and Acts amendatory thereof and supplementary thereto, which are not otherwise reserved and which have been improved by and at the expense of the reclamation fund for administration or other like purposes, are no longer needed for the purposes for which they were withdrawn and improved, the Secretary of the Interior may cause said lands, together with the improvements thereon, to be appraised by three disinterested persons to be appointed by him, and thereafter sell the same, for not less than the appraised value, at public auction to the highest bidder, after giving public notice of the time and place of sale by posting upon the land and by publication for not less than thirty days in a newspaper of general circulation in the vicinity of the land; not less than one-fifth the purchase price shall be paid at the time of sale, and the remainder in not more than four annual payments with interest at 6 per centum per annum, payable annually, on deferred payments.