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Garden soil in pots

Also known as Topsoil in containers, Native soil

Growing mediumDepends
Potting soil with perlite in a trowel beside a pile of garden soil Photo: Neslihan Gunaydin · CC0

In short

Garden soil works well in the ground but usually badly in pots. In a container it compacts, holds too much water in its small pores, loses air and can drown roots, so extension services recommend soilless potting mixes instead.

How it looks

Garden or topsoil is a mix of mineral sand, silt and clay with organic matter; it is heavy, may contain stones, roots, weed seeds and soil animals, and often feels sticky when wet. Bagged potting mix, by contrast, is light and fluffy, usually with visible perlite, bark or coir.

Signs

Soil in pots sets hard, cracks or pulls away from the pot walls, drains slowly, and stays wet at the bottom; plants may wilt despite wet soil, show yellowing or develop root rot. Weeds, fungus gnats or other soil pests may appear.

What favours it

Soils compact and lose pore space after repeated watering in containers and can be too heavy. Because a pot is shallow, water collects at the bottom (a perched water table), which is worse with fine, dense soil.

Prevention

  • Use a soilless potting mix for containers.
  • If you add topsoil, keep it to a small share; University of Maryland Extension suggests no more than 10% of the volume in very large containers.
  • If using native soil indoors, Clemson Extension advises pasteurising it by heating until the coolest part reaches 180 °F (82 °C) for 30 minutes.
  • Loam-based composts such as John Innes use sterilised loam balanced with peat and sharp sand.

Treatment

Start with the first step and move down only if the problem persists.

  1. Repot plants that are struggling in garden soil into a fresh, well-draining potting mix.
  2. Do not add gravel or crocks to the bottom to fix drainage; this raises the wet zone rather than removing it.

Pros and cons

Pros

  • Free and heavy, which helps stabilise large pots.
  • Contains minerals and soil life; loam is the base of the long-established John Innes composts.

Cons

  • Compacts and holds too much water, starving roots of air.
  • Heavy to move.
  • May carry weed seeds, pests and diseases.
  • Variable pH and nutrients.

Look-alikes

Compost
Compost is organic and lighter, without the sand, silt and clay of soil.
Potting mix
Potting mix is light, soilless and made to drain in containers.

Typical uses

All container plants; the problem is worst in small or shallow pots and for plants that need sharp drainage.

Sources

  1. Growing Media (Potting Soil) for Containers — University of Maryland Extension
  2. Potting soil — Wikipedia
  3. Indoor Plants – Soil Mixes — Clemson Home & Garden Information Center
  4. Gardening myths — University of Missouri Integrated Pest Management
  5. John Innes compost — Wikipedia

Written in our own words from these sources. Check a product label before using any treatment.