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 fertilizer company Terra Nitrogen (NYSE: TNH) fell as much as 13% today on nearly three times the normal volume.

So what: News wires from Terra were quiet, but Seeking Alpha published an article suggesting selling Terra Nitrogen and buying competitor CVR Partners (NYSE: UAN). Could it just be coincidence that CVR is up today and Terra fell through the floor?

Now what: The author cites a call provision for Terra from CF Industries (NYSE: CF), the company's majority owner, as a major reason to sell. But the provision calls for a purchase price of an average of the previous 20 days, a nice premium after today's drop. I don't see a big reason to sell today, but being aware of such provisions is something investors should keep their eyes on.

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