Shares in K+S lose as much as 5.2 percent, making the stock the worst performer on Germany's blue-chip index, after credit rating agency Moody's cuts the potash miner's rating by two notches to a non-investment grade.
"(The) Moody's step doesn't come as a surprise, but it's bad news in any case," says one trader.
The shares are down 4.2 percent at 0953 GMT, paring gains of 11 percent over the previous three trading sessions.
Traders also point to Canadian rival Agrium Inc's disappointing forecast for the current fourth quarter as weighing on K+S stock.
Agrium, part of the Canadian potash export alliance around Potash Corp. , forecast fourth-quarter earnings of 80 cents to $1.25 per share, well below analysts' expectations of $1.51.
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