Ohalos, Chapter 18, Mishnah 2. Our discussion until this point has revolved around the beis haperas, a field whose soil must be treated with suspicion because a plow once passed through a kever there and splinters of bone may now lie anywhere in it. The Mishnah before us introduces a fresh idea: the term beis haperas covers more than one situation. Several varieties exist, and their halachos are not identical.
The wording is "Shlosha beis prasos hein": three distinct kinds of beis haperas exist (beis prasos being the plural form). The one named first is "hachoreish es hakever", ground in which a person plowed over a grave. That is the very scenario we have been occupied with, where we fear that a chip of bone was thrown up out of the kever and scattered across the field. The Bartenura previews the other two, both of which the Mishnah itself will detail further on: a field containing a grave whose location has been forgotten, and a sdei bochim, ground where people would assemble to cry over the meis and to hear the hesped delivered. Only the first of the trio is treated here.
What may be planted there
Concerning ground in which a grave was plowed, the ruling is "nita'as kol neta": trees of every variety may be planted in it. A tree stays rooted where it is, so nothing leaves the field. On the other hand, "ve'eino nizra'as kol zera": not every crop may be sown there. The reason is that the usual way of gathering such crops is to draw them up out of the ground, and whatever is drawn up brings its roots along, and clinging to those roots is soil from that very field. Vegetables illustrate the point perfectly. Consider what such a person accomplishes: he takes earth from a beis haperas and moves it elsewhere, and he may thereby be relocating tumah and spreading it to new places.
Hence the qualification "chutz mizera niktzar": a crop that is reaped with a blade is excluded from the prohibition. When the plant is severed rather than yanked up, not a particle of the field's earth departs with it. The Rosh points to wheat and barley: at harvest one cuts the stalk while the roots remain below.
A difficulty from the earlier Mishnah
A problem presents itself immediately. The Mishnah preceding this one recorded Rebbi Yosi's ruling that a vineyard may not be planted in a beis haperas, together with a penalty imposed on someone who planted deliberately. If so, how can we now be told that trees of every variety are permitted? The Bartenura suggests two resolutions. One: our Mishnah may not be following Rebbi Yosi's view at all, and in his opinion no such blanket permission exists. Two: Rebbi Yosi's stringency may have applied to a vineyard alone, on account of the rule habotzer lagas huchshar, that grapes gathered for delivery to the winepress are treated as having been rendered susceptible to tumah, as though liquid had already come upon them. Regarding other trees, to which that added severity in tumah does not extend, Rebbi Yosi may well have been lenient.
When the grain came up with its roots
Picture a man who sowed wheat in such a field, as he was fully entitled to do, wheat being reaped by cutting rather than by hauling the plant from the soil. This time, though, the stalk was not cut: he tore it out, and up came the root with earth stuck to it. Conceivably an etzem kisse'orah, a fragment of bone no larger than a barley kernel, is lodged in that clod. What is he told to do?
The Mishnah answers "tzover es garno lesocho": his threshing floor is to be established within the field. Tzover carries the sense of heaping together, related to the word tzibbur. The grain stays where it grew; the heap is made on the spot, and according to the Bartenura the threshing is performed there as well. He may not haul it to some other field for threshing, since the whole object is to keep this tumah from traveling.
Once threshed: "vechover bishtei chavaros", the grain is passed through a sifter twice, and only afterward may it leave the field. Two chavaros means one sifting followed by a second. The Tiferes Yisroel describes how this works: a sieve with wide openings comes first, clearing away the coarser matter, and then a sieve with narrow openings removes what is finer. Both stages are obligatory. "Divrei Rebbi Meir", such is Rebbi Meir's position.
"Vachachomim omrim": the Chachomim distinguish according to what is being cleaned. Hatevuah, grain of the wheat and barley sort, is sufficiently cleaned by bishtei chavaros, two passes. Kitniyos, however, the legumes such as beans, are not: "beshalosh chavaros", they demand a third pass before anything may be taken out of the beis haperas. Legumes emerge from the ground with considerably more soil adhering to them than grain does.
The Bartenura notes something essential here: the crop itself has no tumah. Nothing ever made it susceptible, no liquid ever came upon it, so it cannot contract tumah in the first place. The sole worry throughout is that soil belonging to this field should not end up somewhere else.
Burning the straw
What becomes of the straw? "Vesoref es hakash ve'es ha'otzah": it is put to the fire. Kash denotes what remains of grain stalks, while otzah is the corresponding refuse of the kitniyos. The Tosafos Yom Tov explains why fire is the remedy in this case: sifting straw cannot realistically be done, because threshing leaves it a jumbled tangle. At the same time we cannot let him remove it, since soil from the ground is certain to be caught somewhere in the mass. Burning is what remains.
The category of this tumah
The closing words define the level involved: "umetamei bemaga uvemasa ve'eino metamei be'ohel". In a beis haperas of this first kind, where a grave was plowed, tumah passes to a person who touches the earth and to one who carries it, even carrying it without any bodily contact, yet it does not pass by way of ohel, whether the earth is above him or he is above it.
Why so? We return to a teaching from the early part of Masechtas Ohalos, Perek Beis, Mishnah Gimmel: "Eilu metamin bemaga uvemasa ve'einan metamin be'ohel", the enumeration of those things that transmit tumah by contact and by carrying while not transmitting through ohel. The item opening that list was the etzem kisse'orah, a barley kernel's worth of bone. Naturally, then, in ground where a kever was plowed, where the fear from beginning to end is a splinter of precisely that measure, the tumah operates by the same rules: maga and masa apply, ohel does not. The soil of such a field is governed exactly as is the bone we suspect is buried in it.
Summary
Three categories of beis haperas are recognized: ground in which a grave was plowed, ground in which a grave was lost, and a sdei bochim where people assembled to eulogize the meis. Our Mishnah addresses the first alone. There one may plant any tree, since it will presumably remain in place forever, but not the ordinary sown crops, which come up together with their roots and with soil that may hold an etzem kisse'orah. Crops reaped by cutting, wheat and barley among them, are permitted. Should a stalk nevertheless be pulled up root and all, the produce stays put: the threshing floor is arranged and the threshing carried out inside the field. Rebbi Meir demands two siftings; the Chachomim demand two for grain and three for legumes, which cling to more earth. The straw, both the kash of grain and the otzah of legumes, is impossible to sift and is therefore burned. And because everything here hinges on an etzem kisse'orah, the tumah takes effect through maga and masa but not through ohel.