That fever I told you about? The world's farmers have still got it.

I'm talking about the demand for fertilizer, and potash in particular. Unlike a certain unfortunate footwear maker, this is no fad, Fools.

Farmers are suddenly flush, and as long as they keep getting strong prices for their products, they have an incentive to increase plantings. Fertilizer is a small part of the overall cost of farming, and it provides a lot of bang for the buck. Yara International, the Nokia (NYSE:NOK) of nitrogen, puts the plant energy impact at 10 to 15 times that required to produce, transport, and apply the fertilizers it sells.

PotashCorp (NYSE:POT) is sitting on the stuff with the most attractive supply/demand picture of all. That's potash, or potassium carbonate. Last quarter, the company's potash segment raked in over $500 million in gross margin, or sales net of selling costs. This time around, that figure lifted to $886 million, nearly matching the entire 2007 contribution.

The crazy thing is that realized potash prices in the quarter, at a little over $400 per metric ton, were a mere fraction of where prices stand today. In April, after a drawn-out negotiation, China signed on for a $400 per ton increase. There's simply much more cash to be made in potash. The supply constraints are just that substantial.

I used to think that the valuations on guys like PotashCorp and Mosaic (NYSE:MOS) were a little nutty. When the former company backed up the truck upon a share price pullback to $120, I started to reconsider. Now it's rather plain that these potash players, and possibly even Intrepid Potash (NYSE:IPI), aren't particularly pricey. Three months ago, the average analyst estimate for PotashCorp's 2009 earnings was around $12.70, and now it's north of $20. I'm glad I'm not the only one that's a little slow on the uptake.

PotashCorp is rated a middle-of-the-road three stars in Motley Fool CAPS. Think it deserves more Foolish fervor? Weigh in on the fertilizer firm's future right here.