Until BHP Billiton came around, the price of potash and PotashCorp were moving pretty much in unison. The mineral’s price increased fourfold in 2008, peaking at more than $800 a tonne. The PotashCorp share price also quadrupled to a high of $241. Both then declined, the mineral to about $400 a tonne and the shares to a pre-bid $110. The parallel price falls would have been even steeper if the potash industry had not cut production by 40 per cent.

The big rally was a bit surprising. Potash, which makes plants grow tall and strong, had long suffered from price wilt: down 35 per cent between 1977 and 2007, adjusted for inflation. During that period, production grew at a modest 1 per cent annual rate.

It hardly sounds like a hot investment. But both the Anglo-Australian mining company and the Canadian producer think investors have missed something: growth in the developing world will produce profits far exceeding those already captured in the pre-bid share price.