Ha,
the answer is in your post. The
worms are what keep the soil open and airy.
I re-pot or up-pot as needed, replenishing the 'soil' as required. 'Soil' it mostly homemade compost, sand, pine fines, vermiculite, cow manure (sometimes goat manure) and some coir.
If plants are being grown for a free give-away I might also use the free potting soil from dumpster dives, like this excellent one-day haul.
This is commercial potting soil which contains vermiculite, perlite, slow-release fertilizer (and who knows what else
).
