About one third of the population is engaged in farming. Wheat, corn, sugar beets, sunflowers, hemp, and flax are the chief crops; the fertile plains of Vojvodina are the most productive agricultural areas. Serbia proper has extensive vineyards and is one of Europe's major regions for fruit growing (notably plums). Manufacturing is the largest contributor to the economy; products include furniture, machinery, chemicals, tires, and clothing, and food processing also is important. Serbia's mineral wealth includes oil and natural gas, coal, iron ore, copper, and zinc. The political turmoil of the 1990s (see under History ) greatly exacerbated Serbia's already severe economic problems. Exports include iron, steel, and other metals, clothing, wheat, and fruits and vegetables. Serbia's main trading partners include the European Union nations, especially Germany, Italy, Hungary, Slovenia, and Austria, as well as Bosnia and Herzegovina and Montenegro.