Hence the seamy side of a city, etc.'Custom is second nature' wrote Plutarch in the 1st century AD.
This had been translated into English by the 14th century, and from this idea that something you habitually do becomes so much a part of you that it is indistinguishable from nature, comes our use of second nature.
Coined by Shakespeare, whose Cleopatra speaks of 'My salad days, When I was green in judgement...' in Antony and Cleopatra, I, 5, lines 73-4.
Both bacon and back are related to the same Old Teutonic word, and this is more likely to account for the expression than the burning of heretics or the preservation of meat from hungry dogs during winter in the days before refrigerators, as some have suggested.
The river was the Mississippi, and 'down' implied the transfer of slaves from north to south: there was a slave-market near the mouth of the river at New Orleans, among others.
Such transfers took place either because of increased or seasonal demand in the cotton or sugar-fields of the south, as distinct from the more northerly tobacco-plantations, or because the harsher slave-owners of the deeper-south were not choosy about accepting - and dealing with - troublesome slaves whom northern owners wished to offload, or domestic ones who could be turned into profit.
The expression comes from the Bible, where Jesus describes his disciples as 'the salt of the earth [i.e. He meant something different, however: salt has preserving and purifying qualities (newborn babies in the east were rubbed with salt to promote health) and so the disciples were being described as the agent by which mortal souls were to be purified and preserved.
The modern meaning, though related, is untheological.