I have a lot of plants in Pots in a very small garden, and like to move them
around quite a bit according to whats in flower.

I like to have a free *draining* soil in the pots, but adding sand ends up
making them very heavy; especially the terracotta ones.

Is there anything I can use to 'add' to the soil to improve drainage, that
will not result in a increase in weight? Thanks.