A three-year field experiment was conducted in the experimental site belonging to the Siedlce University of Natural Sciences and Humanities. Nitrogen ¹⁵N at 10.3% enrichment was applied in the form of ¹⁵(NH4)₂SO₄ at the amount of 1.66 g per 1 m² in early spring. In each year of the study three cuts of the test plant were harvested at the stage of budding. The total content of P and S in the plant and soil was determined by emission spectrophotometr with inductively coupled plasma (ICP – AES). The uptake of phosphorus and sulphur with the yield of the test plant was then calculated. Under the conditions of ¹⁵N application there was a reduction in the yield and in phosphorus content in the biomass of successive cuts of eastern galega and in the subsequent years of the study, and a reduction of phosphorus and sulphur uptake with the yield. The sulphur content increased in the consecutive cuts with respect to the first cut. In the subsequent years of the study decreased levels of phosphorus and sulphur were assayed in the soil.