There are ups and downs in fertilizer markets. I haven't seen the March 13 study you were referring to, but over a number of years Agriculture and Agri-Food Canada has done an annual report on Canada-U.S. price differentials on fertilizers. And the conclusion in that report has traditionally been that there are no significant differences between the price of fertilizer in Canada and the United States.
On May 7th, 2009. See this statement in context.