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Chinas economy underwent great changes since 1949,especially since China initiated economic reforms and the open-door policy in the 1980s.The growth of agricultural production has been one of the main national accomplishments.By 1999 China was feeding 22%of the global human population with only 7%of the worlds arable land.It has been acknowledged widely that the crop yield increase was accomplished by greater inputs of fertilizers,irrigation,new crop strains,and other technologies of the "Green Revolution".