What? CF Industries Holdings' (NYSE:CF) shares fell 11.6% last month. That's a painful drop, unless you compare it to CF's full-year 2015 drop of 26%. Interestingly, however, the real declines didn't start until the middle of the year...

So What? CF Industries sells fertilizer, that's been an increasingly difficult market as of late. For example, the company's earnings were down roughly 36% through the first nine months of 2015. And while management noted in the quarterly release that it expects 2016 demand for its primary product nitrogen to be up year over year, it also explained that the market is currently being driven by supply. In other words, there's too much fertilizer and not enough demand to sop it all up.

Investors are reacting as you would expect to a commodity producer looking at weak pricing. In fact, shares of other fertilizer players are faring even worse; Potash (NYSE:POT), for example, was down over 50% in 2015. But here's the thing, Potash shares have been falling all year. CF Industry's shares were holding up relatively well until the announcement that CF would acquire parts of European competitor OCI.

This deal might be a good call in a down market since asset prices are relatively cheap, but it complicates the CF story in a tough operating environment. Worse, the purchase will lead to a change in CF's country of incorporation, in what has increasingly been dubbed a tax inversion. The Treasury Department stepped in on this big-picture issue in December, leading CF to alter its plan to adjust for the Treasury's comments.

And since the deal isn't done yet, there's still a lot up in the air here. Which helps explain the continued weakness in the shares. All in all, CF Industries has two headwinds. A poor operating environment and a complicated acquisition that could draw regulatory scrutiny. Investors are probably right to be leery of the shares.

What Now? Despite current headwinds, CF Industries is still making money. In fact, it looks like it's using the downturn to become a better company. However, it also looks like fertilizer prices will remain weak as the company heads into 2016. Add to this a complicated acquisition and one can only conclude that conservative investors should probably stay away. For more aggressive individuals, CF might be worthy of consideration.