Fallow ground is ground that has been plowed and readied for sowing, but then is withheld and allowed to lie useless and unproductive. God, through His prophet, had to rebuke His people, not only because they had left their prepared ground unused, but because they were actually sowing their seed on thorn-choked ground. That is, they were turning to idols and forsaking God.
The word "fallow" occurs one other time in the Bible and to the same effect: "Sow to yourselves in righteousness, reap in mercy; break up your fallow ground: for it is time to seek the LORD, till he come and rain righteousness upon you" (Hosea 10:12). It is poor stewardship and a bad testimony, at best, for the people of God to ignore His righteousness and fail to cultivate His grace in their lives, choosing instead the philosophies and pleasures of the ungodly world around them.
Now, if the ancient Israelites had much unfruitful fallow ground in their lives, many modern Christians are still more blameworthy, for we have far greater opportunities and privileges than the people of ancient Israel.
Most of all, we have the complete Word of God and the indwelling Holy Spirit, yet our lives are even more cluttered with the thorny ground of worldliness and paganism than theirs. We urgently need to break up our fallow ground, to sow righteousness, and reap mercy.
"Be not deceived; God is not mocked: for whatsoever a man soweth, that shall he also reap. For he that soweth to his flesh shall of the flesh reap corruption; but he that soweth to the Spirit shall of the Spirit reap life everlasting" (Galatians 6:7-8). HMM