Each lease shall describe the leased lands by the legal subdivisions of the public-land surveys. All leases shall be conditioned upon the payment to the United States of such royalties as may be specified in the lease, which shall be fixed by the Secretary of the Interior in advance of offering the same, at not less than 5 per centum of the gross value of the output of phosphates or phosphate rock and associated or related minerals. Royalties shall be due and payable as specified in the lease either monthly or quarterly on the last day of the month next following the month or quarter in which the minerals are sold or removed from the leased land. Each lease shall provide for the payment of a rental payable at the date of the lease and annually thereafter which shall be not less than 25 cents per acre for the first year, 50 cents per acre for the second and third years, respectively, and $1 per acre for each year thereafter, during the continuance of the lease. The rental paid for any year shall be credited against the royalties for that year. Leases 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 readjustment 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.