S.D. Codified Laws § 43-30 app. 14-10
If the holder of a mortgage acquires the estate of the mortgagor, the mortgage interest may be merged in the fee and the mortgage extinguished, although it is only when the fee and the lien center in the same person without any intervening claims, liens or equities that a merger of the title and the lien will take place.
However, merger depends upon the intent of the parties. In determining intent, equity is not limited by the rules of law, and under unusual circumstances, a mortgage may be extinguished where it would continue to exist at law, or a mortgage may be preserved where it would be merged at law. Because of the difficulty in determining what a court might decide in any particular case, if merger cannot be determined from the instruments, then a merger is not ordinarily assumed so long as the mortgage remains unsatisfied of record.
If there is an outstanding or intervening lien or title, the foundation for the merger does not exist and no merger will be declared.
SDCL app TO CHAPTER 43-30 § 14-10