Biological processes which shape soil fertility are affected by microorganisms and enzymes they produce as well as the rate of biogeochemical transformations in the cycling of elements. One of the enzymes is arylsulphatase (EC 3.1.6.1.), which hydrolyses sulphate esters with aromatic radical, releasing sulphate ions according to the equation: R-C-O-SO3–+ H2O → R-C-OH + SO42– + H+. The enzyme plays an essential role in the sulphur cyclein soil and it can be an indicator of sulphur mineralization in soil. For his study, soil wassampled from a field under potato fertilized with different doses of farmyard manure (0, 20, 40, 60 and 80 t⋅ha–1) and mineral nitrogen (0, 45, 90, 135 kg N⋅ha–1). The activityof arylsulphatase was assayed according to Tabatabai and Bremner, while sulphate(VI) sulphur was determined as described by Bardsley and Lancaster. The content of organiccarbon in the soil ranged from 8.168 to 10.96 g⋅kg–1 and depended on FYM fertilization, while the content of total nitrogen ranged from 0.889 to 1.012 g⋅kg–1 with an average of 0.960 g⋅kg–1 for FYM and mineral nitrogen doses. The effect of fertilisation on changes in the amount of sulphate sulphur and the activity of arylsulphatase in the soil was noted. The content of sulphate sulphur throughout the research ranged from 21.49 to 24.83 g⋅kg–1. The higher the FYM doses, the higher the content of the fraction of sulphur available toplants. The soil provided a good supply of sulphur to plants. The activity of the enzyme ranged from 0.010 to 0.024 μM pNP⋅g–1 ⋅h–1. Its highest activity (an average 0.018 μMpNP g–1⋅h–1) was recorded in the samples fertilised with nitrogen at the amountof 45 kg⋅ha–1. Both parameters changed during the potato vegetation period.