Rashi: Refer to 3:18:1:2.

Radak: "Lo Sashchis Es Etzah" applies only when besieging a city. The verse gives the reason - "Ki Mimenu Sochel." Even though our Rebbeyim say that the Mitzvah applies in all places and at all times, the simple meaning is only during a siege. Perhaps this was a Hora'as Sha'ah, like Eliyahu, who offered on Har ha'Karmel when Bamos were forbidden.

Rashi: It is a seeded field.

Radak: This is a borrowed expression (it is as if you pain it). The same applies to "ha'Yichyu Es ha'Avanim" (Nechemyah 3:34), "va'Yrap'ei Es Mizbach Hashem he'Harus" (Melachim I, 18:30).

Radak citing Targum Yonasan: You will close up.