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Figure 1. Sugarcane trucks outside an ethanol and sugar millClick to Enlarge ImageBefore nations pin big hopes on biofuels, they must face stark realities, warns crop physiologist Thomas R. Sinclair. No matter what techniques are developed to expand biofuel feedstock, some basic physical and physiological limitations will apply. Plants cannot be grown without three crucial resource inputs: light, water and nitrogen. Each of those inputs will be needed in substantial quantities, yet their availability in the field is limited. As important, so far plants make use of those resources only at established rates. In fact, the close relationship between the available amounts of these resources and the amount of plant mass they can produce—not human demand—will determine how much biofuel the world can produce.

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