Cheap drainage material for up-cycled oil drums
I'm trying to start a community garden on a local patch of land and we're thinking about using clean open-top oil drums as containers. I know we need to punch/drill holes for drainage in the bottom. Rather than completely fill them with compost (they're 45 gallon) I thought we could save money by filling the bottom half with something else e.g. polystyrene chips. Does anyone have any ideas about other materials we could use, either free or cheaply and readily available ?