The type of the applied nitrogen fertiliser plays a significant role in plant nutrition. Supplying plants with potassium in the form of chloride, sulphate or salpetre significantly modifies the chemical composition of plants, as anions accompanying potassium serve different functions. The vegetation experiment on the cultivation of red pepper cv. ‘Cyklon’ was conducted in 2005 and 2006 in a plastic tunnel in rings (V = 5 dm³) filled with mineral soil and highmoor peat (3:2). The response of plants to three potassium salts KCl, K₂SO₄ and KNO₃ was determined at two levels of nitrogen-potassium fertilization of 250 mg N and NH₄NO₃ and 300 mg K, as well as 350 mg N and 400 mg K·dm^-3 substrate, at constant levels of the other macro- and microelements. Biometric measurements of plants were taken and the volume of fruit yield was determined. It was found that the type of potassium fertilizer at the two levels of nitrogen and potassium fertilization did not have a significant effect on total and marketable yields of fruits in red pepper cv. ‘Cyklon’, plant height, total and marketable fruit numbers as well as fresh weight of a single fruit. It was shown that a higher level of nitrogen and potassium fertilization (350 N and 400 K mg dm^-3 substrate) had a positive effect on the analyzed biometric parameters of fruits, except for fresh weight of a single fruit. Results of these investigations, confirmed by other authors, critically refer to the division of vegetable crops developed by Th. Geissler and Buchner into chloride- and sulfate-loving vegetable crops.

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