Why does the Pasuk use the expression "Shabbas Shabbason"?
Seforno: To add preparatory Malachos 1 to the prohibition.
Seforno: Such as sticking manure to the bark and anointing and holing young figs. as listed in Avodah-Zarah, 50b.
What does "la'Aretz" refer to?
Rashi and Seforno: It refers to the fields and vineyards - with reference to the Melachos that pertain to them, such as planting and reaping. 1
Digging pits per se (and building) are not prohibited during the Shemitah (Sifsei Chachamim).
What is the meaning of "Karmecha Lo Tizmor"?