"That presumably worked fine in a primitive economy before decent fertilizer, but shmita presented problems for the new Jewish state. Zionism was founded on the notion of a return to the land, but a modern country cannot live on what falls to the ground.
So respected rabbis from both the Ashkenazic and the Sephardic communities compromised. Charged with interpreting religious law, or halacha, they devised the 'heter mechira,' or sale permit, which allows Jews to temporarily 'sell' their land to non-Jews for the shmita year, so the land may be cultivated." - The New York Times, October 8, 2007