A long-term fertiliser experiment (LTFE) initiated during 1972 was evaluated after 51 yr (118th crop; 2024–25) at Tamil Nadu Agricultural University, Coimbatore, India, to quantify changes in soil potassium (K) fractions, K depletion dynamics and yield sustainability of finger millet grown on an Inceptisol. The experiment comprised ten fertiliser treatments, including control, nitrogen (N) alone, nitrogen and phosphorus (NP), recommended dose of NPK (100 % NPK), higher NPK levels and integrated nutrient management (INM) with farmyard manure (100 % NPK + FYM), arranged in a randomised block design with four replications. Surface soil (0–15 cm) samples were analysed for water-soluble K (WSK), exchangeable K (exch. K), non-exchangeable K (NEK), lattice K, total K and available K using standard procedures. Integrated nutrient management (100 % NPK + FYM) maintained significantly higher available K and exchangeable K compared with sole chemical fertilisation, whereas continuous K omission treatments showed substantial depletion of labile K pools, indicating progressive mining of native reserves. Lattice K constituted the dominant fraction, followed by NEK, exch. K and WSK. Available K showed strong positive associations with grain and straw yield. The Sustainable Yield Index (SYI) was highest under integrated fertilisation, reflecting greater yield stability over time. Net change analysis showed a negative K balance under omission treatments and improved K retention under balanced fertilisation. The results demonstrated that long-term balanced fertilisation combined with organic manure is essential for sustaining soil K fractions, preventing reserve depletion and maintaining productivity in intensive cropping systems on inceptisol.