DES MOINES, Iowa --- The skyrocketing price of soybean oil is taking a toll on the once-expanding biodiesel industry, making it difficult for refineries to turn a profit, the Associated Press reported.

In the U.S., about 20 of 170 biodiesel plants are not operating.

For example, construction on a new $63 million plant in Algona was completed in December. But the plant isn't operating today because of the current economics. Over the past two years, soybean oil prices have tripled.

Back in 2006 when the Algona plant was under consideration, soybean oil was trading between 18 and 21 cents per pound. Last week, soybean oil traded at about 58 cents a pound on the Chicago Board of Trade.