Deuteronomy 14:22

We have here a part of the statute concerning tithes. The productions of the ground were twice tithed, so that, putting both together, a fifth part was devoted to God out of their increase, and only four parts of five were for their own common use; and they could not but own they paid an easy rent, especially since God's part was disposed of to their own benefit and advantage. The first tithe was for the maintenance of their Levites, who taught them the good knowledge of God, and ministered to them in holy things; this is supposed as anciently due, and is entailed upon the Levites as an inheritance, by that law, Num 18:24, etc. But it is the second tithe that is here spoken of, which was to be taken out of the remainder when the Levites had had theirs. I. They are here charged to separate it, and set it apart for God: Thou shalt truly tithe all the increase of they seed, Deu 14:22. The Levites took care of their own, but the separating of this was left to the owners themselves, the law encouraging them to be honest by reposing a confidence in them, and so trying their fear of God. They are commanded to tithe truly, that is, to be sure to do it, and to do it faithfully and carefully, that God's part might not be diminished either with design or by oversight.

— Matthew Henry's Concise Commentary (public domain)

Thou shalt truly tithe all the increase of thy seed--The dedication of a tenth part of the year's produce in everything was then a religious duty. It was to be brought as an offering to the sanctuary; and, where distance prevented its being taken in kind, it was by this statute convertible into money.

— Jamieson-Fausset-Brown Commentary (public domain)