I don't know where they get there data from..
Potash prices are quite cloak and dagger...there is no spot market and estimates are usually drawn from pricing arrangements from suppliers/customers
I think potash peaked in 07 around $1100, and bottomed recently around $300... Also read that intrepid/potash corp have been signing around the 400-450 mark as of late, so not sure where they got $900/mt for Jan this year from