Fallow was the condition of farmland allowed to rest in between plantings.
In 2375, "Anjohl Tennan" told Winn Adami that the cure for Rogath blight was to burn contaminated crops at the first sign of the blight, then allow the land to lie fallow for a year, which would restore the land's fertility. As Winn described it, "The poison must be purged away so the land can be reborn." (DS9: "'Til Death Do Us Part")
In ST: "The Girl Who Made the Stars", the failure of the first people to reliably grow crops was said to be due to always farming in the same location, ergo, not allowing to ground to lie fallow.