Farmers’ fields in South America are among the most prized assets in a new global market for agricultural land that has sprung up alongside soaring commodity prices. Private equity and fund managers at a farm investing conference in Geneva named South America a top place to buy, lease and manage agricultural lands for profit. “The South American marketplace is really booming along right now,” said Mark Zenuk, managing director of the $3 billion NGP Global Adaptation Partners fund. Black…