Identium
Houseplant Soil: Why Bagged Compost Kills Plants and What to Mix Instead
Most houseplants are sold in a mix that suits the nursery, not your living room. Changing it is the cheapest upgrade in the hobby.
People obsess over watering schedules and ignore the thing that decides how long water stays in the pot. Soil is half the watering equation, and getting it right removes most of the guesswork.
The short version: plain bagged potting compost holds too much water for too long for most houseplants. Adding one or two cheap ingredients fixes it.
Why plain compost is a problem indoors
Potting compost is designed for outdoor containers, where sun and wind dry the surface fast and rain drains through. Indoors there is no wind and much less light, so the same mix stays wet for days.
Roots need air as much as water. They breathe through the gaps between soil particles, and a mix that stays saturated has no gaps. That is why overwatering kills: not drowning exactly, but suffocation, followed by root rot moving into the dead tissue.
Nursery mixes are worse in this respect than you would expect. Growers keep plants in dense, water-retentive peat because it makes watering hundreds of plants on a bench efficient. It is the right choice for them and the wrong one for a shelf in your flat.
You are not choosing soil. You are choosing how fast the pot dries out.
Rule of thumbWhat the ingredients actually do
Potting compost or coir. The base. Holds water and nutrients. Coir is coconut fibre, a peat substitute, and worth using since peat extraction destroys bogs that took millennia to form and store enormous amounts of carbon.
Perlite. The white polystyrene-looking bits. Volcanic glass popped like corn. It creates air pockets and does not break down. This is the single most useful thing you can add to anything.
Orchid bark. Chunky pine bark. Creates large air gaps, which is what climbing aroids want because their roots evolved clinging to trees rather than sitting in soil.
Coarse grit or pumice. Heavier than perlite, so it does not float to the surface when you water. Better for succulents and for top-heavy plants that need ballast.
Worm castings. Gentle slow nutrition. A handful improves almost any mix.
Horticultural charcoal. Optional. Keeps a mix sweet in pots without drainage holes, which you should not be using anyway.
Three mixes that cover nearly everything
- General houseplants: 3 parts compost or coir, 1 part perlite. Pothos, philodendron, spider plant, peace lily, dracaena, ferns, calathea. If you only ever make one mix, make this.Most plants
- Chunky aroid mix: 2 parts compost, 2 parts orchid bark, 1 part perlite. Monstera, large philodendrons, anthurium, alocasia, syngonium. Dries fast and lets you water generously.Climbing aroids
- Succulent and cactus: 1 part compost, 1 part coarse grit or pumice, 1 part perlite. Snake plant, ZZ plant, jade, aloe, echeveria, cacti. Should feel gritty and pour rather than clump.Succulents
Do not measure carefully. Handfuls are fine. The proportions are a direction, not a recipe, and the difference between three parts and two and a half will not register.
Special cases
Orchids. Do not use soil at all. Phalaenopsis roots are adapted to clinging to tree bark in open air, and they rot fast in potting mix. Use straight orchid bark.
Carnivorous plants. Sphagnum moss and perlite only. No compost and no fertiliser ever, since they evolved in nutrient-poor bogs and normal compost will kill them.
African violets. Light and airy, roughly half compost and half perlite. They rot easily at the crown.
Two myths worth dropping
Gravel in the bottom of the pot for drainage. This is the most persistent piece of bad advice in the hobby and it does the opposite of what people think. Water does not cross freely from fine material into coarse material, so a gravel layer actually raises the saturated zone in the pot closer to the roots. Use a mix that drains and a pot with a hole.
Garden soil is fine for pots. It is not. It compacts into a brick in a container, drains badly, and brings in pests, weed seeds and pathogens. Container growing is a different problem from ground growing.
Should you repot a new plant into better mix?
Usually yes, but not on day one. Give it a month to adjust to your light and humidity first, then repot into fresh mix in the same size pot or one size up.
The exception is if it arrives sitting in sodden soil, or you tip it out and find dense wet peat packed around the roots. Then do it now, and take the chance to check the roots while you are there.
You do not need to wash all the old soil off. Loosen the root ball, shake off what falls away easily, and pot on. Bare-rooting is stressful and rarely necessary unless you are dealing with rot or a pest in the soil.
The practical payoff
A better mix does not mean your plant needs less attention. It means the consequences of getting watering slightly wrong are much smaller, because the mix drains instead of holding water against the roots for a week.
That is the whole argument. You buy yourself margin for error, for about the price of a bag of perlite.
Different species want genuinely different mixes, and the labels almost never say which. If you are not sure what you have, photograph a leaf with the stem visible and Identium will name it and tell you what it wants from its soil, which is the part that decides everything else.
Frequently asked questions
What is the best soil for houseplants?
For most houseplants, three parts potting compost or coir to one part perlite. That single mix suits pothos, philodendron, spider plants, peace lilies, dracaena, ferns and calathea. Climbing aroids such as monstera prefer a chunkier mix with orchid bark, and succulents want grit or pumice instead of extra compost.
Can I use garden soil in pots?
No. Garden soil compacts into a dense block in a container, drains poorly, and brings weed seeds, pests and pathogens indoors. Growing in a pot is a different problem from growing in the ground, and container mixes are built to stay open and drain.
What does perlite do?
Perlite is volcanic glass heated until it pops, and it creates permanent air pockets in the mix so roots can breathe and water can drain. It does not break down or add nutrients. Adding perlite to plain compost is the cheapest and most effective improvement you can make to almost any houseplant soil.
Should I put rocks in the bottom of a plant pot?
No. It is a persistent myth that does the opposite of what people expect. Water does not move freely from fine soil into a coarse layer, so gravel at the bottom raises the saturated zone closer to the roots rather than draining it away. Use a free draining mix and a pot with a drainage hole.
Should I repot a plant as soon as I buy it?
Wait about a month unless it arrived in sodden soil or you find dense wet peat packed around the roots. A new plant is already adjusting to a change in light and humidity, and repotting adds a second shock. After that, fresh mix in the same or one size larger pot is a genuine upgrade.