Soil & Substrate Basics
Potting soil and potting mix are not the same product, and in a pot indoors that difference decides whether a plant thrives or slowly declines. This is the foundation: what is in the bag, why a container changes the rules, and how to match a mix to a root.
The short answer
1. Potting soil contains soil. Potting mix does not. Indoors, the one without soil almost always wins.
2. Air is the point. Roots need oxygen. Compaction, not underwatering, kills most houseplants.
3. Structure has a lifespan. A good soilless mix holds its shape for 12–18 months. Bagged soil collapses in a season.
Potting soil vs. potting mix
They sit side by side on the same shelf, at roughly the same price, and they behave nothing alike once there is a root system in them.
| Potting soil | Soilless mix | |
|---|---|---|
| Base | Screened topsoil plus amendments | Bark, perlite, pumice, coir, biochar |
| Drainage | Variable; holds water at the pot base | Fast and predictable, batch to batch |
| Air space | Falls away as particles compact | Held open by rigid screened particles |
| Pests & pathogens | Can carry fungus gnats and soil-borne disease | No soil, so no soil-borne passengers |
| Useful life in pot | One season | 12–18 months |
| Best for | Outdoor beds, raised beds, annuals | Anything living in a pot indoors |
What’s actually in the bag
Six components do nearly all the work. Every Molly’s blend is a different ratio of these, screened to a consistent particle size.
- Pinus radiata bark. The skeleton. Chunky, slow to break down, and it holds the air pockets everything else depends on.
- Perlite. Popped volcanic glass. Almost weightless, permanently porous, and it stops the mix from settling into a brick.
- Pumice. Perlite’s heavier cousin. Adds ballast so top-heavy succulents and cacti stay upright.
- Coconut coir. The moisture buffer. Rewets easily where peat repels water once it dries out, and it is renewable.
- Coco chips. Chunky coconut husk. Slower to break down than coir and it keeps large air pockets open deep in the pot.
- Biochar. Sweetens the mix. Its pore structure holds moisture and beneficial microbes, and buffers salts so a closed pot does not turn sour.
Match the mix to the root
Roots are not interchangeable. An orchid root evolved clinging to bark in open air; a succulent root evolved in grit that dries within hours. Same principle, three ratios.
Aroid mix — monstera, philodendron, pothos
Chunky and bark-forward. Thick tropical roots want to grip something and dry between drinks. Shop aroid →
Succulent mix — echeveria, haworthia, cacti
Gritty and mineral-heavy. Drains almost immediately and adds weight so rosettes do not tip. Shop succulent →
Orchid mix — phalaenopsis, cattleya, oncidium
Nearly all bark and air. Epiphyte roots are built for open structure, not a dense root ball. Shop orchid →
Read next
Start here
- Potting soil vs. potting mix — the most common confusion in plant care, finally settled, and when each one is the right call.
- A guide to soil alternatives — when bark mixes, gritty mixes, LECA, sphagnum and coir blends beat traditional potting soil.
- What to know about soilless mix — a primer on ingredients, performance, and why a good mix lasts 12–18 months in the pot.
Going deeper
- How to choose the best soilless mix — a decision framework rather than a single recommendation.
- Best soil for indoor plants — match the mix to your plant, by group.
Common questions
Is potting mix the same as potting soil?
No. Potting soil contains actual topsoil plus amendments. Potting mix is engineered from components like bark, perlite, coir and biochar, with no soil in it at all. For anything in a pot indoors, mix almost always outperforms soil — it drains better and resists compaction.
What does “soilless” actually mean?
That the substrate contains no soil whatsoever — it is built entirely from bark, perlite, pumice, coir and biochar. The payoff is predictable drainage, a consistent screened medium, and a structure that holds for 12–18 months. Standard potting soil compacts and degrades within a single season.
How long does a bag of soilless mix last?
In a pot, 12–18 months before the structure breaks down enough to need refreshing. Unopened and stored dry, the bag stays usable for two to three years or more. There is no meaningful shelf life concern — it is in-pot performance that tells you when to repot.
Do I still need to fertilise?
Yes. Molly’s mixes ship free of fertiliser on purpose, so you control the feed rather than inheriting an unknown charge. Feed at quarter to half strength during active growth and you will not burn new roots.
Can I use one mix for every plant?
You can get away with it, but you will water around the mismatch forever. Aroid mix in a succulent pot stays wet too long; succulent mix under a monstera dries before the plant has drunk. Matching the ratio to the root is the cheapest care upgrade there is.
Find the right mix for your plant
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More care guides
Orchid care · Succulent & cactus care · Propagation · Flowers & container gardening
