Garden Soil vs Potting Soil: Why the Bags Differ

The garden soil against potting soil question sounds like marketing and it is not. They are different products for different physics, and the mistake in each direction costs money: garden soil in a pot kills the plant, and bagged potting compost in a raised bed empties your wallet. Here is what each one actually is.

The short answer

Potting compost is engineered to hold air and water at once in a small volume, with a few weeks of feed. Garden soil and topsoil are soil, which needs the ground beneath it to drain properly. So: potting compost in containers, never garden soil alone. For a raised bed, half topsoil and half compost bought loose, not bags. To improve an existing bed, compost rather than topsoil. Loam-based compost for anything staying more than a season.

What each bag actually is#

Multipurpose or potting compost

A blend of organic matter, usually coir, bark, wood fibre or peat, plus a few weeks of nutrients. Light, open, drains freely. Made for containers.

Loam or soil-based compost

The same plus sterilised loam, which adds weight, structure and nutrient holding. Better for anything staying in a pot more than one season.

Topsoil

Soil. Sand, silt, clay, some organic matter, no added feed. For filling holes, building levels and bulking out a raised bed.

Garden compost or well rotted manure

Decomposed organic matter. The best soil improver there is, and it feeds soil biology rather than the plant directly.

Ericaceous compost

Acidic, for blueberries, azaleas, camellias and rhododendrons. Only useful for those and their relatives.

Seed compost

Fine textured and low in nutrients, so seedlings are not burned. Worth buying if you sow a lot; multipurpose sieved is a fair substitute.

Why garden soil fails in a pot#

This is the bit that explains the whole distinction, and it is about drainage rather than nutrition.

In a bed, garden soil sits on more soil. Water moves down through it and keeps going, and air follows into the spaces left behind.

In a pot, there is nothing below except a hole. The fine particles in garden soil settle and pack, water collects in the lower part of the pot because it has nowhere to go, and the roots in that zone sit in saturated compost and suffocate. The plant looks like it is short of water when it is drowning.

Potting compost is built from coarser, more open material precisely so it holds both water and air in a small volume with a hard bottom.

Garden soil in a pot also brings you weed seed, slug eggs, vine weevil larvae and whatever fungal problems were in that bit of the garden.

Why potting compost is the wrong thing for a raised bed#

The other direction, and it is a money mistake rather than a plant one.

A raised bed 2 m by 1 m and 40 cm deep needs about 800 litres. In 50 litre bags of multipurpose that is sixteen bags, which is a startling amount of money for something that will sink by a fifth in the first year as the organic matter decomposes.

The standard answer is roughly half topsoil and half garden compost or well rotted manure, bought loose by the cubic metre from a soil supplier. It costs a fraction of bagged compost, it does not collapse the same way, and it improves every year as you top it up.

Filling a new raised bed

Half topsoil, half compost or manure, bought loose. Add grit if drainage is poor. Never all bagged potting compost.

Topping up a bed each year

Two or three centimetres of compost on the surface. The worms take it down. No digging.

Improving a poor existing bed

Compost, generously, on the surface. Adding topsoil adds volume without adding life.

Filling a hole or raising a level

Topsoil, which is what it is for.

Filling containers

Potting compost, and loam-based if the plant is staying more than a year.

Perlite, vermiculite and grit: when they earn their place#

These get bought far more often than they are needed.

Perlite

Adds air and drainage. Genuinely useful for cuttings, seed sowing and plants that resent wet feet. Not needed in general container growing.

Vermiculite

Holds water. Useful as a light covering on sown seed. Expensive and largely unnecessary elsewhere.

Horticultural grit

Adds weight and sharp drainage. The one I use most: for Mediterranean herbs, alpines, and anything on heavy soil.

Sand

Only sharp or horticultural sand. Builders' sand is too fine and will make drainage worse, not better.

For ordinary container growing, a decent compost in a big enough pot does what people buy perlite to achieve. The bigger pot is a better investment than the additive.

The peat question, stated plainly#

Peat based composts perform well and their extraction destroys carbon-storing bog habitat that took thousands of years to form. That is not a close call environmentally, and peat-free is now the default in many markets and legislated in some.

Peat-free composts vary considerably in quality, and the honest complaint about them is real: many are lighter, dry out faster, and go water-repellent sooner. Two practical adjustments deal with most of it.

Use a bigger container, because the failure mode of a light peat-free mix is drying out and volume fixes that. And mix in a third loam or garden compost, which restores the weight and water holding the mix lacks.

Reusing compost#

Container compost from last year is spent rather than useless.

Break it up, because it will have compacted. Mix in a third fresh compost or a slow release feed to replace the nutrients. Add a little grit if it feels dense.

Two exceptions. Do not reuse compost from a plant that had root disease or vine weevil grubs, and do not reuse it for seed sowing, where you want something clean and low in nutrients.

What it costs#

Multipurpose compost, 50 litres

$8 to $15. Fine for annuals and salad.

Loam-based compost, 35 litres

$12 to $22. Heavier, more expensive, and the right choice for anything permanent in a pot.

Topsoil, loose, one cubic metre

$50 to $110 delivered. That is a thousand litres, against roughly $200 for the same volume in bags.

Garden compost, loose, one cubic metre

$60 to $130 delivered, or free if you make your own.

Horticultural grit, 20 kg

$8 to $18. The additive most worth having.

Perlite, 10 litres

$8 to $16. Buy it when you are taking cuttings or sowing, not as a habit.

The one line version of each decision#

Key points

• Container: potting compost. Loam-based if the plant is staying more than a season.
• New raised bed: half topsoil, half compost, bought loose. Not bags.
• Improving an existing bed: compost on the surface, not topsoil, and no digging.
• Filling a hole or raising a level: topsoil.
• Seed sowing: seed compost, or multipurpose sieved.
• Blueberries, azaleas, camellias: ericaceous, and only those.

Where the guidance here comes from#

The drainage explanation is standard soil physics rather than my opinion, and it is the part that most product labelling leaves out. For compost choice, peat-free performance and bed filling ratios the RHS pages on soil, composts and mulches are the reference I check, and Garden Organic is the better source on peat-free alternatives and on making your own, which sidesteps most of this.

The short version#

Potting compost holds air and water in a small volume with a hard bottom; garden soil needs the ground under it. So compost in pots, never garden soil alone, and up to a third garden soil in a mix if you want the weight. Fill raised beds with loose topsoil and compost, not bags. Improve beds with compost, not topsoil. Buy grit, and mostly skip the perlite.

Related reading#

• Container gardening: pot volume, which matters more than which compost you chose.
• Mulching a garden: how compost improves a bed from the surface down.
• Composting for beginners: making the most useful of these products for nothing.
• Square foot gardening: why I reject the prescribed peat and vermiculite bed mix.