BRASILIA/SAO PAULO Dec 19 (Reuters) - Vale SA's $2.5 billion sale of its fertilizer business to Mosaic Co, the latest step in the Brazilian iron ore miner's strategy to reduce debt, also makes it the U.S. company's biggest shareholder. Mosaic, which made the deal to improve its access to Brazil's vast agricultural markets, will pay $1.25 billion in cash and $1.25 billion in newly issued shares for the unit. In return, a Mosaic spokesman said, Vale will receive an 11 percent stake in Mosaic, bigger than those held by investment companies such as Vanguard, Franklin Advisers and BlackRock Inc .