* Russia's Uralkali says to stop fixing prices with partner
* Uralkali says potash prices could slump below $300/t
* Uralkali shares down 21.6 pct, K+S down 24.5 pct
* Britain's Sirius off 12 pct, Israel Chem loses 16.2 pct (Adds analyst profit estimate, updates prices, N.American stocks)
FRANKFURT, July 30 (Reuters) - The breakup of the biggest player by volume in the global potash market, a Russian-Belarusian joint venture, sent a shockwave through shares of European potash miners on Tuesday on fears of falling prices for the key fertiliser mineral.
Russia's Uralkali said it would no longer fix potash prices and volumes with Belaruskali, adding this may push global potash prices down below $300 per tonne in the second half of 2013 - a drop of more than 25 percent.
Shares in Uralkali plummeted 21.6 percent by 1106 GMT. Those of German rival K+S, the world's fourth-largest potash miner, slumped 24.5 percent to a six-year low, its biggest one-day drop ever.
One trader described the news as "a catastrophe" for K+S, while Commerzbank analyst Lutz Grueten said he would review his target price and recommendation.
"Assuming a 30 percent lower potash price would lead to a circa 50 percent cut in our K+S group EBIT (earnings before interest and tax) forecasts," Grueten said.
K+S has said its C$4.1 billion ($4.0 billion) new mining project in Canada would require a price of at least $420-$460 per tonne of potash, including freight, to secure the project's targeted premium on costs of capital of 15 percent.
"If potash prices indeed fall below $300, a number of new mining projects in the industry would certainly be at risk," Metzler Equities analyst Lars Hettche said.
London-listed Sirius Minerals, which plans to build a $1.7 billion potash mine in North Yorkshire, fell 12 percent.
Israel Chemicals, which extracts the fertiliser mineral from the Dead Sea, plunged 16.2 percent.
The break-up of the Belarus Potash Company (BPC) leaves North America's Canpotex as the ruling potash export venture.
BPC and Canpotex had accounted for a combined 70 percent of global trade in potash, and the duopoly had set identical prices in key markets such as China and India.
Frankfurt-traded shares in Canpotex members Potash Corp of Saskatchewan and Agrium Inc fell 10.2 percent and 5.9 percent, respectively.
$1 = 1.0261 Canadian dollars Reporting by Ludwig Burger and Daniela Pegna; Editing by Dale Hudson