All land sold under the provisions of this act [19-11-1, 19-11-3 to 19-11-9 N.M. Stat. Ann.] shall be so sold with the proviso and a covenant running therewith, that the purchaser or purchasers thereof shall encourage, protect and conserve the growing of young timber and timber products thereon, such as are particularly adapted to the said lands; provided, however, that any part of such lands sold that are suited for agricultural purposes may be used for that purpose.