(Reuters) - Canadian fertilizer producers Potash Corp of Saskatchewan Inc and Agrium Inc agreed to combine to navigate a severe industry slump by boosting efficiency and cutting costs, but the new company's potential pricing power may attract tough regulatory scrutiny. The proposed all-stock tie-up comes as industry profits have fallen due to excessive supply and soft farm incomes. It would combine Potash's crop nutrient production capacity, the world's largest, with Agrium's farm retail netw