Shares of top fertilizer makers Mosaic (MOS), Potash Corp. (POT) and Agrium (AGU) rose early after they collectively reached a deal with a major Chinese buyer. A subsidiary of China's Sinofert Holdings said it will buy their potash fertilizer at a steep discount of $70 a ton. Although the Hong Kong-based company didn't specify the exact price it will pay, the last contract for the fertilizer was for $470 a ton. So the purchase price under this