Wichita, Kansas-based Koch, one of the largest U.S. private companies, disclosed in a regulatory filing that it owned 5.2 million shares of Intrepid as of Sept. 30, or 6.9 percent of the company's outstanding shares. It had owned 1.38 million shares as of June 30, while its subsidiary, Spring Creek Capital, owned 4.125 million shares, or 5.44 percent, according to an Aug. 19 filing.

Koch's businesses include Koch Fertilizer LLC, which along with its subsidiaries owns fertilizer plants in the United States, Canada and Trinidad and Tobago.

A Koch spokesman could not be reached for comment.

Shares of Denver, Colorado-based Intrepid fell sharply with other potash miners after the July 30 breakup of Belarusian Potash Company, which triggered a dip in potash prices.

Intrepid shares have since recovered.

The stock gained 10.7 percent in New York at $15.75 on Monday. The company has a market capitalization of about $1.2 billion.

The fertilizer sector also got a lift on Monday when HSBC upgraded several companies including Potash Corporation of Saskatchewan, Agrium Inc, K+S AG and Mosaic Co.

K+S shares jumped more than 9 percent in Frankfurt, while Agrium and Potash Corp each gained about 3.6 percent and 3.3 percent in New York and Toronto, respectively.