earth worms vs vegetables

thanks for all the replies,
we've still got 2 more boxes to fill, but i won't bother laying a weed block membrane down first.
think i was being a bit paranoid about the worms contaminating my premium top soil with clay   :blink:
the first box has been getting a lot of worms added as we dig them up around the garden so they will do their magic in there

ps oliver, our first box is 40cm deep, but i'd still be delighted with a parsnip half that size !