Soil acidity severely restricts agricultural productivity across large portions of the tropics, limiting crop yields and threatening food security for smallholder farming communities. In Ethiopia, nearly half of all agricultural lands suffer from soil acidification driven by intensive cultivation, heavy rainfall, and a lack of residue return. While agricultural lime is traditionally used to correct soil pH, its high cost and the logistical challenges of transportation make it largely inaccessible to rural farmers. Consequently, adopting locally available organic amendments like biochar and vermicompost provides a practical path toward restorative soil management.

Biochar produced through the slow pyrolysisPyrolysis is a thermochemical process that converts waste biomass into bio-char, bio-oil, and pyro-gas. It offers significant advantages in waste valorization, turning low-value materials into economically valuable resources. Its versatility allows for tailored products based on operational conditions, presenting itself as a cost-effective and efficient More of abundant coffee husk waste offers an alkaline, carbon-rich matrix with a high surface area and intrinsic liming capabilities. When incorporated into the soil, biochar helps buffer re-acidification, adsorb metal ions, and improve structural stability. Simultaneously, vermicompost supplies a rich matrix of humic substances, beneficial microorganisms, and essential macronutrients that stimulate biological activity and nutrient cycling. Evaluating how these two conditioners interact when applied together provides crucial data for optimizing land productivity without relying heavily on synthetic fertilizers.

Incubation experiments demonstrate that applying biochar and vermicompost, both alone and in combination, yields major improvements across key soil fertility indicators. In the intensely acidic clay soils of the Hula district, the highest combined treatment of six tons per hectare of biochar and five tons per hectare of vermicompost raised soil pH by 18.4 percent, moving it from a strongly acidic 4.9 up to a much more favorable 5.8. Similarly, in the Gorche district, a combination of three tons per hectare of biochar and five tons per hectare of vermicompost increased soil pH by 15.7 percent, shifting it from 5.1 to 5.9. These combined treatments also slashed exchangeable acidity by over 80 percent at both study sites.

Beyond correcting acidity, the integrated amendments substantially boosted soil organic carbon, total nitrogen, and available phosphorus. The presence of biochar reduces phosphorus fixation by removing toxic aluminum ions from the soil solution, while vermicompost introduces active microbial pathways that enhance phosphorus and nitrogen availability. Furthermore, exchangeable base cations—including calcium, magnesium, and potassium—increased significantly alongside cation exchange capacity and percent base saturation, reflecting a vastly improved nutrient retention capacity. Conversely, excessive concentrations of extractable micronutrients like iron and manganese were safely reduced to optimal levels through pH elevation and chelation. Ultimately, these findings confirm that co-applying coffee husk biochar and vermicompost serves as a highly effective, circular-economy strategy for reclaiming tropical acidic soils.

Source: Dogiso, E., Lemma, B., Yoseph, T., & Haile, W. (2026). Effects of biochar and vermicompost application on the physicochemical properties of acidic soils in Sidama Region, Ethiopia. Scientific Reports, 16(1), Article in Press.