South America Seeks to Fill the World's Table
South America, led by Brazil, has had explosion of farm exports over past decade; growth is fueled by combination of market-friendly economic policies and advances in agronomy that have brought formerly unusable tropical lands into production and increased productivety levels beyond those in United States and Europe, challenging their traditional dominance of global farm trade; in addition to Brazil, trend is being felt in parts of Argentina, Bolivia, Paraguay and Uruguay; photos; map; graphs (L)