Last year, about a third of the biodiesel plants in the country went idle and output fell by half. But now federal tax credits and renewable energy mandates mean that biodiesel is booming again and plants are opening back up.

Their hold on success is tenuous, though: It depends, the industry says, on Congress extending a tax credit that pushes fuel blenders to include biofuel. The current boom started when Congress restored that credit back in December. But that was only a one-year reboot. For the industry to revive completely, producers say they need a longer extension.

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Although plants are moving—slowly—away from corn- and soy-based biofuels, biofuel production still sucks up energy and land. It might be worth keeping the industry alive until it's commercially viable to make biofuels from corn stalks and other more sensible fuel stocks. But, given how unclear the benefit of biofuels are, it might not be.

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