Biochar and Kitchen Stove Ash for Improving Nutrient Availability and Microbial Functions of Tropical Acidic Soil

Tropical acidic soils exhibit inherently low fertility and reduced microbial activity, driven by low pH and accelerated organic matter mineralization, phosphorus (P) fixation, and aluminum (Al3+) and iron (Fe3+) toxicity. These constraints limit agricultural productivity, necessitating sustainable and low-cost soil amendments essential for improving the soil fertility in such regions. This study investigated the effects of biochar, kitchen stove ash (KSA), and their combined application on the soil chemical properties, nutrient dynamics, and microbial functions in a tropical acidic soil. The treatment included the unamended control and two doses of 0.25% w/w (B10) and 0.5% w/w (B20) corncob biochar, 0.03% w/w kitchen stove ash (Ash), and 0.027% w/w commercial-grade calcium carbonate (Lime). Each biochar dose was added alone or in combination with either ash (Ash + B10 and Ash + B20) or calcium carbonate (Lime + B10 and Lime + B20). After eight weeks of laboratory incubation at 20 °C, the soil pH, N and P bioavailability, microbial biomass, and extracellular enzyme activities were measured. The combined application of 0.5% w/w biochar with 0.03% w/w KSA (Ash + B20) resulted in the most significant improvements in all of the examined soil fertility indicators than the individual amendments. Specifically, the soil pH was increased by 40% (+1.9 pH units) compared with the unamended control. Available phosphorus, mineral nitrogen, and total potassium were increased by 49%, 22%, and 36%, respectively, compared with the unamended control. Regarding the microbial parameters, the Ash + B20-treated soil showed the highest microbial respiration (+56%), microbial biomass (+45%), and extracellular C- and N-cycling enzyme activities compared with the unamended soil. The ash supplied minerals (P, K, and Mg) provided a more beneficial effect on the soil’s nutrient content and microbial functions than the calcium carbonate. The study demonstrated that underutilized kitchen ash may supplement biochar’s liming and nutrient supply potentials, even at a lower application rate, to improve the fertility of weathered acidic soil.