River floodplains are the state's most fertile cropland, and floodplain farming in this region long predates Missouri statehood in 1821, when American Indians raised varieties of corn, beans, squash, and sunflowers. Today these fields produce soybeans, corn, alfalfa, grain sorghum, and in sandy soil sunflowers and pumpkins, with Missouri soybean and corn production generally ranking between fifth and tenth in the country. The U.S. soybean harvest represents almost half of the world's crop, and soybeans, once used almost entirely for livestock feed and fertilizer, are also grown for human consumption and for dozens of industrial products such as printing inks, lubricants, cleaning solvents, adhesive removers, and herbicide additives. Corn, the only U.S. crop larger than soybean, often reaches 9 feet tall with a 4-foot-deep root system in just two months; most field corn is used as livestock feed, but its byproducts also go into ink, alcohol, shoe polish, ethanol fuel, and paint. Because corn removes valuable nitrogen from the soil, fertilizer must be applied regularly or legumes such as soybeans and alfalfa must be included in crop rotation, so soybean and corn usually alternate from year to year in these fields. The tradeoff for fertile soil deposited by the river is the possibility of flooding, and levees were built around large areas of fertile low-lying soil to lessen the frequency of floods. During the Great Flood of 1993, the river breached levees on this stretch and inundated the fields; although river sand deposited elsewhere ruined productivity on other bottomland fields, damage here was minor. Farmers still accept the risk of periodic crop loss in return for exceptional harvests, but after the severe floods of the 1990s some bottomland farmers sold their land to state and federal agencies, leading to the creation of the Big Muddy National Wildlife Refuge and the Overton Bottoms Conservation Area upriver. Conservation agencies are now working together to restore some of the river's meandering character with islands, side channels, backwater habitat areas, and bottomland forest, both to restore productive natural communities and to decrease flooding in the agricultural areas that remain.