Adam Clarke on Deuteronomy 14:22

Thou shalt truly tithe - Meaning the second tithe which themselves were to eat, Deu 14:23, for there was a first tithe that was given to the Levites, out of which they paid a tenth part to the priests, Num 18:24-28; Neh 10:37, Neh 10:38. Then of that which remained, the owners separated a second tithe, which they ate before the Lord the first and second year; and in the third year it was given to the Levites and to the poor, Deu 14:28, Deu 14:29. In the fourth and fifth years it was eaten again by the owners, and in the sixth year was given to the poor. The seventh year was a Sabbath to the land, and then all things were common, Exo 23:10, Exo 23:11, where see the notes, Exo 23:11 (note), and see Ainsworth on this verse.