Shevi'is, Chapter 1, Mishnah 2. In the previous mishnah we learned the law of tosefes shevi'is - that a person may not plow his field at the end of the sixth year, and in a field that contains trees one may plow only until Shavuos according to Beis Hillel. Now the mishnah comes to clarify the precise scope of this allowance: what qualifies as a 'field of trees,' in which one may plow the entire field until Shavuos. This question goes to the heart of the law, for if the field is not defined as a field of trees or an orchard, it is considered a 'white field' - an expression meaning a grain field - and that may be plowed only until Pesach, not until Shavuos.
The text of the mishnah:
"Eizehu sdeh ha'ilan? Kol shloshah ilanos l'veis se'ah" - a field of trees is a field that contains three trees within an area of a beis se'ah, that is, fifty amos by fifty amos.
"Im re'uyin la'asos kikar develah" - the mere presence of the trees is not enough; they must also be capable of producing a certain yield: a kikar develah, a round cake of pressed figs, weighing sixty maneh by the Italian maneh. Each maneh is one hundred dinar, and if the trees are able to produce a cake of pressed figs of this weight according to the Italian standard, the place is considered an orchard.
"Choshrin kol beis se'ah bishvilan" - in such a field one may plow the entire area of the beis se'ah for the sake of those trees, until Shavuos.
Less than this:
"Pachos mikan ein choshrin lahen ela melo ha'oreh v'salo chutzah lo" - if the field does not contain that number of trees, or if they do not produce that measure of figs, the allowance is limited to the area immediately around the single tree: the space of the oreh - the one who picks the figs, who is called an 'oreh' - and his basket, which is located outside the tree. Only this area may be plowed until Shavuos, while the rest of the field is treated as a white field, like any grain field, which may be plowed only until Pesach.
A closer look at the measure of sixty maneh:
The commentators disagree about how to understand this measure: some explain that each one of the three trees in the beis se'ah must be capable of producing a cake of pressed figs weighing sixty maneh, while others explain that it refers to the three trees together, and it is enough that each tree produce a third of sixty maneh.
In summary: This mishnah defines what constitutes a field of trees in which the allowance to plow the entire area until Shavuos applies - three trees in a beis se'ah that are capable of producing a cake of pressed figs weighing sixty maneh. In the absence of this measure, the allowance is limited to the space of the oreh and his basket around the tree, and the rest of the field is treated as a white field, which may be plowed only until Pesach.