Ephraim is a trained heifer that loves to thresh; So I will put a yoke on her beautiful neck. I will set a rider on Ephraim. Judah will plow. Jacob will break his clods.
The fallow ground is the ground that is become hardened because of the lack of cultivation. The soil has not been broken up, not been loosened, and thus by the rains and all the soil has become compacted. And becoming compacted, becomes very hard so that the seed cannot really take root. So God is saying, "Break up the fallow ground within your heart so that the seed, the Word of God can begin to take root in your life."