In the years 1987-1991 a field experiment on brown acid soil was carried out. Two grass species were involved in the study (Dactylis glomerata and Bromus unioloides). Each year they were fertilized with progressive doses of potassium and nitrogen. After three and five years of cultivation, samples of soil were collected and were determined: a reaction (pH in 1 mol KCl), available aluminium content (Al⁺³), hydrolytic acidity (Hh), sum of available bases (S) and degree of saturation of soil with the bases (V). With growth of nitrogen dose increased soil acidity and available aluminium content as well hydrolytic acidity but the sum of available bases and the degree of saturation with the bases decreased. Fertilization with potassium in a double dose caused especially unfavourable changes in chemical composition at cultivation of brome grass.