Charles Ellicott Commentary Genesis 41:34

"Let Pharaoh do [this], and let him appoint overseers over the land, and take up the fifth part of the land of Egypt in the seven plenteous years." — Genesis 41:34 (ASV)

Take up the fifth part of the land. —Heb., let him fifth the land, that is, exact a fifth part of the produce. It has been supposed that it had been customary in Egypt to pay the king a tithe of the crop, and the doubling of this tax would not press very heavily on the people in these years of extraordinary abundance. As the reason for the enactment would be made known, it would also induce all careful people to store up a portion of their own superabundance for future need. Subsequently, a fifth of the produce was fixed by Joseph permanently as the king’s rent.