So far spokesman for the companies have denied any wrongdoing. Prices of fertiliser have risen in a result of tight supplies and increasing demand for fertiliser by farmers due to expanding grain and food production.

Companies included in the lawsuit are:

• Mosaic of Plymouth, Minnesota, USA,
• Agrium of Calgary, Alberta, Canada,
• Potash Corp. of Saskatchewan Inc., Canada,
• JSC Uralkali of Moscow, Russia;
• RUE PA Belaruskali, of Soligorsk, Belarus,
• RUE PA Belarusian Potash Co. of Minsk, Belarus,
• JSC Silvinit of Solikamsk, Russia, and
• JSC International Potash Co. of Moscow, Russia.

In several countries, obscure laws shield makers of potash and phosphate -- two key ingredients in fertilizer -- from certain antitrust rules.

In the US, for example, phosphate makers are among a handful of industries empowered by the 1918 Webb-Pomerene Act to talk with competitors about pricing and other issues.

The allegations come as the fertilizer companies have profited form the global grain-price boom of the past two years.

Price rocketed sky high
The price of phosphate has climbed to about $1,100 a tonne, up from $430 last year, while the price of a tonne of potash is now more than $930, up from $275.

The Minnesota suit alleges, among other things, that the companies exchanged "sensitive, non-public" information about prices and demand, allocated market shares, and coordinated output.

Farmers around the world have cried foul as they've watched the prices rise. The North Dakota Farmers Union, a trade group, also asked to investigate the price increases. Farmers in India and Russia have complained to their countries' regulatory bodies.

Government intervention
In March, Russian antimonopoly regulators required the country's largest potash maker, Uralkali, to cut domestic prices of the plant nutrient, a key ingredient in fertilizer, after wrangling with the company over its pricing behaviour in court.

Brazil's government is considering nationalizing the country's fertilizer deposits to help reduce farmers' production costs.

In 1993 Canadian and US potash producers were similarly accused in a number of class-action lawsuits.

The lawsuits were later dismissed.

For regular updates on feed news subscribe here to our free newsletter