Your clay substrate sounds a lot like Florite. I have a tank that is
sand with laterite and tanks that are florite with a thin layer of
sand on top. Why the sand on top? I had trouble getting some plants
to stay down in the florite when I first planted it and the sand
helped make them stay down. I only used only about 1/2 inch sand.

The sand only approach has it's positives and negatives. I think it
is much easier to vacuum and plant in. However, since you have to add
laterite and/or root tabs, things become more difficult in the long
run. Laterite is messy when (not if) it gets uncovered from the sand
like when you're replanting or aerating it (you can't completely rely
on snails for aerating). I can never remember where I've put the root
tabs and I seem to need more and more all the time. It can get
expensive in the long run.

In the end I prefer the florite to sand. You should definitely plan
for the long term. If you enjoy keeping a planted aquarium you will
regret not getting the best from the start. It is a *lot* of trouble
to change your mind.

Cris