On Jan 24, 7:59*pm, "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)

--http://www.gillsmith999.plus.com/

Your native soils is exactly the same as what's underneath plus
organic matter.
As for China, it's nothing to do with cultivation. It's a geological
phenomenum.
http://en.wikipedia.org/wiki/Loess#Sites