Despite the recent collapse of an international cartel that could cause a plunge in prices of crop nutrient potash, BHP Billiton (ASX: BHP) is still open to the idea of constructing a $14 billion potash mine in Saskatchewan, Canada.

BHP Chief Executive Andrew Mackenzie said the completion of preparatory work for the proposed Jansen mine would add a fifth commodity and further diversify the world's largest mining company's operations beside coal, copper, iron ore and oil and gas.

The Russian producer of potash, Uralkali, which ended a marketing alliance with Belarusian Potash Co that led to the collapse of the international cartel, forecast the move could cut price of potash by up to 25 per cent to $300 per metric tonne by the end of 2013.

Analysts said BHP would think twice in investing in the planned Saskatchewan potash mine such a big amount of money if the expected return would be $300, not $400 per tonne.

"The impacts of what we've seen recently are interesting, but we have to look forwards tens of year in terms of potash supply to really understand what, if any, investment we'll make and at what pace ... We saw this evolving in time to being a more open market, and therefore you'd expect that would be in any of our plans," The Wall Street Journal quoted Mr Mackenzie.

BHP attempted in 2010 to go into potash mining by making a $39 billion hostile takeover attempt on the Potash Corp of Saskatchewan but was blocked by the Canadian government.

The collapsed cartel and a second major player, Canpotex, control almost two-thirds of the global supply of the $22-billion potash market. BHP said it plans to sell the commodity independently of Canpotex which markets the crop nutrient for Potash Corp, Mosaic Co and Agrium.

BHP has so far spent over $1 billion in preparatory work at Jansen, including construction of mine shafts to ferry workers and materials.

Meanwhile, Mr Mackenzie said BHP's yearly $9 billion tax bill in Australia is fair, which is at 45 per cent tax rate. He did not comment further on the High Court's ruling that upheld the Mineral Resource Rent Tax and the plan by the Opposition to cut corporate tax rate.

"Tax is one of the many issues we need to work through with whoever wins the next election," he added.

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