43 U.S.C. § 424b

All “permanently unproductive” and “temporarily unproductive” land now or hereafter designated under sections 423 to 423g and 610 of this title, shall, when sold, remain subject to sections 423 and 423b of this title. The exchange provisions of section 423c of this title , shall not be applicable to the land purchased under sections 424 to 424e of this title.