PETROVINA, Brazil - The six-seat Embraer airplane glides from a cloudless sky onto a red-dirt runway. Views of scrub-brush savanna stretching to the Amazon River give way to fields of 10-foot high corn and boll-bursting cotton. It's a farmer's wonderland, where the fecund soil can be had for as little as $200 a sun-drenched acre and a Maryland-sized chunk of land is cleared each year for cotton, corn, soybean and cattle farms. Agriculture is booming in Brazil, and U.S. farmers are taking notice. Buffeted by high production costs, low market prices and the World Trade Organization, Americans increasingly look to...