By Rod Nickel

Feb 20 (Reuters) - Canadian fertilizer company Agrium Inc reported a 72 percent decline in quarterly profit on Thursday as crop nutrient prices fell and nitrogen sales volumes weakened.

Calgary, Alberta-based Agrium, the third-largest global producer of nitrogen and a major North American potash producer, said crop nutrient sales had slipped due to cold weather during a period when U.S. farmers usually apply fertilizer to fields after harvest.

Grain prices dropped off year-over-year, taking fertilizer prices down with them. Global potash prices were further thrown into a tailspin last year after Russia's Uralkali OAO broke away from its joint venture with Belaruskali in July, sparking competition between producers that had previously maintained a high discipline on pricing.

Net earnings for Agrium's fourth quarter fell to $99 million, or 66 cents per share, from $354 million, or $2.34 per share, a year ago.

Net earnings from continuing operations, excluding its purchase of Viterra Inc farm retail outlets, were $126 million or 87 cents per share. The company on Jan. 20 warned earnings from continuing operations would be at the lower end of its guidance of 80 cents to $1.25 per share.

On that basis, analysts on average had expected Agrium to earn 86 cents a share in the fourth quarter, according to Thomson Reuters I/B/E/S.

Revenue for the company dipped seven percent to $2.9 billion versus expectations for $2.975 billion.

Agrium's U.S.-listed shares edged up 0.5 percent to $89.26 after normal trading hours.

The company sold 907,000 tonnes of wholesale nitrogen products at an average selling price of $458 per tonne, versus sales of 966,000 tonnes at an average price of $561 a year ago.

Agrium sold 344,000 tonnes of wholesale potash at an average price of $313 per tonne, slightly more than the 341,000 tonnes sold a year ago but at a much lower price than the average $449 per tonne.

Agrium's retail sales to farmers, which include fertilizer, seed and chemicals, climbed 6 percent to $2.1 billion.

Rival nitrogen producer CF Industries Holdings Inc posted on Tuesday a fourth-quarter profit that declined 31 percent, but the drop from record-high earnings a year earlier was not as steep as analysts had expected.