Soil mix calculator
The right mix for your species, scaled to your pot, measured in cups — and adjusted for whatever you already have.
Chunky aroid mix · 2.9 gal
Aroids grow on tree bark in the wild — they want air around the roots, not dense soil.
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The recipes
Chunky aroid mix
Aroids grow on tree bark in the wild — they want air around the roots, not dense soil.
4 parts bark · 3 parts perlite · 2 parts potting mix · 1 parts coir
Standard houseplant mix
Bagged potting soil alone stays too wet in an indoor pot; perlite fixes the drainage.
7 parts potting mix · 3 parts perlite
Fast-draining succulent mix
Succulents and dracaenas rot in moisture-retentive mixes. Grit is the whole point.
4 parts potting mix · 4 parts pumice · 2 parts sand
Palm mix
Palms want moisture retention with enough structure that the roots still breathe.
6 parts potting mix · 2 parts perlite · 2 parts bark
Orchid bark (no soil)
Phalaenopsis are epiphytes. Potting soil suffocates and rots their roots.
8 parts bark · 1 parts charcoal · 1 parts perlite
Questions
How much soil does a pot actually hold?
Less than people expect, because a pot is a truncated cone filled to below the rim. A 6-inch pot holds roughly 2 quarts, an 8-inch about 4.5, a 10-inch about 10 and a 12-inch about 17. Volume rises with the CUBE of width, which is why going up two pot sizes more than doubles the soil — and why it drowns plants.
Why not just use bagged potting soil?
Because bagged mix is formulated to hold water, and in an indoor pot with no sun and no wind it holds it for far too long. Most houseplant deaths attributed to overwatering are really soil that never dries. Cutting it with 30% perlite or bark fixes the drainage without changing anything about how you water.
What is the difference between perlite and pumice?
Both open the mix up and neither holds much water. Perlite is lighter and cheaper but floats to the surface when you water. Pumice is heavier, stays put, and is the better choice for top-heavy plants and anything you water from above. They substitute freely.
Do I need to sterilise or bake the soil?
No, for houseplants in bagged mix. Sterilising is for seed-starting where damping-off fungus kills seedlings. For established plants it destroys the beneficial soil life along with everything else.
Can I reuse old potting soil?
Not for a plant that was rotting or infested — that soil carries the problem forward. Otherwise you can, but old mix has broken down into fine particles that compact and drain badly. Cutting it half-and-half with fresh mix and adding perlite restores the structure.