The Sabbatical Year
One of the reasons for the Sabbatical year is to allow the land to rest for a year, to enhance its fertility (Guide for the Perplexed 3:39). From this it follows that after six consecutive years of intensive agriculture, the land is at its least fertile point in the seven-year cycle. So the Torah’s promise, that the land “will yield produce (sufficient) for three years” in the naturally infertile sixth year, is totally irrational, and requires a person to accept an authority which is higher than his mortal understanding.
The Talmud (Sanhedrin 97a) compares the six agricultural years to the six millennia of this world, and the Sabbatical year to the seventh millennium (when the Redemption will have arrived). Since the Jewish people suffer from a gradual regression in spiritual stature as the generations pass, a person might ask: How could the efforts of the spiritually weak and “infertile” sixth millennium bring the true and complete redemption? The Torah answers: it is the suprarational self-sacrifice and commitment to Judaism, of the final generations of exile, that will bring the blessings of the Redemption.
(Based on Likutei Sichos, vol. 27, pp. 189-190)