As noted in the first two shiurim, Parashat Behar's presentation of the laws of shemitta is characterized by prominent and repetitive mentions of the land. The word "aretz" (land) is mentioned in several different contexts, and the prohibitions listed focus upon the type of work performed upon the land, not the way in which humans partake of its crops. Indeed, almost all Rishonim recognize that apart from specific forms of labor which are forbidden (namely, planting, pruning, reaping, and harvesting grapes), a general injunction against all forms of work can be derived from the Torah's generalization that the "land itself should rest" (Vayikra 25:2). In shiur #3 I mentioned a debate amongst Rishonim as to whether plowing was forbidden mi-de'oraita or not. Regardless of whether it is included in the list of forbidden forms of work, plowing is certainly forbidden on shvi'it because it disrupts the concept of the land resting.
From a gemara in Avoda Zara, however, it appears as if this notion takes on a very unique definition. Typically, we view prohibitions as relating to human beings and the actions they perform. In our situation the Torah proscribes four activities by stipulating them in a negative form and then bans all other activity by demanding that the land rest. Man is obligated to avoid violating the negative commandments by withdrawing from these four forms of labor and to observe the positive directive by restraining himself form other forms of labor. The focus is clearly upon the person and the actions he performs. In some situations, however, prohibitions address objects rather than designated actions. The most similar example is Shabbat, which is dominated by thirty-nine acts which man cannot perform. In addition to this list, the Torah prohibits a persons animal from working, even if someone else (a goy) works with the animal. This halakha, known as "shevitat behemto" is described in Shemot 23:12 and amplified by the gemara in Avoda Zara (15). That same gemara ponders the notion of "shevitat sadeihu" (resting of an individual's field) during shvi'it. Aside from prohibitions relating to man himself, must a field lay fallow during this year in the same manner that animals must rest on Shabbat?
The gemara is debating the selling of animals to Gentiles (which might be forbidden because selling might lead to lending, and a Gentile might work with a borrowed animal on Shabbat). The gemara relates an episode about Rav Huna, who did sell such an animal but assessed that the Gentile would eat the animal and not work with it on Shabbat. As a proof the gemara cites a mishna in Shvi'it in which Beit Hillel allows selling an animal to a Gentile during shemitta if we can "assume" that the animal might be eaten (and not dedicated to field labor). The gemara then rejects the proof since a person is not commanded to prevent his animal from working during shemitta (hence there are less potential problems surrounding the sale of such an animal during shvi'it). The gemara responds that Beit Hillel also allowed the sale of land to a Gentile during shvi'it since the Gentile might "skip" the year of shemitta and not labor. From this we can infer that even when a prohibition relates to your object (in this case land), you are still allowed to sell that object to a Gentile. If so, Rav Huna's conduct can be vindicated. The simple reading of the gemara suggests that during shemitta, one cannot allow his land to be worked, even by a Gentile. The mishna allows the legal sale of this land to a Gentile only because it is not (concerned with the possibility that instead of selling, the Jew might loan the land and the Gentile would end up working a Jewish field).
Indeed, the Tosafot Rid explicitly infers from the gemara the halakha that Jewish land must rest during shemitta. Rashi also seems to accept this basic reading of the gemara when he cites the verse, "The land shall have a shabbat of complete rest" (Vayikra 25:4). By contrast, Tosafot (s.v. mi) explain the gemara's debate in a manner which does not mandate any unique mitzva pertaining to the land itself. According to Tosafot the only potential prohibition relating to the land once it leaves a Jew's possession is the possibility that another Jew will obtain ownership and work that land. In THIS instance a violation will occur (since a Jew will perform a prohibited action). If a Gentile were to work Jewish land, Tosafot would not realize any prohibition.
An interesting ambiguity in the Rambam merely fuels this debate. As mentioned earlier, in addition to forbidding certain actions during shemitta, the Torah also issues several general verses about allowing the land to rest. Should these verses be interpreted as a mitzva pertaining to the person's actions upon the land or a mitzva about the state of the land itself?
The Rambam issues different signals in different locations. In his title (known as the "koteret," a prelude to each section of Mishna Torah in which he lists the mitzvot contained in that section) to Hilkhot Shemitta Ve-Yovel, he lists the mitzva "that the LAND should rest from work during the seventh year." Yet in his description of that mitzva in perek 1 halakha 1, he writes, "There is a mitzva that [a person] should refrain from work during the seventh year," seemingly devolving the mitzva upon the person. The same tone emerges from the his language in Sefer Ha-mitzvot (positive commandment 135). (Many have commented upon this apparent contradiction; see especially the commentary of Rav Yerucham Fishel Perle to the Sefer Ha-mitzvot of Rav Sa'adia Goan mitzva 61.)
If we recognize this additional commandment during shemitta, what would be some of its legal connotations? The Minchat Chinukh (mitzva 112) concludes from the gemara in Avoda Zara that this mitzva would prohibit a Gentile from working land owned by a Jew. As stated earlier, this mitzva would clearly cover all forms of agricultural labor, many of which are not forbidden based upon the specific prohibitions of work. Even if we might not forbid all forms of labor, we certainly would ban plowing, even if it is not included in the four types of labor to which the Torah applied a negative commandment. In fact, the Minchat Chinukh suggests that even women might be obligated in this mitzva of allowing the land to rest. Even though this mitzva is a time-dependent positive mitzva (from which women are generally excluded), it might still apply to women. Since the mitzva relates to the land and its state during shemitta, women must also insure this condition. Women are only excluded from time-dependent mitzvot which are directed at people. If the mitzva relates to the field, women might still be obligated not to disrupt that state.
In mitzva 326 the Minchat Chinukh raises another possible outcome of a commandment for the land to rest. As mentioned in the shiurim addressing tosefet shvi'it, all laws of tosefet are suspended without a Beit Ha-mikdash. The Rambam does retain one vestige of tosefet when he prohibits planting trees forty-four days prior to shemitta because of marit ayin (people will claim that the tree was actually planted during shvi'it). All other forms of labor may be continued until Rosh Hashanah itself. The Minchat Chinukh wonders about planting seeds within three days of Rosh Hashanah in a manner that the actual "rooting" will occur during shemitta. Even if a person has not actively performed a violation (since he completed his physical labor prior to the onset of shemitta), the land itself will "work" during shemitta, and its shevita has been violated. A similar claim is lodged by the Mikdash Dovid (siman 59) as the reason that the mishna in Shevi'it prohibits planting trees thirty days prior to shemitta; as this mishna recognizes thirty days as the minimum period for a tree to root, it requires this process to take place entirely before shemitta so as not to disturb the resting of the land. (Most Rishonim, on the other hand, interpret the prohibition to plant during this period as a violation of the prohibition of tosefet shvi'it.)
Rav Yerucham Fishel Perle develops another interesting consequence of this notion. What would happen if a person would plant a tree toward the tail end of shemitta so that the rooting would take place AFTER shemitta? Would he still violate the resting of the land? Surely he would violate the prohibition pertaining to the act of planting since this was performed during shemitta proper. In as much as the rooting occurs after shemitta, though, he might not violate the additional mitzva of letting the land rest!
Shiur #3 introduced the famous position of Rabbenu Tam regarding tosefet shvi'it. This is not an early start to shvi'it, according to Rabbenu Tam, but rather a period during which plowing which would improve the land during shvi'it is forbidden. Not only is work forbidden during shvi'it, but one cannot even bring the land to a state of readiness during the shemitta year. Though Rabbenu Tam did not directly associate his halakha with the resting of the land, the correspondence seems very likely. Logically, resting of the land would be the basis for any prohibition of readying the land. Full "rest" would not only mandate the withdrawal from work. It would also require the land to be completely detached from the human agricultural experience. Even readying the land might contradict the spirit of the land's rest.