My substrate recipes, plant by plant
Sphagnum, perlite, bark, biochar, coco chips. For Sonerila/Sarcopyramis: 50/30/10/10. For Alocasia: 25/25/20/15/10/5. For Begonia: 40/30/20/10. Every formula I use at the nursery.
The right substrate is not the one that feeds most. It is the one that lets the roots breathe while they stay damp. In understorey tropicals, which in the wild grow on a mat of decomposing leaves and not in soil, that distinction is everything: nearly every problem you write to me about is suffocated roots, not starved ones.
That is why there is almost never any actual soil in my mixes.
The components, and what each one does.
Long-fibre sphagnum holds a lot of water and gives it up slowly, staying airy because the fibres do not compact. It is the base for anything that wants high, steady humidity. Soak it twelve hours ahead and then wring it out well: it should be damp like a sponge just squeezed, not dripping.
Perlite is what creates the air pockets. I use the 2-6 mm grade: the fine stuff compacts and does nothing. Rinse it before use, because the white dust it makes is unpleasant to breathe and clogs the pores.
Pine bark gives structure and stops the mix collapsing over time. It is the component that makes a substrate last two years instead of six months.
Biochar absorbs excess salts and houses the useful bacteria. A little is enough, between 5 and 10 per cent.
Coconut chips sit halfway between sphagnum and bark: they hold water but drain. I use them where a middle way is needed.
Worm castings are the only organic input I add, and only where the plant is a heavy feeder.
The three recipes I actually use.
For Sonerila and Sarcopyramis, the tiny terrarium melastomes: sphagnum 50, perlite 30, bark 10, biochar 10. These live in very high humidity and have extremely fine roots that do not forgive dryness. The mix is deliberately sphagnum-heavy because in a closed terrarium the risk of drying out is low.
For Alocasia
bark 25, perlite 25, coconut chips 20, worm castings 15, biochar 10, sphagnum 5. Alocasia are the hungry exception, they eat a good deal, but they also rot fast: hence the very open structure with only a touch of sphagnum to hold the moisture.
For Begonias
sphagnum 40, perlite 30, bark 20, biochar 10. Rhizomatous Begonias have shallow roots and hate having the rhizome wet. This mix stays damp underneath and dries at the surface, which is exactly what they want.
The proportions are by volume, not by weight. You measure them with a cup, not with scales.
The mistakes I see most often
Buying general-purpose compost and stirring in a handful of perlite: the compost compacts anyway and the perlite all ends up at the surface. Putting dry sphagnum straight into the pot: it takes up the water you pour in and then holds it unevenly. Making a perfect mix and then putting it in a glazed ceramic pot with no holes.
One last thing: the pot matters as much as the substrate. I use clear pots for almost everything, not for looks but because seeing the roots means noticing a problem three weeks earlier.