as it happens mo just came a cross this resently:::
Salt of the earth
The exact meaning of the expression salt of the earth is disputed, in part because salt had a wide number of uses in the ancient world. There are several different possibilities for the originally intended meaning of the salt metaphor:
Exodus, Ezekiel, and Kings present salt as a purifying agent
Leviticus, Numbers, and Chronicles present it as a sign of God's covenant.
The most important use of salt was as a preservative[dubious – discuss] and hence the most common interpretation of the metaphor is as asserting the duty to preserve the purity of the world.
In the Rabbinic literature of the period salt was a metaphor for wisdom.
Salt was a minor but essential ingredient in fertilizer and so a few scholars such as Gundry believe that earth should be translated as soil (i.e. salt of the soil), and hence the metaphor asserts that the audience should help the world grow and prosper.
One interpretation of salt of the earth is that it orders the audience to take part in the world rather than withdraw from it
Among the ancient Hebrews salt...was used as a preservative, in seasoning food, and in all animal sacrifices. Lev. 2:13; Ezek. 43:24; Mark 9:49–50. So essential was it to the sacrificial ordinance that it was the symbol of the covenant made between God and His people in connection with that sacred performance. Lev. 2:13; Num. 18:19; 2 Chron. 13:5. 
From stone tablets to html code, it's not lost on me.