| Summary: | Between 2012 and 2015, a field experiment was conducted at the Brody Experimental Farm, Poznan University of Life Sciences, Poland. The following two experimental factors were used in duplicate: calcium sulfate (CaSO<sub>4</sub>) fertilization—two levels (0 and 500 kg ha<sup>−1</sup>); and potassium (K) fertilization—four levels (0, 30, 60, and 120 kg ha<sup>−1</sup>). The soil pH (in H<sub>2</sub>O) and enzyme activity (dehydrogenase, acid phosphatase, alkaline phosphatase) were determined. The potassium fertilizer had a significant influence only on the dehydrogenase activity, which increased with the dose of potassium. The research showed that the fertilization of the sward mixture of alfalfa and grasses with potassium doses of 60 and 120 kg ha<sup>−1</sup> in the K 60 and K 120 combinations resulted in higher yields of fresh matter than in the combination without the potassium fertilizer (K 0). In the last year of the research, the additional CaSO<sub>4</sub> fertilization resulted in the highest increase in the yield of the mixture of alfalfa and grasses, as compared with the variant without this fertilizer. The application of this fertilizer in the first years of the research also increased the yield of fresh matter.
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