Salting the earth
Salting the earth refers to the practice of spreading salt on fields to make them incapable of being used for crop-growing. This was supposedly done in ancient times at the end of some wars as an extremely punitive area denial measure.
Perhaps the most famous example of salting the earth supposedly occurred at the end of the Third Punic War in 146 BC between the Romans and Carthage. After sacking the city of Carthage and forcing the few survivors into slavery, an area 50 miles around the city was salted. It is an open issue as to whether this event actually occurred. Aside from the logistics involved (it would require a tremendous amount of salt, very precious at the time) the first reference to the event is found in the Middle Ages, thus making it possible that the story was a later fabrication.
Today the term is used in a variety of ways, referring in general to any sort of poisoning. This varies from the direct in the use of area denial or radiological weapons, to the philosophical, where it is often used to describe business strategy to avoid takeovers (similar but broader in scope than a poison pill).
For a more detailed explination of why salt kills/discourages plant life, see (edwardgoldsmith.com).
