Sheviis, Chapter 3, Mishnah 1. When farmers went about the work of fertilizing a field, their practice was to bring the manure out into the field and pile it up in one or more very large heaps, each called an ashpah (dungheap). From that dungheap they would later spread the manure over the entire field.
The restriction the Sages imposed:
The basic law is straightforward: one does not fertilize a field during the Shemittah year. But even when a person wishes to bring out the manure for the sake of the year following Shemittah, the Sages imposed a restriction: he may not bring it out and form it into a dungheap or dungheaps until a certain point in the Shemittah year has arrived, a point from which it is clear that the work is not being done for the benefit of the Shemittah year itself. It is with this question that our Mishnah opens.
The question of the Mishnah:
"Me'eimasai" - from what point in the Shemittah year is it permitted to bring out the manure, the dung, from where the animals are kept to the dungheaps, those large piles, in order ultimately to spread it over the whole field and fertilize it in the year after Shemittah?
The measures given by the Tannaim:
Rabbi Meir: from the time that the "ovdei ha'avodah" - those who work the field - stop bringing out manure as they normally would in that season. In other words, had this not been a Shemittah year, the workers would have been bringing out manure for the needs of that very year; once the season for bringing it out for the current year has passed, it is permitted to bring it out for the coming year, since then it is clear that the work is being done for the following year. Some have a different reading in the Mishnah: "ovrei aveirah" - those who transgress the laws of Shemittah - from the time that they stop doing so; and in practice this comes to the very same measure.
Rabbi Yehudah: from the time that "hamasok" (the sweetening) dries up. Sweetness is an allusion to manure and fertilizer, because it is what adds sweetness to the fruit and to the produce. From the time it dries up it is permitted, whenever that point may fall.
Rabbi Yosi: "mishe'yikshor" - from the time that the manure begins to form knots, that is, it begins to dry out and to look as though it has knots in it. That is the point from which one may begin bringing the manure out to the large piles at the end of the Shemittah year.
In summary: in this Mishnah we learned when it becomes permitted to bring manure out to the dungheaps during the Shemittah year, for the purpose of fertilizing the field after Shemittah: according to Rabbi Meir, from the time the field workers (or, according to the other reading, the transgressors) stop bringing out manure for the needs of that year; according to Rabbi Yehudah, from the time the sweetening dries up; and according to Rabbi Yosi, from the time it forms knots, that is, once the manure begins to dry out and to look as though it has knots in it.