There are several mitzvos assei (e.g., to be mafkir one’s produce) and lo sa’asseh (e.g., not to plant the land) associated with shemitas karka, which parenthetically begins this Rosh Hashanah. The Chinuch, in mitzvah 84, says that both men and women are obligated to observe these mitzvos.
In mitzvah 112, the Minchas Chinuch asks why women are not exempt from the mitzvos assei of shemitah since they are generally exempt from mitzvos assei she’hazeman gramma (time-sensitive mitzvos). While some Rishonim opine that when a mitzvas lo sa’asseh is associated with a mitzvas assei, women are obligated, Tosafos (in Kiddushin 34a) maintains that women are still exempt. It is thus imperative to explain why, according to Tosafos, women are obligated to observe the mitzvos assei of shemitah.
The Minchas Chinuch first suggests that the obligation to be mafkir one’s produce may not be a time-sensitive mitzvah. Since the fruits, once grown, will be forbidden forever, perhaps the mitzvah is not technically time-sensitive. He argues that this reasoning is dependent on a machlokes Rishonim regarding the mitzvah of tzitzis. According to the Rambam (Hilchos Tzitzis 3:7), a man is only obligated to keep the mitzvah of tzitzis during the day. At night one is exempt from this mitzvah, even with a garment generally worn during the day. This is why women are exempt from the mitzvah of tzitzis.
The Rosh disagrees, suggesting that a garment generally worn during the day must have tzitzis on them – even when worn at night. Only garments generally worn at night need not have tzitzis affixed to them. According to the Rosh, the reason why tzitzis is a time-sensitive mitzvah is because time dictates which garment is included in the mitzvah.
According to the Rambam, since fruit grown in a shemitah year will remain forbidden forever, we can suggest that that mitzvah to be mafkir one’s produce is not time-sensitive. But according to the Rosh, who only exempts women from the mitzvah of tzitzis because time decides which garments must have tzitzis, women should be exempt from observing the mitzvah of being mafkir one’s produce. This is because the time during which the produce is grown dictates which produce will be included in the obligation.
Why, then, according to the Rosh, are women are obligated in all the mitzvos assei of shemitah?
In Kiddushin 29a, the Rishonim ask why there is a pasuk in the Torah exempting women from the obligation to perform bris milah on their sons. After all, women should obviously be exempt since it is a mitzvas assei she’hazeman gramma. The Ritva answers that women are only exempt from mitzvos assei she’hazeman gramma for mitzvos that pertain to themselves. But when the mitzvah requires them to do something for someone else, they are not exempt. That’s why the pasuk is necessary. Without it, women would be obligated to perform (or arrange) the bris milah of their sons.
The Minchas Chinuch (mitzvah 112:3) understands the Ritva’s answer to mean the following: Mitzvos can be placed into two categories: those that an individual is obligated to perform, and those requiring that a certain situation take place. The Minchas Chinuch explains that the mitzvah for parents to perform a bris milah on their son is not a mitzvah requiring them to perform a certain act; rather, the mitzvah requires that they ensure that a certain act is accomplished – namely that their son has a bris milah. Regarding these types of mitzvos, women are not exempt – even if the mitzvah is a mitzvas assei she’hazeman gramma.
That’s why, explains the Minchas Chinuch, women are not exempt from the mitzvah of shemitah. The essence of this mitzvah is ensuring that the land is granted a sheveisa. It is not a mitzvah devolving on a person; rather, the mitzvah requires that a certain situation be arranged – namely that the land receive a rest.