Plowing over a grave creates what’s called a beis hapras (a plowed field that may or may not contain scattered pieces of bone; such a field is of doubtful ritual purity/impurity). Such a field is rendered a beis hapras a furrow’s length, which is 100 cubits (about 150’), an area in which four seah (about 14 gallons of seed) can be planted. Rabbi Yosi says an area of five seah (17.5 gallons of seed). On a downward or upward slope one puts a quarter-kav of vicia seed (about a pint) on the knee of the plow and the place where three vetches will grow next to one another is the end of the beis hapras. Rabbi Yosi says that a beis hapras can be made by a plow going downwards but not by one going upwards.
If a person was plowing and he struck a rock or a fence, or if he shook off the plowshare, he only makes a beis hapras up to that spot. Rabbi Eliezer says that one beis hapras can create another beis hapras; Rabbi Yehoshua says sometimes it can and sometimes it can’t, as follows: If one plowed half a furrow and then later plowed another half-furrow, or if he plowed on the sides, then he creates a beis hapras. If he plowed an entire furrow and later continued plowing from that point on, he does not create a beis hapras.