Potting Mix Basics

Understand why container crops need a clean, airy growing medium rather than dense garden soil alone.

Why container mix is not soil

Potting mix is built to hold moisture while keeping air spaces open and letting excess water leave. Garden soil is built to work in the ground, where structure, organisms, and drainage extend far beyond the pot. In a container, soil alone compacts, holds water at the base, and suffocates roots.

What a good mix does

It should feel light and springy, wet through evenly rather than shedding water, and drain freely within minutes. Most commercial mixes combine a moisture-holding component with something coarse to preserve air. You are looking for structure more than fertility, because you can add nutrients later but you cannot easily fix compaction.

Check whether it is already fertilised

Many mixes include a starter charge or slow-release fertiliser, which typically feeds a plant for the first few weeks. Feeding on top of that early on is unnecessary and can scorch roots and leaf edges. Read the bag before you add anything.

Reusing old mix

Old mix is usually depleted, often compacted, and may be full of the previous plant's roots. It can also carry over a pest or disease problem. Assess it rather than assuming: if it smells sour, is packed hard, or came from a plant that struggled, start fresh. Otherwise loosen it, mix in a good proportion of new material, and use it for something undemanding.

Special cases

Seedlings and microgreens do better in a finer, cleaner medium than mature plants. Mediterranean herbs such as rosemary, thyme, and oregano want sharper drainage than a standard mix provides, so adding coarse material is worthwhile. Moisture-loving crops go the other way.

Common mistakes

Filling a pot with garden soil is the big one. Putting gravel in the base is a close second: it does not improve drainage and it reduces the usable volume. Letting mix dry to the point where it shrinks from the pot wall is the third, because water then runs down the gap and out without wetting anything.