Sorry for replying to self, but I just found one of my potato day
notes which will help here...

At the Q&A panel, someone asked what kind of medium do they recommend
for growing potatoes in containers, with a view to growing Mimi. The
panel said 1/3 leafmould, 1/3 compost and 1/3 loam is what they use.
The initial thought is that commercial compost is too fine to hold on
to enough moisture.

Also a lot of containers such as potato barrels don't work, especially
those in which you plant several layers of tubers. If you keep adding
compost to a deep barrel as the plant grows, this causes severe
compaction of the lower layers and this stops oxygen from reaching the
roots. They said one should add perlite to open up the structure, or
use 12/14" pots and have only one layer of seed potatoes. Don't bury
too deep or the yield will be disappointing.


--
jane

Don't part with your illusions. When they are gone,
you may still exist but you have ceased to live.
Mark Twain

Please remove onmaps from replies, thanks!