The phrase 'salting the earth' used to refer to an action which is needlessly vengeful... I believe that it dates back to ancient times when armies would salt the earth of conquered cities to prevent crops from growing there again (along with other things such as burning, pillaging, raping, sewing seeds of thorny plants (Thanks Dan!)).

However, it seems unlikely to me as salt is somewhat valuable and I'm not sure where they would have obtained such large quantities of the stuff to actually poison the soil.

I would like to know how common the practice was in ancient times and why it was practiced?