Albert Barnes Commentary Judges 9:45

"And Abimelech fought against the city all that day; and he took the city, and slew the people that were therein: and he beat down the city, and sowed it with salt." — Judges 9:45 (ASV)

Sowed it with salt - Expressing by this action his hatred, and his wish, that when utterly destroyed as a city, it might not even be a fruitful field. Salt is the emblem of barrenness (see the marginal references).