Most repotting advice stops at "use fresh potting soil", which is the part that decides whether the plant thrives or sulks for a year. A cactus and a fern both live in brown material in a pot. Almost nothing else about what they need is the same.
You do not need eight bags of separate ingredients. You need one bag of decent general mix, two amendments, and an understanding of what you are adjusting.
What a potting mix has to do
Four jobs, and one of them gets forgotten.
It anchors the plant. It holds water. It supplies nutrients. And it holds air, which is where most failures start. Roots take up oxygen from the gaps between soil particles, and when those gaps stay full of water the roots suffocate and rot organisms move in. Every recipe below is a decision about how much air to build in.
That is why the answer differs by plant. A cactus evolved in grit that drains in seconds. A fern evolved in leaf litter that stays damp. Both need air at the roots; they differ in how long they tolerate water sitting there.
Why bagged houseplant compost is a starting point
Open a bag labelled houseplant compost and you generally get a peat or coir base with a little sand or perlite and a starter fertiliser. It is made to hold water, because a shop plant that dries out on a shelf is a dead sale, and because it has to work for everything.
Straight from the bag it is too dense for most houseplants, and it compacts further over a year of watering. Adding an aerating amendment fixes it, and that single habit prevents more root rot than any watering schedule.
The amendments and what each one does
Perlite. White volcanic granules, very light, holds almost no water. The default aerator. Cheap, available everywhere, and it floats to the surface over time, which is cosmetic rather than a problem.
Pumice. Does the same job as perlite and holds a little moisture and nutrient. Heavier, which suits top-heavy succulents that tip over in a light mix. Harder to find in some countries and dearer.
Orchid bark. Chunky pine bark that creates large air pockets and breaks down slowly. Essential for epiphytes, useful in any mix for climbing aroids.
Coco coir. Coconut husk fibre, holds water well, wets more easily than dry peat. A common peat replacement. Buy it buffered or rinse it, because unrinsed coir can carry salts.
Leaf mould and worm castings. Fine, rich, water-retentive. Castings add nutrition; use them as a small proportion, around 10 percent, rather than a base.
Coarse sand or grit. Adds weight and sharp drainage for cacti. It has to be coarse horticultural sand or grit. Fine builder's sand fills the gaps between particles and sets like concrete, which is the opposite of the intent.
Horticultural charcoal. Optional. Adds structure and is said to sweeten a mix that stays damp. Harmless, not essential.
Four recipes worth memorising
General tropical foliage
Pothos, philodendron, monstera, dracaena, rubber plant, spider plant, peace lily.
Two parts general potting mix to one part perlite or fine bark.
This one recipe covers most of a typical collection. It drains within a minute of watering, then holds enough moisture to get through several days.
Climbing aroids and anything epiphytic
Monstera adansonii, philodendron in the climbing group, syngonium, anthurium.
Equal parts potting mix, orchid bark and perlite.
These grow up trees in the wild with roots in bark and leaf debris rather than soil. The coarser mix looks alarmingly loose in the pot and is exactly right; water runs straight through and the roots get the air they are built for.
Succulents and cacti
Half potting mix, half coarse grit, pumice or perlite.
Some growers go further, to one third soil and two thirds mineral. Err towards more grit, because drainage matters more than nutrition here and these plants are killed by water sitting at the roots far more often than by hunger.
Terracotta suits them: the pot dries the mix from the sides as well as the top.
Ferns, calatheas and moisture lovers
General mix with coco coir or leaf mould added, plus a handful of perlite.
The aim is a mix that stays damp without going airless. These plants want moisture and still rot if the roots sit in water, which is the balance people find hardest.
Orchids
Phalaenopsis and most shop orchids grow in bark alone, with no soil at all. Their roots are photosynthetic and adapted to open air and periodic drenching. Potting an orchid into houseplant compost kills it within a season. Buy a bagged orchid mix and use it as it comes.
Ready-made mixes: when they are fine
Species-specific bags for cactus, orchid and citrus are worth buying, because the base differs rather than being the same compost with a different label. For everything else, a general bag plus perlite gets you closer than most specialist products at a lower price.
Three things to check on a bag. Squeeze a handful: if it stays in a dense ball and drips, it is too wet and heavy. Look for visible structure, some perlite or bark rather than uniform fine brown. And check whether it contains fertiliser, because if it does you should not feed for the first month or two after repotting.
Signs a plant is in the wrong mix
Water sits on the surface for more than a few seconds before soaking in. The mix has compacted or dried into a water-repellent state.
The pot is still wet a week later in reasonable light. Too dense, too water-retentive, or the pot is too big for the root ball.
It dries out in a day. Either the plant has outgrown the pot, or the mix is so coarse that nothing holds moisture. In a plant that also wilts every couple of days, check the roots before you change the mix.
A white crust on the surface or the pot rim. Mineral and fertiliser salts accumulating, common with frequent small waterings that never flush the pot through.
Green film or permanently dark, damp surface. The mix never dries, which usually means an oversized pot, a dense substrate, or too little light for the amount of water going in.
Fungus gnats in numbers. They breed in the top few centimetres of constantly moist substrate, so treat them as a reading on the soil as much as a pest.
A slow decline in the weeks after repotting. If a healthy plant goes downhill after a repot, suspect the pot size and the mix before anything else.
The pot changes the recipe
Substrate and container work as a pair, and adjusting one lets you get away with more in the other.
Terracotta loses water through the walls as well as the surface, so a mix in terracotta runs perhaps a third drier than the same mix in plastic. If you tend to overwater, terracotta plus a normal mix forgives you. If you forget to water, plastic plus an airy mix suits you better.
Pot size matters at least as much. A large volume of mix around a small root ball stays wet for weeks whatever the recipe, because there are not enough roots drinking from it. Going up one size at a time is the fix, not more perlite.
Depth counts too. Wide shallow pots dry faster than tall narrow ones of the same volume, which is why shallow bowls suit succulents and deep pots suit thirsty plants with long roots.
Mixing, storing and what to do with the old soil
Mix in a bucket or a builder's tray, dry, before you add anything to the pot. Judging proportions once the material is wet is guesswork, and a half-mixed pot has dense pockets that hold water while the rest drains.
Dampen the mix slightly before potting. Bone-dry peat and coir repel water, so a pot filled dry can take several waterings before it wets through evenly. A few splashes and a stir until it feels like a wrung-out cloth is enough.
Leftover mix keeps in a closed bag out of the sun. Old soil from a healthy plant goes on the compost heap or into outdoor beds. Soil from a plant with root rot or root mealybugs goes in the bin, along with a wash of the pot, because both survive in it.
Two myths worth dropping
Gravel at the bottom of the pot improves drainage. It does not. Water does not cross from fine material into a coarse layer until the fine material is saturated, so a gravel layer raises the waterlogged zone and takes away usable root space. Use a pot with a drainage hole instead.
Repotting means going up a size every time. Fresh mix in the same pot renews structure and nutrition without adding volume, which is what most plants need after two or three years. Size up only when the roots have filled the pot.
Peat, and what to use instead
Peat holds water well and comes from bogs that took thousands of years to form. Coir behaves similarly, wets more easily once dry, and is a byproduct of an existing crop. Composted bark and leaf mould also work as part of the base. If you want to move away from peat, coir is the closest swap and needs no change to your watering routine beyond the first month while you learn how it feels.
How much of this to worry about
If you own ten plants and one bag of soil, buy a bag of perlite and mix it in at roughly one part in three. That single change fixes most substrate problems in a normal collection. Save the more exact recipes for the plants that keep failing.
When you repot, that is also the best look at the roots you will get all year: firm and pale is healthy, brown and soft means rot, and both are easier to judge with the plant out of the pot than through any symptom on the leaves.
If you are unsure what a particular plant wants, photograph it in Gardenix. A scan identifies the species and returns substrate and repotting recommendations for it, along with a watering interval and light requirements, so you can match the mix to the plant rather than to a general rule. Scan the same plant again a few weeks after repotting and the merge option keeps it on one timeline, so the health chart shows whether the change helped.
For the timing and the technique, we covered the signs a plant needs a bigger pot and the step-by-step in when and how to repot a houseplant.