5b discusses a field in which a grave was lost or plowed over. Having worked on a farm many years ago the question arises in my mind about the effect of plowing a field in which the dead have been buried. I cannot see how the tumah of a kever could possibly dissipate thru plowing inasmuch as the depth of a plow would probably not exceed a foot deep. Were burials done differently in the time of the gemorah? Today burial is effected several feet under the surface of the ground and, therefore, it would appear to me that no plough would in any way disturb the grave per se.
Yitzchak, Thornhill, Canada
You are correct in what you surmise about the manner of burial in the times of the Gemara. TOSFOS (5b, DH Menape'ach) points out that they would not bury the body in the ground very deep at all (normally, a proper burial was done in a burial cave, but the body was first buried in the ground in order to decompose quickly, and thus it was not buried so deeply; alternatively, a body buried in a field was usually buried there temporarily, in order to protect it from Nochrim or the like). (Tosfos writes further that even in those days, when they did not bury the body deep into the ground, it should still have been deeper than the depth reached by the plow. Therefore, the concern that a bone from the body was plowed up is only d'Rabanan.)