Brazilian mining company Vale, the world's top iron ore miner, said on Friday it is not seeking to buy fertilizer assets following rumors it was planning a bid for U.S. fertilizer company Mosaic. Mosaic, based in Plymouth, Minn., owns Mosaic Fertilizer of Riverview and is one of the world's major producers of finished phosphate products. Mosaic's stock spiked almost 9 percent Thursday, sparking rumors of a possible acquisition interest by Vale after a mention in one of Brazil's largest newspapers. Mosaic shares fell slightly on Friday after Vale's denial. Global interest in fertilizer has risen sharply in the last couple of years, with a very small group of companies controlling about 75 percent of the global supply of key ingredient potash.