Bonsai Soil Mix Calculator
Work out akadama, pumice, and lava rock amounts for conifer, deciduous, or tropical soil ratios, plus a pot volume estimate.
That pot holds about 2.7 L of soil.
How it works
Bonsai soil is almost always a blend of a few inorganic components rather than potting soil, because it needs to drain fast while still holding enough moisture between waterings. The classic ratios are 1:1:1 akadama, pumice, and lava rock for conifers, and a richer 2:1:1 for deciduous and tropical trees, since broadleaf trees tend to want a bit more water retention than pines and junipers. Pick a total volume and a tree type, and the calculator splits that volume across the three components by their ratio.
Worked example: 4 liters of soil for a deciduous tree at 2:1:1. The parts add up to 4, so each "part" is one liter, and the mix comes out to 2 liters of akadama, 1 liter of pumice, and 1 liter of lava rock. For a conifer at 1:1:1 with the same 4 liters, it splits evenly: about 1.3 liters of each. If you're not sure how much soil a pot needs in the first place, the second calculator here multiplies length by width by depth in centimeters and divides by 1000 to estimate liters, for example a 25 by 18 by 6 cm pot holds about 2.7 liters.
FAQ
Why not just use regular potting soil?
Potting soil is built to hold water and stays wet for days in a shallow bonsai pot, which suffocates fine roots and invites rot. The akadama, pumice, and lava blend drains within minutes while still holding water on the particle surfaces, which is closer to what a tree's compact root system actually needs.
What does each component actually do?
Akadama is a fired clay that holds water and nutrients and breaks down slowly over a year or two, which is part of why repotting on a schedule matters. Pumice is a light, porous volcanic rock that adds drainage and aeration without breaking down. Lava rock is similar to pumice but a bit heavier and more durable, and it helps keep the mix from compacting over time.
Do tropical and indoor trees really need the same ratio as deciduous?
Close to it. Most indoor tropicals like ficus and jade do well on the same 2:1:1 mix used for deciduous trees, though some growers swap the lava rock for a bit of fine pine bark since indoor air tends to dry soil out faster than an outdoor bench does.
Can I premix a big batch and store the extra?
Yes, dry akadama, pumice, and lava rock all store fine in a sealed bucket. Just sift out the fine dust before you use it, since that dust is what clogs drainage and causes the soggy-soil problems the mix is designed to avoid in the first place.
For more on soil and repotting, see bonsai soil explained: what mix to use, how to repot a bonsai tree, and how to water a bonsai tree the right way.