Sheviis, Chapter 4, Mishnah 2. Our mishnah deals with a field in which forbidden labors were performed during the seventh year, and with the question of whether it may be sown after the year ends - and then with the law of eating the produce of that field and the question of expressing gratitude to its owner.
"Sadeh shenitkavtzah":
A field from which the thorns were removed during Shemittah, where the work was done for the sake of the year following the seventh year - so that the field could be used after it. Even though it was not permitted to perform this labor, the mishnah rules: "tizara b'motza'ei sheviis" - it is permitted to sow the field once the seventh year has ended, despite the work having been done during it.
"Sadeh shenitaivah":
That is, a field that was thoroughly plowed - and this refers to an extremely thorough plowing. To understand the law, we must first note that our mishnah speaks of a time of danger, when the government demanded a certain sum of tax from every single field. Because of the danger that threatened the Jews, they were permitted to plow their fields even during Shemittah, so that they could produce the crop needed to pay the tax. However, this permission was granted for one plowing only; a second plowing, which is the thorough working of the soil, was not permitted, since it was not needed for the payment of the tax.
Hence the two cases in the mishnah:
"Shenitaivah" - one who plowed a second time even in the time of danger, when only one plowing was permitted to him.
"O shenidayrah" - one who fertilized his field by setting up pens for animals in a manner that was not permitted. As was explained earlier in the tractate, the method of pens is based on the animals producing the manure, but it was permitted only in certain ways, and here the person acted in a manner that is not permitted according to halachah.
In both of these cases - whether he plowed twice or fertilized in a forbidden manner - the law is "lo tizara b'motza'ei sheviis", and he may not sow this field after the seventh year has ended.
Eating its produce during the seventh year:
The mishnah then returns to the field that was thoroughly plowed, and discusses its status during the Shemittah year itself rather than afterward:
Beis Hillel say: "ein ochlin peiroseha basheviis" - one may not eat the produce of that field during Shemittah.
Beis Shammai say: "ochlin" - it is permitted to eat the produce during the seventh year.
The Rosh explains that this refers to fruit of trees, and with them eating is permitted during the seventh year because fruit trees produce their fruit in any case. Therefore, even though an additional plowing was done in the field, since this is fruit of trees - it may be eaten.
Expressing gratitude to the owner of the field:
From here the mishnah moves to another dispute between Beis Shammai and Beis Hillel: a field that produces on its own, whether vegetables or fruit and the like, and everyone is permitted to eat from it - may those who eat express gratitude to the owner of the field and thank him?
Beis Shammai say: one does not express gratitude to him, since the Torah declared the produce ownerless.
Beis Hillel say: it is permitted to express gratitude and thanks to the owner of the field, even though the Torah declared his fruit and his produce ownerless.
Rabbi Yehudah says precisely the opposite, and according to him this is one of the examples in which Beis Shammai were actually the lenient ones - and permitted expressing gratitude, while Beis Hillel were the stringent ones and said that it is forbidden to express gratitude.
In summary: in this mishnah we learned that a field cleared of thorns during the seventh year for the sake of the following year may be sown at the close of the seventh year, whereas a field thoroughly plowed with a second plowing during the time of danger, or fertilized by pens in a manner that was not permitted, may not be sown at the close of the seventh year. We also learned the dispute between Beis Shammai and Beis Hillel regarding eating the produce of a field that was thoroughly plowed during the seventh year, and the Rosh's explanation that the permission was stated regarding fruit of trees; and their dispute regarding expressing gratitude to the owner of the field, and the opinion of Rabbi Yehudah, who reverses the positions.