Thought I would add my twopennyworth:

I live in near Monmouth where there is about 12" of soil/subsoil and
then solid red clay! A couple of years ago I planted a pieris in a
border with no thought as to its needs. My neighbour soon pointed out
that the one he had was not thriving and it needed an ericaceous
compost.

After a year of it looking ever more sad, I came across a length of
corrugated drainage piping 2 feet in diameter. Cutting an 18" length, I
let this into the ground (whew, that's a lotta clay) and filled it with
suitable compost. It is now flourishing brilliantly and the size of the
tube means I don't have to water (rainwater) unless there has been a
serious drought.

--
Zak