The aim of the study was to determine soil abundance in nutrients after three and five years of legume-grass mixture application, depending on the mineral fertilizer rate. The experiment was performed under conditions of the Olsztyn Lakeland. The first experimental factor was a legume-grass mixture (50% legumes + 50% grass): A – Trifolium pratense + Festuca pratensis; B – Lotus corniculatus + Dactylis glomerata; C – Trifolium repens + Lolium perenne; D – Medicago lupulina + Festuca rubra. The other experimental factor was a different level of mineral fertilization: 1 – control: 0 kg N, P and K/ha; 2 – 0 kg N, 34.88 kg P and 99.6 kg K/ha; 3 – 60 kg N, 34.88 kg P and 99.6 kg K/ha; 4 – 120 kg N, 34.88 kg P and 99.6 kg K/ha. An analysis of soil fertility made after three and five years of legume-grass mixture application showed a significant increase in organic matter content; after three years the highest increase was observed for mixture A, and after five years – for mixtures C and D. The total nitrogen content of the soil increased as well, except control treatments with mixture D. After three and five years of legume-grass mixture application the ploughing layer was characterized by medium and high available phosphorus concentration, low and medium potassium concentration, and low or very low magnesium concentration.