Converting grain or other biomass into ethanol is currently a popular idea, but it is not a new one. It requires fertile soil, large quantities of water, and sunlight for green plant production. Green plants in the United States collect about 53 exajoules of energy per year from sunlight. Americans consume slightly more than twice that amount, however. Enthusiasts suggest that ethanol produced from corn and cellulosic biomass could replace much of the oil used in the United States. Yet the 18 percent of the US corn crop that is now converted into 4.5 billion gallons of ethanol replaces only 1 percent of US petroleum consumption. If the entire corn crop were used, it would replace only 6 percent. And because the country has lost over a third of its agricultural topsoil, no large increase in the corn crop is possible.