In the previous chapter we learned until when a person may plow his field in the year before Shemittah: an orchard - a field of fruit trees - may be plowed until the Sabbatical year itself, and a grain field until Pesach. In our Mishnah we will see that this is only one opinion, and it does not represent all the views.
The wording of the Mishnah:
"Ad eimasai chorshin bisdeh halavan erev shevi'is? Ad shetichleh haleichah" - Until when may one plow a grain field in the year before the Sabbatical year? Until the moisture is gone. A "white field" is, as mentioned, a field that produces grain, and the limit set for plowing it in the year before Shemittah is until the moisture in the field is gone, that moisture which accumulated from the rains.
The Mishnah adds a practical sign for this time: "Kol zeman shebnei adam chorshim lita mikshaos umidlaos" - as long as people are plowing in order to plant cucumber and gourd patches. Mikshaos are cucumbers, and delu'os are gourds, and these crops require a great deal of moisture. Therefore the time during which people are still plowing for their sake is the measure of "until the moisture is gone."
Rabbi Shimon's objection:
"Amar Rabbi Shimon: nasata Toras kol echad ve'echad beyado" - Rabbi Shimon said: you have placed the law of each and every person in his own hands. Once a general and imprecise time has been set, "until the moisture is gone," the measure and the halachah have been handed over to every individual: each one will decide for himself and make claims that are not necessarily honest as to whether the time has come that the moisture is gone or not.
Therefore a precise time must be stated: "Ela bisdeh halavan ad haPesach, uvisdeh ha'ilan ad Atzeres" - rather, in a grain field until Pesach, and in an orchard until Shavuos:
A grain field - a field of produce - may be plowed until Pesach.
An orchard - may be plowed until Shavuos.
An orchard grove - a field full of fruit trees - may be plowed until the Sabbatical year itself.
In summary: in this Mishnah there is a dispute about the limit for plowing in the year before Shemittah. According to the first opinion the limit is "until the moisture is gone," and a practical sign was set for it: as long as people are plowing in order to plant cucumber and gourd patches, which require a great deal of moisture. Rabbi Shimon disagreed and argued, "you have placed the law of each and every person in his own hands," since an imprecise measure is left to each person's own judgment, and he therefore set precise times: a grain field until Pesach and an orchard until Shavuos.