As Innatz Mouse pointed out, agriculture is HEAVILY DEPENDENT ON PETROCHEMICALS. All the fertilizer the majority of farms use is nitrogen, which is from petrochemicals. All the heavy machinery to plant and harvest the crops don't run on good intentions, but on gas/diesel. The arrigation necessary to keep the crops from drying up (especially noticable these days) is powered by petrochemicals. And to transport the crop, the global economy can't function if done by wheelbarrel.
Also, no one has mentioned that with repeated use of soil, on top of soil erosion, the fertile top soil that takes hundreds of years to create is used up.
Our current lifestyle demands our arable land is used as efficient as possible, which means crops can't just be thrown into dirt, and then come back when the plants are done growing. Pests like locuts, beetles, fungi, and what not, will destroy everything in their path. And since pestcides are made from petrochemicals, ethanol is not a viable alternative.
At the end of the day, if it takes more energy to grow the crops, than the energy stored in ethanol, even if you switch all of the oil dependent agricultural needs, you're going to run into a dead end.