Want to transfer a few plants to big garden pots to keep them there for the
long term.

I was told a while ago that the composts you buy from garden centres do not
contain all the minerals found in ordinary garden soil, so even if you feed
the plants regularly they are still not getting all the necessary minerals.

Would this apply to a compost like John Innes No 3 ? Or should I mix the
John Innes half and half with ordinary garden soil? Novice grateful for
any advice.