Potting soil and compost are not two versions of the same product. Potting mix (often sold as “potting soil”) is the whole root environment in a pot. Compost is decomposed organic matter, and extension services treat it as one ingredient of that mix. Garden soil is a third thing again, and it behaves badly in containers.

If you are about to repot, the short answer is: buy or make a potting mix, add compost only as a minority part, and leave garden soil outside.

Why a pot needs a different mix from the garden

Iowa State Extension explains the difference in terms of pores. Soil in the ground drains through a deep column. Dig it up and put it in a pot, and the shallow depth slows drainage. Many of the large air-holding pores are destroyed in the process.

Iowa State describes a good garden soil as roughly half pores and half solids. High-quality potting media are roughly 85% pore space and much lighter. That extra air space is what keeps container roots from suffocating after watering.

University of Maryland Extension makes the same point from the root’s side. Water moving through a pot pushes air out of the pores. If excess water cannot drain, fresh air cannot get back in and the roots suffocate.

Garden soil: why the sources say no

  • Iowa State: un-amended garden soil “becomes very compacted and poorly drained” in a container.
  • Illinois Extension: garden soil alone will not drain adequately in a pot, and it can carry disease and insect organisms. Use it only if sterilized and amended with peat moss, perlite or vermiculite.
  • UW–Madison: garden soils often contain root rot fungi. Use a pasteurized commercial potting mix instead.
  • University of Maryland: garden soils hold water in small pores and can drown roots, especially in shallow containers.

Illinois also warns about cheap bags labelled “top soil” that are largely sedge peat. They look good but are poorly drained and poorly aerated once potted.

Compost: useful, but as a minority ingredient

Compost has a place in houseplant mixes. Iowa State’s composting guide says it can be incorporated into potting mixes for houseplants. Mix it with perlite, vermiculite and other components so the final mix is about 30% compost.

Iowa State’s potting media article lists the trade-offs:

  • Compost provides a low dose of nutrition, but supplemental fertilizer will likely still be needed.
  • It varies with the source.
  • It is often sold before it is completely composted, and it keeps decomposing in the pot, often faster than peat moss.
  • It may carry weed seeds, bacteria and fungi, some helpful and some not.

University of Maryland Extension notes that compost contains the major and minor plant nutrients but releases them slowly. Many commercial soilless mixes already include composted bark or compost, so check the ingredient list before adding more.

Maryland’s guidance that plants can grow in 100% compost is written for outdoor container vegetables. None of the houseplant guidance we read recommends pure compost for indoor pots.

Signs your compost is finished

Iowa State describes finished compost as dark brown to black, crumbly and uniform, and pleasantly earthy-smelling. You should not be able to recognize the original materials. Leave anything that still smells sour or shows identifiable scraps on the pile.

What goes into a potting mix

ComponentWhat it does (Iowa State, Illinois, Maryland)Watch for
Peat mossHolds water; traditional base of most mixesBlack amendment-grade peat is too fine and holds too much water. Hard to rewet once dry
Coir (coconut fibre)Peat replacementAlso becomes hydrophobic if allowed to dry out
Composted barkAdds structure and air spaceUse processed potting bark, not bark mulch
PerliteLong-lasting air-filled poresDry perlite is dusty
VermiculiteHolds water and some nutrientsOften unnecessary with a good mix
Coarse sandAdds weight and some porosityUse coarse builder’s sand, never play sand or beach sand
CompostOrganic matter and slow-release nutrientsVariable; keep to a minority of the mix
Garden soilWeight; holds nutrients longerCompacts; may carry pests, weed seeds and root rot fungi

Mixes the extension services suggest

You do not need a recipe for most houseplants. Iowa State notes that most home gardeners use a pre-made all-purpose potting soil, with special mixes for orchids and succulents. If you want to mix your own:

  • General houseplant mix (Illinois Extension): one-third soil, one-third peat moss and one-third perlite or vermiculite. The soil should be sterilized.
  • Soil-based container mix (Illinois Extension): one part garden soil, one part peat moss and one part perlite or coarse builder’s sand.
  • Compost-based mix (Iowa State): compost blended with perlite, vermiculite and other components to about 30% compost.

Then match the texture to the plant. Illinois suggests coarser mixes with more bark, perlite or sand for succulents. For tropical foliage plants, use more peat and less coarse material. The RHS adds two refinements: a heavier loam-based compost for top-heavy plants, and grit or sharp sand where drainage matters.

If you would rather choose a ready-made mix by plant type than blend one, our guide to the best potting mix for indoor plants covers what each group needs.

A note on UK labels

In the UK, “compost” on a bag usually means potting compost. RHS advice uses the word that way, recommending peat-free compost for houseplants and loam-based compost for top-heavy plants. That bag is a complete growing medium.

Garden-bin or green-waste compost is something else: the decomposed organic matter discussed above. Read the intended use on the bag rather than relying on the word “compost”.

Preparing and using the mix

  1. Pre-wet soilless mixes. Maryland says soilless mixes tend to repel water when dry; work water in by hand until the mix is evenly damp. Illinois suggests fluffing it in a tub with water before planting.
  2. Use a pot with a drainage hole. Iowa State says every container must have one, or the soil stays wet and roots rot.
  3. Skip the gravel layer. UW–Madison says rocks or gravel in the bottom of a pot can inhibit drainage. Iowa State adds that it creates a wetter zone of soil near the roots.
  4. Go up one size. Iowa State says to move up one container size, maybe two. Illinois suggests a pot 2.5–5 cm (1–2 in) wider in diameter.
  5. Plant at the same depth and leave a gap below the rim for watering (Illinois, Iowa State).

Can you reuse old potting mix?

Sometimes. Illinois says soilless mix from healthy plants can be reused for a while. Its organic materials eventually break down until drainage and aeration are lost; then it goes to the compost pile or garden.

Never reuse mix from a plant that had root rot. UW–Madison advises against reusing houseplant potting mix or drainage water, because both can carry root rot fungi. If you see a white salt crust on the surface, Illinois suggests scraping off the top inch of old mix when you repot.

Compost as a top-up, not a fix

You can use compost lightly on an established plant. University of Maryland Extension suggests a small amount of well-composted, screened leaf mold or other compost as one way to supply micronutrients. It is not a remedy for a pot that stays wet. If the mix is compacted or roots are rotting, repot into fresh mix first. Our root rot guide covers the checks.

If white fuzz or fungus appears on the surface after repotting, see mold on houseplant soil.

Compost, potting mix and fertilizer together

Count every nutrient source before you feed:

  • Iowa State notes most potting soil contains a small amount of fertilizer. Wait at least 2–4 weeks after repotting before regular feeding.
  • Illinois says soilless mixes have very little nutritional value, so proper fertilizing matters once that starter charge is used.
  • Compost adds a slow, variable supply on top.

For products and strengths, see our plant food guide. For how a mix holds water and air, see soil science for houseplants.

References