This study examined the impact of biochar concentration variability and soaking duration in LOF on the chemical properties of Spodosols and Ultisols. The biochar was applied at percentages of 0, 2, 5, 10, 20, 25 and 100%, with soaking times of 0, 1, 12, and 24 hours. The results indicated significant improvements in the soil pH, organic carbon, total nitrogen, available phosphorus, available potassium, and cation exchange capacity in both soil types. The application of biochar at the 100% application rate resulted in the greatest improvement in the soil chemical properties, but the application rate of 10--25% was optimal for agricultural soils, indicating economic feasibility and increased sustainability. The optimal dosage and soaking time varied for Spodosols; a 12-hour soaking period was best for pH improvement, whereas 24 hours maximized organic carbon and total nitrogen. In the Ultisols, no soaking was necessary for the optimal pH, whereas 1 hour was ideal for organic carbon and total nitrogen. Available phosphorus and potassium were maximized at 24 hours for both soils. In the case of CEC, it was best improved, with 12 hours for Spodosols and 24 hours for Ultisols. Thus, the results of this work demonstrated the effectiveness of individually selected biochar treatments for improving the soil pH, structure, and fertility, which is crucial for the sustainable use of soil in agriculture. Further studies should help confirm these results in long-term field tests and evaluate their economic feasibility for extensive applications.