The Prairie province has one of the world's largest deposits of the mineral, and much of its tax and royalty revenues comes from mining it. On Tuesday, credit rating agency Standard and Poor's revised PotashCorp's outlook to negative from stable. The company's credit rating remains at A-. "Uralkali is one of the world's largest potash producers and its exit from BPC and intention to increase production volumes could fundamentally alter the potash market structure and result in intense price competition, in turn leading to lower industry-wide profitability," said S&P credit analyst David Fisher.