Lands known to contain valuable deposits enumerated in this subchapter and not covered by permits or leases shall be held subject to lease by the Secretary of the Interior through advertisement, competitive bidding, or such other methods as he may by general regulations adopt, and in such areas as he shall fix, not exceeding two thousand five hundred and sixty acres; all leases to be conditioned upon the payment by the lessee of such royalty as may be fixed in the lease, not less than 2 per centum of the quantity or gross value of the output of potassium compounds and other related products, except sodium, at the point of shipment to market, and the payment in advance of a rental of 25 cents per acre for the first calendar year or fraction thereof; 50 cents per acre for the second, third, fourth, and fifth years, respectively; and $1 per acre per annum thereafter during the continuance of the lease, such rental for any year being credited against royalties accruing for that year. Any lease issued under this subchapter shall be for a term of twenty years and so long thereafter as the lessee complies with the terms and conditions of the lease and upon the further condition that at the end of each twenty-year period succeeding the date of the lease such reasonable adjustment of the terms and conditions thereof may be made therein as may be prescribed by the Secretary of the Interior unless otherwise provided by law at the expiration of such periods. Leases shall be conditioned upon a minimum annual production or the payment of a minimum royalty in lieu thereof, except when production is interrupted by strikes, the elements, or casualties not attributable to the lessee. The Secretary of the Interior may permit suspension of operations under any such leases when marketing conditions are such that the leases cannot be operated except at a loss. The Secretary upon application by the lessee prior to the expiration of any existing lease in good standing shall amend such lease to provide for the same tenure and to contain the same conditions, including adjustment at the end of each twenty-year period succeeding the date of said lease, as provided for in this subchapter. In the discretion of the Secretary of the Interior the area involved in any lease resulting from a prospecting permit may be exempt from any rental in excess of 25 cents per acre for twenty years succeeding its issue, and the production of potassium compounds under such a lease may be exempt from any royalty in excess of the minimum prescribed in this subchapter for the same period.