The recent Midwestern floods have caused all manner of misery: Burst levies, lost homes, ruined crops, higher food prices, a gusher of agrichemicals and god know what else flowing into streams.

One way to soothe the sting is to own shares in giant fertilizer companies like Potash Corp. of Saskatewan and Mosaic. These companies have seen their share prices jump over the past week.

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Investors may be bidding them up because the floods represent a sales opportunity. To maximize yield on what’s left of the 2008 corn crop, farmers will be scrambling to reapply fertilizer to make up for what’s run off and evaporated due to soggy conditions.

How much will they be laying on? I asked John Sawyer, an Iowa State University agronomist who works with farmers as an extension agent. He said it’s too early to tell, because farmers are still assessing their fields to figure out what can be salvaged.

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He added that in salvageable fields, farmers will likely be applying about 50 pounds of nitrogen per acre. Before planting, farmers typically 150 and 170 pounds per acre. So these reapplications will likely represent a significant income boost to the fertilizer giants — and yet another source of nutrients to leach into streams, clear down to the Mississippi.