The biggest four-month jump in corn prices in 2 1/2 years is signaling to investors BHP Billiton Ltd. will raise its $40 billion bid for Potash Corp. of Saskatchewan Inc., the world's largest fertilizer producer.

Corn has climbed 35 percent and soybeans 19 percent in Chicago since Aug. 17, when Saskatoon, Saskatchewan-based Potash Corp. rejected BHP's bid as too low and said it's seeking other offers. Higher grain prices can prompt farmers to use more fertilizer to boost crop yields, pushing up the value of Potash Corp., which has 20 percent of global capacity in its namesake product.

BHP, the world's biggest mining company, has also gained $5.8 billion by scrapping a deal with Rio Tinto Group this month after opposition from regulators, giving it cash to increase its bid, according to National Australia Bank Group Ltd.

"It's a given that BHP will raise its offer," said Monem Salam, who helps manage the $1.7 billion Amana Growth Fund, including shares of Potash Corp., at Saturna Capital Corp. "Potash Corp. still has to make the case to BHP that the price is too low, but the wind is at their back with grain prices going up."

David Fuller's view As a BHP Billiton shareholder, I will be pleasantly surprised if the company succeeds in taking over Potash at $150 or lower. There appears to be too much political opposition and BHP should have come in with its bid much earlier, when stock markets were reeling.