Mischna zu Wajikra 25:4

If one of the trees was capable of bearing a cake of dried figs [weighing sixty manehs], and the other two unable; or, if two could do so, but one cannot, then they may plow them only for their own needs. [This is the rule if the number of trees is] from three to nine, but if there were ten trees or more, whether they produce [the requisite amount of fruit or not] the whole bet seah may be plowed on their account. As it says: “In plowing and in harvesting, you shall rest” (Ex 34:21). It was unnecessary to state plowing and harvesting in the seventh year, rather [what it means is] the plowing of the year preceding which encroaches on the sabbatical year, and the harvest of the seventh year which extends into the year after. Rabbi Ishmael says: just as plowing is an optional act, so harvesting [referred to in the verse] is optional, thus excluding the harvesting of the omer [which is obligatory].