Hi, maybe not exactly the info you're looking for but the RHS magazine 'The Garden' had a really interesting feature in it recently about a study of plants experimentally grown on various diferent piles of 'waste material', ie broken bricks, cement etc. Lots of things will thrive apparently! It was one of the later issues of last year, maybe September or October. Perhaps you might be able to chase this up and work out a way to not use any top soil at all!

Cheers, Bob