The paper presents the results of research into the activity of alkaline and acid phosphatase against selected chemical parameters in the selected humus horizons of arable soils of the Unisław Basin. It was found that the soil demonstrated a higher activity of alkaline phosphatase by 35% on average, compared with the activity of acid phosphatase, which is due to the high soil richness in CaCO3. Considerable amounts of calcium carbonate determined the neutral or slightly alkaline reaction of the horizons. The soil investigated represented the soils of a low or average content of available phosphorus, which classifies it to classes IV and III of richness in that element. Changes in the activity of the enzymes were found depending on the horizon the soil was sampled from on which various crop species were grown.