FRANKFURT (Reuters) - Canada's Potash Corp of Saskatchewan (>> Potash Corp./Saskatchewan Inc.) has hired a third investment bank as adviser in its effort to take over German fertilizer maker K+S (>> K&S AG), a German newspaper reported on Sunday.

Bank of America Merrill Lynch will join Morgan Stanley and Deutsche Bank in advising the Canadians in the effort, the Frankfurter Allgemeine Zeitung newspaper said, without citing sources.

K+S has also taken on Rothschild as an adviser, alongside Goldman Sachs, the paper added, again without citing sources.

K+S rejected a 7.9 billion euro ($8.81 billion) offer from Potash but said it would consider any improved offer after its position was boosted by stronger than expected quarterly earnings.

Potash and K+S were not immediately available for comment. The banks all declined comment, the Frankfurter Allgemeine Zeitung said.

(Reporting by Jonathan Gould in Frankfurt and Rod Nickel in Winnipeg)