Although we don't believe in timing the market or panicking over market movements, we do like to keep an eye on big changes -- just in case they're material to our investing thesis.

What: Shares of major potash stocks plummeted as much as 23% today after Russia's Uralkali, the world's largest potash producer, broke up a cartel with Belarus that controlled about 43% of global exports.

So what: Uralkali is making the volume-over-price move to steal market share and boost sales to China, completely turning the global potash market on its head. In fact, Uralkali's decision is expected to cause potash prices to plunge to January 2010 levels, sending the stocks of PotashCorp (NYSE:POT), Mosaic (NYSE:MOS), and SQM (NYSE:SQM) -- which will now have to compete at the discounted price -- crashing along with the forecast.

Now what: Uralkali sees potash prices plunging 25% to about $300 per ton by the end of year. "In the near future we expect (global) competition to become stronger -- that will push prices down," Uralkali CEO Vladislav Baumgertner said in a conference call. So while it's always tempting to pounce on severely beaten-down stocks, there might be too much uncertainty surrounding potash right now to make the bet a smart one.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.