Adam Clarke on Ecclesiastes 5:9

The profit of the earth is for all - The earth, if properly cultivated, is capable of producing food for every living creature; and without cultivation none has a right to expect bread.

The king himself is served by the field - Without the field he cannot have supplies for his own house; and, unless agriculture flourish, the necessary expenses of the state cannot be defrayed. Thus, God joins the head and feet together; for while the peasant is protected by the king as executor of the laws, the king himself is dependent on the peasant; as the wealth of the nation is the fruit of the laborer's toil.