Increased nutrient availability, especially nitrogen and phosphorus from fertilization and pollution, affects plant communities. Nutrient concentrations in plant tissues and their stoichiometric ratios can indicate nutrient availability. This study aimed to evaluate the use of nutrient concentrations and stoichiometric ratios as indicators of nutrient addition effects on productivity, species composition, and functional group proportions in a mountain meadow. In a long-term grassland experiment in Polish Carpathians various fertilizer combinations (90NPK, 180NPK, N, P, PK, control) and liming were applied. Over 39 years, species composition was assessed, and nitrogen, phosphorus, and potassium concentrations in plant biomass were measured. Liming had no effect on vegetation parameters or nutrient concentrations. The N:P ratio remained low across all treatments, ranging from 4.9 in 90NPK to 11.7 in 90N, suggesting nitrogen limitation. Despite significant variations in species composition and productivity between NPK-fertilized treatments and others, the N:P ratio showed little variation. The N:P ratio was influenced not only by nutrient availability but also by other factors, such as the ability of some species of oligotrophic habitats to accumulate excess nitrogen in the 90N treatment, and the addition of potassium in the PK treatment, which increased nitrogen-fixing legumes share. Conclusion: Stoichiometric ratios can be useful for assessing nutrient effects on vegetation but must be interpreted cautiously due to various environmental influences.