Remove roots and big lumps, loosen it up, and add fresh perlite if it has compacted. Used soil is usually low in nutrients, so mix in compost or worm castings, or a dedicated amendment, and let it sit moist for a week or two so the biology settles. Check pH and EC before the next run.
Living soil is built for this and gets better with every cycle. See our living soils, for example Monster Living Soil Concentrate 50 L for refreshing an existing mix. Soil with fungus gnats, root aphids or root rot is not worth the risk.
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