The plaintiff, within thirty days after the entry of final judgment, must pay the sum of money assessed, except where school or public land upon which no contract is outstanding is taken for public use under this chapter, the plaintiff shall pay for such land as follows: one-fifth of the sale price in cash at the time of the sale; one-fifth of the purchase price each five years thereafter on the anniversary date of the sale, with interest at the rate of not less than three percent per annum, payable annually in advance.