Mineral and organic fertilization is one of the most important factors affecting activity of soil enzymes. It is commonly accepted that organic fertilization is more beneficial to the soil biological activity than the mineral. The objectives of this investigation were to assay C_org, N_tot, P_tot content and to determine urease and amylase activities in the soil fertilised for a long time with various rates of farmyard manure and slurry. Soil samples were taken from the experiment established on a typical lessive soil. FYM was used at the following rates: 20, 40, 60, 80 t ha^–1 fresh weight once in the rotation on the plots with potatoes, while slurry was applied in four doses corresponding to manure fresh weight. Soil samples were taken after potato harvest in the 8^th year after the experiment start-up from two depths: 5-15 and 25-35 cm of the soil profile. Usually, higher amounts of C_org and P_tot were found in the soil manured with FYM in comparison with the pig slurry-manured soil. These amounts increased when the doses of both fertilizers were the highest. The content of Corg ranged from 5.5 to 8.2 g kg^–1 of soil when 80 t ha^–1 of FYM was used. Total nitrogen ranged from 0.45 to 0.95 g kg^–1of soil, average for doses of both fertilizers, whereas total phosphorus content ranged from 0.26 to 0.48 g kg^–1of soil. The highest amylase activity was observed in the soil when FYM was used (0.48 µg starch g^–1 16 h^–1) as compared to the soil where slurry was applied (0.41 g starch µg^–1 16 h^–1). No increase of amylase activity was observed when the doses of both fertilizers were the highest. A higher urease activity was noted when FYM was used than in the case of the soil manured with slurry. A decrease of urease activity was noted in the control samples (3.0 mg NH₃ 10 g^–124 h^–1), while the urease activity increased when the doses of both fertilizers were the highest. The highest activities were noted when 80 t ha^–1 of FYM or slurry were used.