Its not just fertilizer, but land and water. To grow any crop as fuel more land needs to be cleared so as not to impact on food supply. This will result in deforestation and a lot of the planet's capacity for sinking greenhouse gases will be removed. Water issues are perhaps even more important.
Agriculture is a very inefficient way of capturing solar energy. We would be vastly better served by using much smaller areas of land to house wind farms, solar plants or any other form of renewable energy generator, and devote research efforts to finding better technologies for storing electricity. Luckily these technologies are becoming viable without the massive subsidies given to biofuels.
