"Israel, you have sinned from the days of Gibeah. There they remained. The battle against the children of iniquity doesn't overtake them in Gibeah.
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."