On Thu, 9 Aug 2012 15:12:21 +1000, David Hare-Scott wrote:
What you propose will create an interface between top soil and
subsoil. Many plant roots will not cross that interface.


OK but it is a flower garden this may not be a problem.

Yes, it can still be a problem. For instance, all the fertilizers and
all the minerals in your water that you dump
on the flower bed over the years -- the remains -- may become more
concentrated because they can't move past that X inches of "good soil",
and then you get plants in a saline soil, which collapsse and die.

What you're proposing makes a sort of giant flower pot. With a real
flower pot, you can tip it over at the end of a season, dump out the old
and replace the soil fairly readily. That's a whole lot more work with
a bed in the garden -- as much or more than you started the project with.


http://septictankinfo.com/Gayman_Clay.JPG is a micrograph of clay particles;
they are flat plates that tend to want to stack or shingle over each other,
forming water-impenetrable layers, particularly if there's enough sodium
in the soil or in what you add.

Treat clay soils with respect... they're difficult to grow on.

Kay