Sforno on Leviticus 25:2/ספורנו על ויקרא כ״ה:ב׳

1כי תבואו אל הארץ, a reference to the land west of the river Jordan (Torat Kohanim 2)

2ושבתה הארץ, .a reference to the soil intended for farming.

3'שבת לה, so that during this whole year the farmer instead of “serving” the soil which requires cultivation, will turn his efforts to serving G’d directly instead. Just as the weekly Sabbath is a day set aside for intensive service of the Lord, so the sh’mittah year is to serve the same purpose. (compare Exodus 20,9)