The mineral and agricultural resources of the United States are tremendous. Although the country was virtually self-sufficient in the past, increasing consumption, especially of energy, continues to make it dependent on certain imports. It is, nevertheless, the world's largest producer of both electrical and nuclear energy. It leads all nations in the production of liquid natural gas, aluminum, sulfur, phosphates, and salt. It is also a leading producer of copper, gold, coal, crude oil, nitrogen, iron ore, silver, uranium, lead, zinc, mica, molybdenum, and magnesium. Although its output has declined, the United States is among the world leaders in the production of pig iron and ferroalloys, steel, motor vehicles, and synthetic rubber. Agriculturally, the United States is first in the production of cheese, corn, soybeans, and tobacco. The United States is also one of the largest producers of cattle, hogs, cow's milk, butter, cotton, oats, wheat, barley, and sugar; it is the world's leading exporter of wheat and corn and ranks third in rice exports. In 1995, U.S. fisheries ranked fifth in the world in total production.
Major U.S. exports include aircraft, motor vehicles and parts, food, iron and steel products, electric and electronic equipment, industrial and power-generating machinery, organic chemicals, transistors, telecommunications equipment, pharmaceuticals, and consumer goods. Leading imports include ores and metal scraps, petroleum and petroleum products, machinery, transportation equipment (especially automobiles), food, clothing, computers, and paper and paper products. The major U.S. trading partners are Canada (in the world's largest bilateral trade relationship), Mexico, China, Japan, Great Britain, Germany, and South Korea. Despite the steady growth in imports, the gross domestic product also has continued to rise, and in 2006 it was easily the largest in the world at about $13 trillion. The development of the economy has been spurred by the growth of a complex network of communications not only by railroad, highways, inland waterways, and air but also by telephone, radio, television, computer (including the Internet), and fax machine. This infrastructure has fostered not only agricultural and manufacturing growth but has also contributed to the leading position the United States holds in world tourism revenues and to the ongoing shift to a service-based economy. In 1996 some 74% of Americans worked in service industries, a proportion matched, among major economic powers, only by Canada.