; neither approach tells you plant-available water directly.

Repeat if the result is surprising. Swap positions to check for an exposure effect. An empty pot has no roots removing water and no canopy intercepting light or rain, so its drying curve is not a watering timetable for a tree. It screens for obvious mismatches and provides a reproducible starting point.