While Fools should generally take the opinion of Wall Street with a grain of salt, it's not a bad idea to take a closer look at particularly stock-shaking upgrades and downgrades -- just in case their reasoning behind the call makes sense.

What: Shares of Mosaic Co (MOS -0.19%) slipped nearly 3% this morning after Goldman Sachs downgraded the fertilizer giant from neutral to sell.

So what: Along with the downgrade, analyst Adam Samuelson lowered his price target to $35 (from $41), representing nearly 30% worth of downside to yesterday's close. While value investors might be attracted to the stock's drop over the past year, Samuelson believes there's plenty of room to fall given his negative outlook for corn and soybean prices.

Now what: Goldman expects the farming environment to remain pressured over the next few years. "We believe the market is overly focused on a near-term bottom in international K pricing and is overly optimistic on 2H14 and 2015 pricing potential given our expectation for limited improvement in industry operating rates in 2015-2016 and the impact of rising North American production on premium domestic pricing," noted Samuelson. With the stock off almost 30% from its 52-week highs and boasting a 2%-plus dividend yield, however, those short-term concerns might be providing patient Fools with a juicy long-term buy-in opportunity.