New issue Rentech Nitrogen Partners (RNF) is producing big profit growth, a plus for a stock that pays a solid dividend. The company makes nitrogen fertilizers and industrial products such as ammonia and urea. Rentech's production facility is in the heart of the Corn Belt, the largest market in the U.S. for nitrogen fertilizer products. The company's earnings bolted 162% in 2011 and it's off to a fast start in 2012. Last month, Rentech delivered a