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Here is a commentary on this verse from:
SOW FOR YOURSELVES RIGHTEOUSNESS, REAP THE FRUIT OF UNFAILING LOVE: God's mercy is still a possibility.
BREAK UP YOUR UNPLOWED GROUND: ...if you turn about and begin diligently to serve Him. Fallow or unplowed ground needs to be broken up -- which is hard work -- thus exposing that which hides under the surface: unconfessed sins.
FOR IT IS TIME TO SEEK THE LORD, UNTIL HE COMES AND SHOWERS RIGHTEOUSNESS ON YOU: "Seek the LORD while he may be found; call on him while he is near. Let the wicked forsake his way and the evil man his thoughts. Let him turn to the LORD, and he will have mercy on him, and to our God, for he will freely pardon" (Isa 55:6,7).
"Our nature at its largest is but a small farm, and we had need to get a harvest out of every acre of it, for our needs are great. Have we left any part of our small allotment uncultivated? If so, it is time to look into the matter and see if we cannot improve this wasteful state of things. What part of our small allotment have we left fallow? We should think very poorly of a farmer who for many years allowed the best and the richest part of his farm to lie altogether neglected and untilled. An occasional fallow has its benefits in the world of nature; but if the proprietor of rich and fruitful land allowed the soil to continue fallow year after year we should judge him to be out of his wits. The wasted acres ought to be taken from him and given to another husbandman who would worthily cherish the generous fields and encourage them to yield their harvests.
"Bad is the man who neglects to cultivate his farm, but what shall be said of the sluggard who fails to cultivate himself? If it be wrong to leave untended a part of our estate, how much worse must it be to disregard a portion of ourselves! Now, there is a part of our nature which many allow to lie fallow. It is not often that they neglect the clay soil of their outward frame. They dress that field which is called the body with sufficient care; and truly I would not that they should be careless about it, for it is worthy to be kept in due order and culture. Albeit that it is a very secondary part of our nature, yet it is so interwoven with the higher that it is most important that the body should not be neglected. See ye well to that field, and by temperance, cleanliness, and obedience to the rules of health let it be as a garden... Few need to be exhorted to pay attention to their bodies... The fault is not that they care for the body, but that it takes an undue share of consideration, and usurps a higher place than it can claim.
"There is another field in man's self-farm... The soil where true religion should flourish in the furrows is left by many to produce the deadly nightshade of superstition, the hemlock of error, or the thistle of doubt... Your hearts, your innermost natures, have been neglected, and from the finest part of your being the Lord has derived neither rent nor revenue. Your best acres lie fallow -- fallow when you have good need to cultivate every inch of the ground.
"Do you know what happens to a fallow field? how it becomes caked and baked hard as though it were a brick? All the fragile qualities seem to depart, and it hardens as it lies caked and unbroken; I mean, of course, if year succeed year, and the fallow remains untouched. And then the weeds! If a man will not sow wheat, he shall have a crop for all that, for the weeds will spring up, and they will, seed themselves, and in due time the multiplication table will be worked out to a very wonderful extent; for these seeds, multiplying a hundred-fold, as evil usually does, will increase, and increase, and increase again till the fallow field shall become a wilderness of thorns and briars, and a thicket of nettle and thistle" (CHS).
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