A slow-release potassium fertiliser (KBC) was prepared from biochar (BC) produced from cotton stalks. Zeta potential (negative charge), SEM (porous, smooth, oval shaped holes), EDX (potassium confirmed by a strong peak 3.31 keV), FTIR (C=C bonds in the Amine group in BC, in impregnated BC unsaturated compounds with C=O and C=C bonds, stretching of C-C bonds), and BET (SSA for BC: 2.566 m²/g & KBC: 2.145 m²/g) techniques were used for characterisation. Adsorption and desorption study of potassium using different models were assessed. Redlich isotherm model (R2= 0.99) outperformed other isotherm models. Tomato plants were utilised to evaluate the impact of BC-based potassium fertiliser on growth metrics. ANOVA was utilised to assess the statistical significance of these growth characteristics. The findings demonstrate that the BC-enhanced slow-release potassium fertiliser increases potassium availability and its use efficiency, and fosters prolonged plant development, providing a sustainable and effective solution for potassium as a slow release fertilizer in agriculture.