Potting Soil: What Is Actually in the Bag, and Which to Buy

Potting mix contains no soil at all, and that is the point. What each ingredient does, why garden soil fails in a container, and how to read a bag honestly.

By The Grow In Backyard Team September 9, 2026 5 min read Save to Pinterest

A handful of dark potting mix with white perlite granules visible through it — Photo: Wikimedia Commons contributor, Public domain, via Wikimedia Commons

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The first surprising thing about potting soil is that it usually contains no soil. It is a blend of organic matter and mineral particles engineered for one situation — a plant in a box with a hole in the bottom.

That situation is harsher than it looks, and it explains every ingredient in the bag.

Why a container is a hostile place

A pot is a small volume of medium with a hard boundary. Three things follow:

Water cannot drain away downward into deeper soil. It sits in the base until the plant uses it or it evaporates, and roots in that zone run out of oxygen.
The medium compacts every time you water. In the ground, worms, roots and freeze-thaw constantly reopen structure. In a pot, nothing does.
Nutrients wash straight out through the drainage holes with every watering.

A good potting mix answers the first two by holding air space even when saturated. It cannot solve the third, which is why feeding is a permanent job in containers — see growing in pots.

What is actually in the bag

The standard ingredients and what each does

Ingredient	Job	Notes
Peat	Holds water, light, sterile	Cheap and effective; environmentally contentious
Coconut coir	Same job as peat	Renewable; rehydrates far better once dry
Composted bark	Structure and air space	Keeps the mix open for longer
Perlite	Air space	White volcanic granules; does not hold water
Vermiculite	Air space plus water and nutrient holding	Softer, holds more moisture than perlite
Compost	Nutrients and biology	Too much alone becomes dense and wet
Sand or grit	Drainage and weight	For cactus mixes and top-heavy pots
Wetting agent	Lets dry mix reabsorb water	Peat is water-repellent once fully dry
Starter fertiliser	Feeds for the first few weeks	Typically exhausted in 4 to 8 weeks

Perlite and vermiculite are not interchangeable. Perlite is pure air space and drains; vermiculite holds moisture and nutrients as well. Use perlite where rot is the risk, vermiculite where drying out is.

Peat, coir, and the trade-off

Peat is harvested from bogs that took millennia to form and that store enormous amounts of carbon, which is why peat-free mixes are now the default in many places. Coir is a byproduct of coconut processing and rewets much more willingly than peat, but it is shipped a long way and often needs extra nitrogen early on. Bark and composted green waste mixes work well and behave differently — they drain faster and need watching in hot weather.

Potting soil, potting mix, garden soil, compost

The names on bags are inconsistent, and the differences matter more than the labels suggest.

Potting mix / potting soil. Soilless, for containers. This is what goes in a pot.
Garden soil. A bulk amendment for digging into beds. Never put it in a container.
Compost. Decomposed organic matter — a nutrient-rich ingredient, and too dense on its own.
Topsoil. Screened mineral soil for filling and levelling ground. Not a growing medium in a pot.
Raised bed mix. A middle ground: soil, compost and bark, for large volumes that sit on the ground and drain into it.

The single most consequential mistake is putting garden soil or straight topsoil into pots. In a bed it is fine; in a container it slumps into a dense, waterlogged block within weeks and takes the plant with it.

Choosing by what you are growing

Seeds. Fine-textured and low in nutrients. Fertiliser salts damage germinating seedlings, which live on the seed’s own reserves until they have true leaves.

Houseplants, general. A standard mix. For aroids — monstera, philodendron — add orchid bark and extra perlite; their roots want far more air than a bag mix provides.

Cactus and succulents. Roughly half standard mix, half grit, sand or pumice. The aim is a medium that dries out quickly.

Vegetables and tomatoes. Richer, with more compost, and a plan for feeding from week six onward. Container vegetables are heavy feeders in a small volume.

Ericaceous plants — blueberries, azaleas, camellias. A dedicated acidic mix. In an ordinary mix they go yellow between the veins and slowly fail, because they cannot take up iron at that pH. Related reading: lowering soil pH.

Orchids. Not a soil at all — bark chunks, and nothing that holds water around the roots.

Reading a bag honestly

Bag marketing is mostly noise, but a few things are real:

“Feeds for up to 6 months.” Usually a coated slow-release fertiliser, and usually genuine, but the release rate depends on temperature and watering. Treat it as a head start, not a season.
“Moisture control.” Extra water-holding material, sometimes a polymer. Useful for hanging baskets, actively unhelpful for succulents.
“Sterile.” Means it should be free of weed seed and pathogens, which matters for seed sowing.
Weight. A suspiciously heavy bag is usually heavy on soil or sand and light on structure. Good potting mix is light for its volume.

Gravel in the bottom of the pot makes drainage worse

This is the most persistent myth in container growing. A layer of gravel does not help water leave — it raises the saturated zone, because water will not move from fine material into coarse material until the fine layer is fully soaked. You end up with a waterlogged band sitting higher in the pot and less root room. Use the right mix throughout, and make sure the drainage hole is clear.

Reusing last year’s mix

Worth doing, with a refresh. Old mix has lost its structure, its air space and its nutrients, and it is full of the previous plant’s roots.

Tip it out and pull out old roots and debris.
Mix in about a third fresh compost or new potting mix to restore air space.
Add a slow-release feed, or worm castings and an organic fertiliser.
Break up any compacted lumps thoroughly.

Do not reuse mix from a plant that had root rot, a soil-borne disease, or a serious pest problem — put that on the compost heap or in the bin instead. And start seeds in fresh sterile mix rather than reused material, where damping off is a real risk.

Common questions

What is the difference between potting soil and garden soil?

Potting soil is a soilless blend of organic matter and aeration material, designed to drain and hold air inside a container. Garden soil is sold as a bulk amendment to dig into beds. Put garden soil in a pot and it compacts into a dense, airless block that drowns roots — the ingredients are chosen for completely different jobs.

Can you use garden soil in pots?

You should not. Real soil relies on the ground beneath it to drain; in a container there is nowhere for water to go, so it compacts, waterlogs, and starves roots of oxygen. It also brings weed seed, pests and disease indoors. If you must, use it as no more than a quarter of a mix with plenty of compost and perlite.

What is potting soil made of?

Usually a base of peat or coir for water retention, perlite or vermiculite for air space, compost or bark for structure and nutrients, a wetting agent so it rehydrates once dry, and a starter fertiliser lasting a few weeks. Specialist mixes vary the ratio — cactus mixes add grit, seed mixes drop the fertiliser and sieve finer.

Can you reuse potting soil?

Yes, for most plants, provided the previous occupant was healthy. Old mix loses structure and nutrients, so refresh it: pull out old roots, mix in about a third fresh compost or new mix to restore air space, and add a slow-release feed. Discard rather than reuse anything that grew a diseased or root-rotted plant.

Do you need special potting mix for seeds?

It helps. Seed-starting mix is sieved finer so it makes good contact with small seed, and it is deliberately low in nutrients, because fertiliser salts damage germinating seedlings that are still living on the seed's own reserves. Once seedlings have true leaves, pot them on into a normal mix.

How long does potting soil last in the bag?

Indefinitely as a growing medium if kept dry and sealed, but the starter fertiliser in it is exhausted or degraded within roughly a year, and damp bags left open grow fungus gnats and mould. Store it sealed and dry, and treat anything more than a season old as unfertilised — feed the plants yourself from the start.

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