Duncan writes
I wonder if anyone has any views, or even knowledge of research, on whether
there is a certain critical mass / volume of planting container above which
it becomes pointless to use man-made potting mixtures and feasible to use
good old garden topsoil; or a minimum volume below which soil becomes
unuseable? I ask because I have a planting area which is entirely
self-contained (solid concrete 2ft down, enclosed by brick walls) containing
soil which seems to have worked fine for the last 3 years (with garden
compost and seaweed added), and also have a big container which is going to
need about 6 sacks of John Innes or whatever to fill it - can I use soil,
keep it watered and put up with weeds? If so, would you add anything to stop
it drying out and/or setting solid?

Totally unscientific, but if the planter is more than about 6 inches
deep I either use garden compost on the bottom or garden compost for the
whole thing, depending on the robustness of the plants I am going to put
in it.
--
Kay