Potting Mix and Garden Soil Are Not Interchangeable

They are not the same product

Garden soil in a container fails, reliably, and the reason is physics rather than fertility.

Soil is dense, with small pore spaces. In the ground that works, because water drains away through a soil column many feet deep. In a pot, that column ends abruptly at the base, and water is held in the small pores rather than draining. The result is a saturated zone at the bottom of the container and roots without oxygen.

University of Maryland Extension puts it plainly: soils are too dense to allow good air and water movement in containers, hold water in their small pore spaces, and can drown roots — especially in shallow pots.

What potting mix actually is

Most potting mixes contain no soil at all. Maryland lists typical ingredients: sphagnum peat moss, perlite, vermiculite, composted bark, compost and coconut coir.

Those materials produce large pore spaces, so water drains freely while the organic fraction holds enough moisture for roots to use. Maryland describes the result as lightweight, free-draining, water- and nutrient-holding, and generally free of weeds, insects and disease.

That last point matters more than people expect. Garden soil brings weed seeds, insect eggs and soil pathogens into a container where the plant has no room to escape them.

Reading the bag

Garden centres sell several similar-looking products with quite different purposes.

• Potting mix / potting compost — for containers. This is what you want for pots.
• Seed-starting mix — finer, lower in nutrients, for germination.
• Garden soil / topsoil / soil improver — for the ground. Not for containers, despite often sitting on the same shelf.
• Ericaceous mix — acidified, for blueberries, camellias and rhododendrons.

The words are not regulated tightly, so read what the bag says it is for rather than trusting the name.

The drainage layer question

A layer of gravel or crocks in the base of a pot is traditional and is usually described as improving drainage.

The conventional explanation is that it does the opposite — water does not readily cross the boundary between fine and coarse material, so it perches above the gravel and raises the saturated zone. A 2025 study in PLoS ONE tested this directly and found that coarse layers usually reduced water retention, though it worked with saturated containers and did not measure plant growth.

The claim is contested. What is not contested is that a drainage hole is essential, and no material placed in the bottom of a pot substitutes for one.

When garden soil is acceptable

In large raised beds sitting on open ground, ordinary soil improved with compost is normal and correct — a raised bed is still connected to the soil profile beneath.

The failure case is a closed container. The smaller and shallower the pot, the more the difference matters.