And if you shall say: What shall we eat in the seventh year? ... But I will command my blessing upon you in the sixth year, and it shall bring forth fruit for three years (Leviticus 25:20-21)
The 7-year shmittah cycle corresponds to the seven millennia of history: for 6000 years man labors in the fields of the material world in preparation for the 7th millennium -- the era of Moshiach.
So the question "What shall we eat in the seventh year?" can be also asked on the
cosmic level. If the spiritual giants of earlier generations failed to bring about a
perfect world, what can be expected of us, we of the exhausted and depleted "sixth year"?
But precisely because our resources are so meager, our every trial and achievement is
so much more meaningful. G-d therefore promises that our efforts shall nourish
the sabbatical millennium and beyond.