Why does the Torah juxtapose Sh'mitah next to the Din of not oppressing a Ger?
Oznayim la'Torah: To teach us that it is not only because you were Geirim in Mitzrayim thatwe should not oppress a Ger, but because he is in no way inferior to you. See how you too are Geirim in the land
How can the land produce fruit in six consecutive years without giving it a rest period?
Mechilta: This is what will happen if Yisrael perform the will of Hashem; if not, they will plow one year and plant the next year
Is it a Mitzvah to work the land six years?
Da'as Zekenim: Yes! A Midrash says that even if one has a small ruin in his garden, he must tend to it every day. R. Moshe says, this is only in Eretz Yisrael, in order to increase separation of Terumah and Ma'aser.