"Gill Smith" wrote ...

my garden contains raised beds of heavy clay

(possibly excavated 5 yards down from a motorway!)

the native soil is friable black stuff

but

doesn't that make clay soil packed full of goodies for plants

(images come to mind of Chinese farmers ploughing amid clouds of sand,

the leftovers of soil exhausted by millennia of cultivation)

Clay subsoil is not fertile unless you have been very lucky. The reason that
clay is good is that it holds on the nutrients and water put into it unlike
a sandy soil where it simply washes through.
Sounds like your natural top soil is wonderful stuff.

--
Regards
Bob Hobden
W.of London. UK