Albert Barnes Commentary 1 Samuel 8:12

"and he will appoint them unto him for captains of thousands, and captains of fifties; and [he will set some] to plow his ground, and to reap his harvest, and to make his instruments of war, and the instruments of his chariots." — 1 Samuel 8:12 (ASV)

This organization was as old as the time of Moses (Numbers 31:14; Deuteronomy 1:15) and also prevailed among the Philistines (1 Samuel 29:2). The civil and military divisions were identical, and the civil officers were the same as the captains of thousands, hundreds, fifties, and tens in time of war.

To ear his ground - Literally, “to plow his plowing.” “To ear” is an old English word, now obsolete, for to plow.