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 CF Industries (NYSE: CF) fell 10% today as investors worry demand will fall along with corn prices.

So what: Corn prices are down today and investors are betting there is some correlation between demand for fertilizer and the price of these products. The move didn't come from management's indications but investor fears about weak demand in the future.

Now what: This looks like a major sell-off for a relatively small drop in corn prices today, but it's worth keeping an eye on. Investors are fretting about everything from high supply levels to Europe's debt crisis as an excuse to crush stocks, and today the fear hit CF Industries. With shares trading at 8.3 times trailing earnings and 6.7 times forward earnings, I would look at this move as a good buying opportunity for the long-term investor.

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