Using Old Bathtub for a Herb Garden Container

I have an old, cast iron bathtub which I would like to use to grow a herb garden. How should I prepare it for my plants?

It would be best if you could bury the tub up to its rim, so that plants could survive in it for the winter, if they are rated to be hardy in your area. Otherwise, only plants that are rated for two zones colder than your zone will survive in your tub over the winter.
If you do bury it, make sure there is plenty of gravel under the tub for good drainage.
Also, in the fall mulch the herbs in the tub and the edge of the tub, to slow down the loss of heat from the contents of the tub.
Put a six inch (15 cm) layer of gravel in the bottom of the tub and then a layer of fine gravel and then a good sandy soil.