Leases under this act [19-8-10, 19-8-11 N.M. Stat. Ann.] may be made for a term of ten years or less and as long thereafter as said salt in paying quantities shall be produced from the leased lands; provided that as to any saline leases existing at the effective date of this act the commissioner of public lands may in his discretion extend the term of any such leases for an additional term of five years and as long thereafter as salt is being produced in paying quantities from the leased lands.