Rentech Inc. reported a 25 percent larger fourth-quarter net loss, largely due to higher costs as the company continues to develop its plant-based clean fuels business.

The Los Angeles clean fuels and fertilizer company on Tuesday reported a net loss of $9 million for the quarter ended Sept. 30, compared with a loss of $7.2 million a year earlier. The loss per share remained unchanged at 4 cents due to a larger number of shares in the 2010 quarter.

Revenue jumped 34 percent to $34.5 million, driven by larger fertilizer sales by its wholly-owned subsidiary, Rentech Energy Midwest Corp. “Over the last few months, we have witnessed a dramatic improvement in fertilizer margins,” Chief Executive D. Hunt Ramsbottom said in a statement. “We believe that the record-high product margins we see now are primarily driven by high demand and prices for grains, combined with continuing low prices for natural gas.”

Rentech’s biomass gasification process converts plant material into ultra-clean synthetic jet and diesel fuels, specialty waxes and chemicals. Its nitrogen fertilizer operation is a compatible side business that brings in revenue while the company builds out its potentially larger fuels business, including a production plant now under construction.

Shares were down 8 cents, or 5.5 percent, to $1.37 in midday trading on the Nasdaq.