You have plowed wickedness. You have reaped iniquity. You have eaten the fruit of lies, For you trusted in your way, in the multitude of your mighty men.
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."