A four-year pot experiment was conducted at an experimental facility of the Siedlce University of Natural Sciences and Humanities; the experiment was set up in a random arrangement in three replications. The following factors were taken into account: I – organic fertilising (no organic fertiliser, sludge from the wastewater treatment plant in Siedlce, chicken litter and brown coal from the coal mine in Turów) at the dose of 2 g C kg-1 of soil; II - liming (no liming and liming at a dose calculated for 1 Hh of soil as CaCO3) ; III – contamination of soil with nickel at different levels (no nickel, 100 and 200 mg Ni kg-1 of soil as aqueous solution of NiCI2 6H2O). Orchard grass (Dactylis glomerata L.) was used as the test crop and was harvested four times during each vegetation season. The soil was analysed in each year of the experiment after the fourth (last) harvest of the test crop. All the factors had a significant diversifying impact on dehydrogenase activity in the soil in the years of the experiment. The addition of organic waste and liming increased the activity of the enzymes in question, whereas increasing the soil contamination with nickel to 200 mg Ni kg-1 of soil had the opposite effect. The toxic impact of nickel was mitigated by the addition of organic fertilisation and liming.