By Alistair MacDonald in Toronto and Eyk Henning in Frankfurt 

An attempt by Canada's Potash Corp. to buy rival miner K+S AG and create a dominant potash player in the Americas faces likely pushback from the German company and regulatory hurdles.

A tie-up between two of the world's largest potash miners would create a company capable of controlling up to 30% of the global market just two years after the collapse of a global cartel system that had helped regulate prices of this key agricultural commodity.

But some analysts say a deal will likely be nixed by German federal regulators, who stopped a similar takeover attempt in 1997.

K+S, Germany's largest miner, said on Thursday that Potash Corp. made an informal approach to its board, a move the Saskatoon-based company confirmed later. That approach was at just over EUR40 ($44.80) a share in cash, according to a person familiar with the matter.

That person said that the German company is likely to reject the advance given it undervalues the dominant position the two could achieve in North America, where Potash has major mines and K+S is developing its own mine.

People close to K+S say a combination with Potash Corp. could yield $370 million in annual synergies by shifting production from K+S's costlier German mines to Potash's underutilized Canadian sites, and by increasing the pricing power of the combined company on the global market.

The people close to K+S said synergies would add around EUR18 to K+S's shares, which were trading at EUR27 before the news of a potential transaction broke Thursday, meaning K+S is looking for at least EUR45 a share.

Shares in K+S climbed by around 30% in Frankfurt, while Potash Corp's shares were trading 5.38% lower in late morning trade in Toronto.

Many analysts say that the combined large market share in North America, and in other markets including Germany, means that German regulators would likely reject the combination. The German Cartel Office rejected a 1997 attempt by Potash Corp to take over K+S, after the company had been spun off by BASF.

Such a combination would have taken away a potential source of competition to K+S's dominant position in Germany, the Cartel Office said in a report at the time.

"At the same time, the global oligopoly would have tightened further and K+S would have been incorporated into [Potash Corp.'s] pricing policy for the world market," the report said.

The Canadian offer for K+S comes as the potash market remains in flux following the July 2013 collapse of an informal cartel system that had helped shape the global price of this $22 billion market. The end of that system sent the price of potash about 25% lower and led potash miners' profits to decline sharply.

The miners, including Potash Corp., that formed the two trading groups often described as cartels say they didn't act to set prices and weren't cartels. They said the groups just marketed, sold and transported the potash.

As new mines come into production during the next five years, supply is expected to surge. Unless demand, especially in China and India, increases as sharply as was expected when companies invested in expansion, prices could suffer further.

Analysts suggested Friday that Potash Corp. would seek to close down K+S's North American production, taking supply out of the market and boosting the price of the fertilizer, which strengthens plants and makes them more resistant to disease.

"If Potash Corp is successful in closing some supply... then all potash-levered equities should rejoice," Scotiabank said in a note.

K+S is spending 3.25 billion Canadian dollars ($2.64 billion) on a project in Saskatchewan that could produce some 2.9 million metric tons of potash a year by 2023, according to the company.

Potash Corp. is expected to mine around 9.2 million metric tons of potash this year, according to Scotiabank, with K+S producing around 3.2 million metric tons. A merger would give the combined company almost 27% of global potash capacity by 2017, when both companies are set to complete new mines.

The price of potash is currently trading 30% below its October 2011 peak, when talk of global population growth and improving diets in developing countries propelled potash into the ranks of other booming commodities. That boom encouraged a round of mine building that is likely to bring new supply onto the market over the next five years, at a time when demand is likely to stay flat.

Analysts believe that Potash Corp. would also try to spin off the salt business of K+S should it be successful.

Ulrike Dauer in Frankfurt contributed to this article.

Write to Alistair MacDonald at alistair.macdonald@wsj.com and Eyk Henning at eyk.henning@wsj.com

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