* TSX falls 62.98 points, or 0.50 percent, to 12,606.06 * Six of 10 main sectors advance * Potash loses a fifth of its market value * Gold miners drop on weaker bullion price By John Tilak TORONTO, July 30 (Reuters) - Canada's main stock index slid on Tuesday to its lowest in nearly two weeks after the breakup of a Russian-Belarusian potash joint venture sent Potash Corp shares tumbling on fears of a drop in the price of the fertilizer ingredient. Russia's Uralkali dismantled the world's largest potash cartel in a move that it expects to slash prices by 25 percent, heralding a reshaped industry and pummeling shares of rival producers. Potash, one of the world's largest fertilizer makers, lost more than a fifth of its market value, dropping 20 percent to C$30.99. With more than 12 million shares changing hands, it was by far the most active listing on the Toronto Stock Exchange. "You just have to look at the stock price" to see the impact of the Russian company's strategic shift on the industry, said John Ing, president of Maison Placements Canada. Agrium Inc, another member of the North American Canpotex potash marketing joint venture, stumbled 5.3 percent to C$88.88. Investors also braced themselves for a policy statement from the U.S. Federal Reserve on Wednesday and the all-important U.S. jobs report later this week. Ing said he does not expect a dramatic shift in policy by the U.S. central bank. "The Fed's been in backtracking mode, so it will be more of the same," he said. The Toronto Stock Exchange's S&P/TSX composite index was down 62.98 points, or 0.50 percent, at 12,606.06, after reaching 12,555.82, its lowest since July 17. Six of the 10 main sectors on the index were higher. The materials sector index, which includes Potash Corp and other miners, dropped 5.2 percent. Gold miners' shares declined as the price of bullion fell, with investors nervous about possible monetary policy moves by the U.S. Federal Reserve. Goldcorp Inc lost almost 2 percent to C$28.83, and Barrick Gold Corp fell 2.2 percent C$17.69. Financials, the index's most heavily weighted sector, were down 0.4 percent. Toronto Dominion Bank declined 1.2 percent to C$87.86, and Royal Bank of Canada slipped 0.5 percent to C$64.92. TD Bank's stock fell after the company warned it expects to report a third-quarter loss in its insurance business due to floods in Alberta and storms in the greater Toronto area, joining a list of Canadian companies hurt by bad weather.