<Tr> <Td> Sugar beets </Td> <Td> 30.7 </Td> <Td> 29.8 </Td> </Tr> <P> The only other crops to ever appear in the top 20 in the last 40 years were, commonly: tobacco, barley, and oats, and, rarely: peanuts, almonds, and sunflower seeds . Alfalfa and hay would both be in the top ten in 2003 if they were tracked by FAO . </P> <Table> <Tr> <Th> Major Crops in the USA </Th> <Th> 1997 (in US $billions) </Th> <Th> 2014 (in US $billions) </Th> </Tr> <Tr> <Td> Corn </Td> <Td> $24.4 </Td> <Td> $52.4 </Td> </Tr> <Tr> <Td> Soybeans </Td> <Td> $17.7 </Td> <Td> $40.3 </Td> </Tr> <Tr> <Td> Wheat </Td> <Td> $8.6 </Td> <Td> $11.9 </Td> </Tr> <Tr> <Td> Alfalfa </Td> <Td> $8.3 </Td> <Td> $10.8 </Td> </Tr> <Tr> <Td> Cotton </Td> <Td> $6.1 </Td> <Td> $5.1 </Td> </Tr> <Tr> <Td> Hay, (non-Alfalfa) </Td> <Td> $5.1 </Td> <Td> $8.4 </Td> </Tr> <Tr> <Td> Tobacco </Td> <Td> $3.0 </Td> <Td> $1.8 </Td> </Tr> <Tr> <Td> Rice </Td> <Td> $1.7 </Td> <Td> $3.1 </Td> </Tr> <Tr> <Td> Sorghum </Td> <Td> $1.4 </Td> <Td> $1.7 </Td> </Tr> <Tr> <Td> Barley </Td> <Td> $. 9 </Td> <Td> $. 9 </Td> </Tr> <Tr> <Th> Source </Th> <Th> 1997 USDA - NASS reports, </Th> <Th> 2015 USDA - NASS reports, </Th> </Tr> </Table> <Tr> <Th> Major Crops in the USA </Th> <Th> 1997 (in US $billions) </Th> <Th> 2014 (in US $billions) </Th> </Tr>

Why are most of the crops grown in the center part of the u.s