The invention provides an eco-friendly process for the preparation of slow release potassium fertilizer from combustion wastes of  biomass based power plants. The invention is based on few simple steps of extraction of potassium from combustion wastes followed  by evaporation and pelletization thereof with gypsum. The final fertilizer product has K content in the range of 20 - 40 %, depending  on the quality of the combustion waste. The prepared fertilizer has other essential plant nutrients as well. Overall this fertilizer is  useful for plant growth and agricultural production. Besides, the process would help the biomass based power plants to sustainably  utilize their waste ashes generated in huge quantities and to generate additional revenue from waste.