Doth the plowman plow all day to sow? doth he open and break the clods of his ground?
Doth the plowman plow all day to sow? - emphatic: he is not always plowing; he also 'sows,' and that, too, in accordance with sure rules (Isaiah 28:25).
Doth he open and break the clods of his ground? - supply always. Is he always harrowing?