No penalty for the prepayment of any indebtedness evidenced by an instrument which is secured by a mortgage on rural property may exceed:
(1) five percent of the unpaid principal balance if prepaid during the first year from the date of the instrument;
(2) four percent of the unpaid principal balance if prepaid during the second year from the date of the instrument;
(3) three percent of the unpaid principal balance if prepaid during the third year from the date of the instrument;
(4) two percent of the unpaid principal balance if prepaid during the fourth year from the date of the instrument;
(5) one percent of the unpaid principal balance if prepaid during the fifth year from the date of the instrument;
(6) no prepayment penalty shall be assessed if prepaid more than five years from the date of the instrument.