Can used potting mix be reused?

Used mix is not automatically worthless, but it is not automatically equivalent to a fresh bag either. Inspect it before planning the next planting.

Ask what happened last time

Was the plant healthy? Were there persistent pests, disease, salt problems, or unexplained root decline? Remove large roots and debris, then assess whether the medium remains open or has become dense and broken down.

Reusing suspect material for delicate seedlings or new indoor plants is a poor experiment. Disease and pest risks require appropriate guidance; simply adding fresh mix does not guarantee that the old problem disappears.

Choose a realistic next use

Healthy, still-usable medium may form part of a refreshed blend for suitable established plants, with nutrition assessed rather than guessed. Some material can have an appropriate role in garden improvement, depending on ingredients and local conditions. Do not dump it into natural areas or drains.

Avoid a universal rule such as replacing exactly half every year. The starting mix, plant, and growing conditions differ. Keep the history with the material, and use fresh appropriate medium when the uncertainty outweighs the saving. Reuse is most valuable when it preserves a workable growing environment, not when it transfers an unresolved problem into the next pot.