Agriculture covers ninety percent of Iowa's land, the highest share of any state in the United States, and the state's dominant crops are corn and soybeans. Corn, originally cultivated in central Mexico as maize, is a major Iowa commodity, with the state leading the nation in production and farmers producing more than 2 billion bushels in a typical year, including 30 million bushels from Harrison County. Grown worldwide except in Antarctica, corn thrives in the U.S. Corn Belt and is harvested from early September through fall, with uses ranging from food ingredients and packaging materials to ethanol, which consumes about 20% of the nation's corn supply. Common varieties include sweet corn, field corn such as dent, flint, and flour corn, and popcorn, which is grown in many places in western Iowa. Soybeans, native to China and introduced as an Iowa crop in the late 1920s, expanded through the 1950s and have since remained steady, with Iowa also leading the nation in soybean production at more than 445 million bushels in a typical year, including around 5 million from Harrison County. Soybeans grow well in the same conditions as corn, are harvested from early September through fall, and are used for cooking oils, industrial products, biofuels, and high-protein livestock feed. Iowa farmers often rotate corn and soybeans on the same land because soybeans leave nutrients that help the next year's corn crop, while rotation also helps increase yields and control weeds, diseases, and harmful insects.