Notes

1. The water demands of some biofuel-producing crops could put unsustainable pressure on local water resources if not managed wisely.
A 2009 study suggests that, in the rush to produce enough corn-based ethanol to meet federal alternative energy requirements, biofuel demand is already putting stress on fresh water supplies in the Great Plains and central Southwest

2. Deforestation

3. But there's another complication that arises with this selection. Many unsaturated oils have undesirable burn characteristics: They'll leave gummy residue in an engine when used as fuel. Hydrogenating, or treating the oil with hydrogen, can mitigate this problem, but increased processing means increased cost [source: Journey to Forever].

4. Monoculture refers to the practice of growing one heavily concentrated crop, rather than the rotation of various crops through a farmer's fields over time. Hundreds -- even thousands -- of unbroken acres of one crop offer an irresistible target for plant pests; pest populations can explode beyond control in such a tempting environment.Long-time monoculture farms have to use much more artificial fertilizer than their more sustainable peers, increasing water pollution.

http://auto.howstuffworks.com/fuel-efficiency/biofuels/10-disadvantages-of-biofuels.htm#page=2