DuPont Co. said it agreed to buy Ralston Purina Group's soy unit for $1.5 billion in stock and assumed debt as the largest U.S. chemical company raised its bet on its future in agribusiness. The acquisition of Ralston Purina's St. Louis-based Protein Technologies International unit, a maker of soy protein and food ingredients with $450 million in annual sales, comes on the heels of DuPont's Aug. 7 announcement of a $1.7-billion investment in Pioneer Hi-Bred International Inc., a supplier of seeds and genetically engineered agricultural products. Shares of Wilmington, Del.-based DuPont, which said it will take a charge of $1 a share this year on acquisitions and asset sales, fell 31 cents to close at $65.63 on the New York Stock Exchange. Ralston shares gained $3.44 to close at $91.69, also on the NYSE. DuPont is going to have to show how it will use Ralston's soy unit and its Pioneer joint venture to make a profit, analysts said. Like its rival, Monsanto Co., DuPont--one of the top five sellers in the world's $32-billion crop-protection market--now wants a shot at the $500-billion food, feed and fiber market.