Why No Agra Fuels?

Why do we still have our tax dollars going to farm subsidies for farmers not to grow crops on their lands? We also subsidize the price of grains so that our farmers can compete with the world prices. Why is it that a Joe six pack can modify his car and drive across the country using "spent oil" from fast food restaurants and the automotive industry can not come up with a way to do this?

The United States is one of the most productive countries for agriculture, yet we are dependant on other countries for oil. I just have a very hard time trying to understand why we can use corn or soy beans to make alcohol and vegetable oil for fuel.