Rashi on Deuteronomy 11:11/רש"י על דברים י״א:י״א

1ארץ הרים ובקעת [THE LAND WHITHER THOU ART PASSING IS] A LAND OF HILLS AND VALLEYS — The mountain is better than the plain because in the plain, in an area that takes a Kor of seed, you can sow only a Kor, but in the mountain the summit of which is an area that takes a Kor, from it you have areas for five Kors of seed: four from its four slopes and one on its summit (Sifrei Devarim 39:3).

2ובקעת — these are the plain lands.