Bava Batra, Chapter Two, Mishnah Eight
Mishnah eight deals with distancing a threshing floor from the city. A threshing floor was the place where farmers would bring their harvested wheat in order to separate the kernels of wheat from the chaff. A threshing floor would produce much waste, as the chaff is thrown to the wind. In order to prevent the chaff from polluting the city the mishnah discusses how far away the threshing floor must be kept.
1) A permanent threshing floor may not be made within fifty cubits of the town.
2) One may not make a permanent threshing floor within his own domain unless his ground extends fifty cubits in every direction.
a) And he must distance it from his fellows plants and ploughed land so that it will not cause damage.
Sections one and two state that a threshing floor may not be placed less than fifty cubits from another persons property or from the town. The reason for this prohibition was explained in the introduction.
Section 2a also restricts a person from placing a threshing floor close to his neighbors plants or ploughed land. Again, the chaff can be damaging to plants and can spoil ploughed land.