Ohalos, Chapter 18, Mishnah 3. The closing chapter of the mesechta deals with beis hapras, land whose status is uncertain because of graves, and it works its way through the different kinds of beis hapras one at a time. The previous Mishnah dealt with a field in which a kever was plowed over, so that bones may have been dragged and scattered through the soil. Now the Mishnah turns to a second type: "sadeh she'ovad kever besocha", a field in which a kever was lost. Here nobody scattered anything. We know with certainty that there is a kever somewhere in this field; we simply no longer know where it is.
Planting Seeds: Two Versions
Regarding the sowing of seeds, our Mishnah rules in the opposite direction from what was taught about the field in which a kever was plowed over. In that case trees were permitted while most vegetables were not, because vegetables come out of the ground with clumps of earth stuck to their roots, and a bone the size of a barley grain could easily be lifted out along with them. Here the Mishnah says "nizra'as kol zera": every variety of seed is permitted, vegetables no less than anything else. There is no worry that soil will be pulled up with something in it.
What accounts for the leniency? The Rambam explains that a grave which has not been disturbed sits below the depth of three tefachim, and the roots of such plants simply do not descend to that level. One could object: the very reason the location is unknown is that the ground was worked over, so perhaps the plow did reach the grave itself. The response is that two separate doubts are in play here. It is possible that the plow never touched the kever; and even granting that it did, there is no assurance that pulling up a vegetable would bring an etzem kisa'orah along with the root. Two doubts together leave room to rule leniently.
The Tosefta, however, has a different girsa. There the text reads "einan nizra'as kol zera": nothing at all may be planted here. According to that version we are not being more lenient than in the previous Mishnah but actually more machmir. Regarding the plowed field the ruling permitted at least those crops that are cut rather than uprooted; here, according to the Tosefta, even those are forbidden.
What stands behind such stringency? We do not want people wandering into this field at all, because, as the Mishnah goes on to teach, the tumah of this type of beis hapras is more severe: here there is even tumas ohel.
Let us gather the two approaches to sowing in this kind of beis hapras. The reading in our Mishnah permits every sort of seed. First, even a plant that comes up with earth attached has roots that do not normally descend to the depth at which a grave lies. Second, it is not established that this ground was ever plowed across the grave. Third, even granting that it was, it remains doubtful whether a barley-grain-sized bone would be lifted out with the plant. All three points distinguish this case from the field of the previous Mishnah, where the plowing over the grave is something we know happened. The Tosefta's reading, by contrast, bars every seed, including the crops that are harvested with a blade, since the entire objective is to prevent anyone from setting foot on this ground.
Trees: Not Only No Planting, but No Keeping
When it comes to trees, this field is unquestionably treated more stringently than the plowed-over field, and here there is no dispute in the girsa. The Mishnah states "ve'eina nita'as kol neta": no tree may be planted in it. And the Mishnah goes further: not only may one not plant a tree here, but a tree that is already standing must be removed. "Ve'ein mekaymin ba ilanos", existing trees are not allowed to remain, "chutz me'ilan srak", with the exception of an ilan srak. An ilan srak is a tree that does not produce fruit.
The Severity of the Tumah Here
The concern in this field is that it certainly contains a kever, we do not know its location, and there is a strong likelihood that the kever is still intact. Therefore the Mishnah teaches "u'metamei be'maga u'bemasa u've'ohel". That it is metamei be'maga, through touching the earth, and be'masa, through carrying it, was already true of the field in the previous Mishnah. What is added here is be'ohel, that it conveys tumah by overhanging as well.
Why is it metamei through touch and through carrying? Because there is a slight possibility that the grave was plowed over and that bones are lying scattered here and there in the soil. And why is it metamei be'ohel? Because we do not know where the kever is, so it may be that an entire intact kever is lying here, and wherever a person walks in this field he may be passing directly over it, forming an ohel above the corpse.
Keeping People Away
It is the far greater severity of the tumah here, tumah be'ohel included, that drives the halachah to keep this field empty of visitors. Nothing should be left that would draw a person to this ground. That accounts for the ban on fruit trees, and, on the Tosefta's reading, on crops as well. The issue is not merely that a plant might come up with soil clinging to it; the deeper concern is that people should have no occasion to enter here at all.
Where there is no fruit and no food to be had, the chances are that people will stay away. That is why the ilan srak is the exception. According to most shittos it may even be planted here to begin with, since it draws no one. Some understand the words "chutz me'ilan srak" more narrowly: an ilan srak that is already standing may be left in place, but lechatchilah one should not plant even that kind of tree in such a field.