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What: Shares of fertilizer and biofuels maker Rentech (AMEX: RTK) plunged as much as 17.9% on extremely heavy volume today.

So what: Rentech reported fourth-quarter results this morning with 6% year-over-year sales growth and negative earnings of $0.04 per share. Since few analysts follow the stock and none seem keen on offering earnings estimates, it took a while for the disappointment to sink in -- Rentech's share price has been sliding all day long.

Now what: Reflecting on the year, CEO Hunt Ramsbottom said that 2011 was "transformative" and that his company is looking to pull back on capital expenses next year. With one foot in the potential hypergrowth of alternative fuels and another in the cash-cow fertilizer industry, Rentech is trying to figure out how to get the best of both worlds. After today's drop, Rentech has underperformed fertilizer specialists such as Terra Nitrogen (NYSE: TNH) in 2011 but crushed synthetic-fuel rivals from Amyris (Nasdaq: AMRS) to Solazyme (Nasdaq: SZYM) -- but I think the time is ripe for alternative fuels to go big in 2012. Keep a close eye on this space.

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