Jamieson, Fausset and Brown on Isaiah 28:29

29. This also--The skill wherewith the husbandman duly adjusts his modes of threshing is given by God, as well as the skill (Is 28:26) wherewith he tills and sows (Is 28:24, 25). Therefore He must also be able to adapt His modes of treatment to the several moral needs of His creatures. His object in sending tribulation (derived from the Latin tribulum, a "threshing instrument," Lu 22:31; Ro 5:3) is to sever the moral chaff from the wheat, not to crush utterly; "His judgments are usually in the line of our offenses; by the nature of the judgments we may usually ascertain the nature of the sin" [Barnes].