TORONTO (AP) — Potash Corp. of Saskatchewan Inc. said yesterday its board has rejected an unsolicited takeover bid from BHP Billiton Ltd. worth about $38.49 billion, saying it undervalues the fertilizer producer.

BHP Billiton, an Australian natural resources company, is offering to buy Potash for $130 per share, a 16 percent premium over Monday's closing price.

Shares of Potash surged above the offer price in pre-opening trading, indicating that investors expect a higher offer. They rose $32.85, or 29 percent, to $145.

Potash said its board has adopted a shareholder rights plan, also known as a poison pill, to give the company time to better develop alternatives to enhance shareholder value.

It said the BHP Billiton bid is non-binding and "opportunistic".

"The fertilizer industry is emerging from the recent global economic downturn, and we feel strongly that PotashCorp shareholders should benefit from the current and potential value of the company," Potash said. "Global demand for food is steadily increasing, creating an attractive operating environment for the entire fertilizer industry and, with our premier position, PotashCorp is uniquely poised to benefit."

Potash consumption has been growing over the past decade because of greater demand for fertilizer from developing nations. Potash is any of several compounds containing potassium, used chiefly in fertilizers. Its main markets are China, the US, Brazil and India.

Last month the Canadian company reported its second-quarter net income more than doubled to $472 million, or $1.55 per share, as revenue surged 68 percent to $1.44 billion.

BHP Billiton confirmed it made an approach to Potash. In a statement, it said Potash's board has not yet agreed to engage in discussions. BHP said it continues to review its options.

BHP has been ramping up its Potash mining business in Canada for the past four years. It holds exploration permits for over 7,338 square kilometers in Saskatchewan, where its best prospect so far — the "Jansen Project" — is expected to begin producing potash from 2015.

BHP bought out its Anglo Potash Ltd. joint venture in Canada for $282 million in 2008 and bought Athabasca Potash Inc. earlier this year for about $320 million, extending its potential exploration acerage in Canada to over 14,000 kilometres. The mining giant has said it sees potash as a relatively low cost business that offers it significant growth.