Historians who argue in favor of soil exhaustion as an explanation for slave importation into the Deep South posit that exporting states emerged as slave producers because of the transformation of agriculture in the Upper South. By the late 18th century, the coastal and Piedmont tobacco areas were being converted to mixed crops because of soil exhaustion and changing markets. Because of the deterioration of soil and an increase in demand for food products, states in the upper south shifted crop emphasis from tobacco to grain which required less slave labor. This decreased demand left states in the Upper South with an excess supply of labor.