BHP won't buy POT anymore because Potash is fairly priced. The CEO is just saying he's going to use restraint in order to save face. If BHP was smart, they would have given a fair bid, instead of a lowball bid. That awoke other interested parties to Potash's (under)value. I used the Agricultural Commodities historical data to shed some light on the Potash-mania.
I charted POT against Agricultural Commodities. The 6 month chart tells a good story:
Look how poorly POT was doing against its economic backdrop. BHP saw this, and the second it put a bid in, POT rocketed up to reflect the move in underlying commodities since July. The trend in Agro Commodities seems to be pointing up, so you know why Potash would decline BHP's offer.