First, there must be enough salt in that nation. Second, that salt must have not lost its savour. God is looking for enough salt that is savoured in a nation so He can spare that nation from destruction. Note what this salt is. Salt represents the righteousness of Christ.
Savoured salt represents personal righteousness.
For I bear them record that they have a zeal of God, but not according to knowledge. For they being ignorant of God's righteousness, and going about to establish their own righteousness, have not submitted themselves unto the righteousness of God. Abraham came to God and begged for the salvation of Sodom. His nephew, Lot, was there, and Abraham did not want Lot and his family destroyed. God agreed to do so.