In article ,
Duncan wrote:
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?

There is no specific volume, and it depends on your soil. My soil
is 60% sand, 18% silt and 22% clay, so is a first-class basis for
a John Innes compost. My standard one is two buckets of soil, one
of garden compost and half a bucket of old soilless (commercial)
compost. If I have none of the last, I up the garden compost.

Compost is as compost does.


Regards,
Nick Maclaren.