I am a farmer, let me give you my thoughts.

First off, I don't believe that the high grain prices we have will last for any more than 18 mos at most, maybe only 6.  Why?  Because they never do!  Ethanol has driven a lot of this, and Germany is selling us all their ethanol plants because they have realized that it is false economy.  Germany is going heavy into solar... I digress there.  This yr, there will be heavy fertilizer usage.  It blows my mind that POT is doing so well, though Brazil and China are major buyers.  Despite high grain prices, farmers are still squeezed with higher fuel, fert, and machinery costs and always look to where they can cut costs.  Potash doesn't give a lot of bang for the buck, though most farmers use at least some.  Nitrogen is #1, phosphorus #2.  N was short here last fall despite having a plant 20 mi away!  I think farmers will pay to put extra N on, maybe some extra P on.  N is made from Natural gas which is at very low prices relative to oil.  Of course most Nat gas isn't for fertilizer, but it is a component.  I think investing in Nat gas co's has upside because the commodity is underpriced, and the fertilizer ind will be putting increased demands on it.  One possible prob with phos long term is that crops use a low % of what is put down, and lots end up in the lakes, causing that nice pea soup in the summer.  I forsee limits on the phos we can apply some time in the future, though it may be years (shoulda been yrs ago IMO).

Of course a weather problem can jack grain prices further, though remember that long term, a poor crop leaves more unused fert in the soil, so if it is a widespread problem, there emay be less demand the following yr... especially if grain prices drop. 

BNN feature a phos producer a bit ago...was it Phoscan?

Good luck!

ps do you see the irony of using one fossil fuel (nat gas) to produce ethanol to replace another fossil fuel (gasoline)?  I shake my head at it every day.