Shares of nitrogen maker Terra Industries tumble

Citi Investment Research analyst Brian Yu said in a note that global urea prices fell sharply this week and that the Sioux City, Iowa-based company gets 100 percent of its profits from nitrogen.

"The risk is that urea prices continue to decline as buyers delay their purchases and draw down inventories deeper than usual in anticipation of even lower prices," he said. "The timing and extent of this feedback loop is nearly impossible to predict or quantify, but could take weeks or months to play out."

He also downgraded shares to "Hold" from "Buy" and slashed his price target to $44 from $64, saying "recent weakness in global nitrogen pricing that could last through the end of 2008."

Shares tumbled $7.48, or 19.7 percent, to $30.42.