Extremely high plasticity soils, such as bentonite, present substantial challenges in geotechnical applications due to their high water retention capacity and expansive behaviour. This study evaluates an integrated physical–chemical stabilization approach using bamboo leaf ash (BLA) to reduce the plasticity of such problematic soils. Bentonite was selected as a sample of soil with extremely high plasticity. BLA, which was made from three types of bamboo and treated through controlled burning, was used as a chemical stabilizer. Mayan bamboo was chosen for soil stabilization owing to its high silica (SiO2) content and pozzolanic reactivity. Numerous geotechnical tests, such as Atterberg limits and compaction tests, were performed following the ASTM standards. Scanning Electron Microscopy (SEM) combined with Energy Dispersive Spectroscopy (EDS), X-Ray Fluorescence (XRF), and X-Ray Diffraction (XRD) investigations were used to check how the soil changes at a microscopic level, particularly the shape and mineral content after stabilization. The results showed that BLA greatly lowered the plasticity index (PI) from 455.41% to 180% and the liquid limit (LL) from 568.70% to 270%, with only small changes in the plastic limit (PL). The microscopic analysis showed the formation of cement-like materials such as calcium silicate hydrate (C-S-H) and calcium alumino-silicate hydrate (C-A-S-H), which means that the pozzolanic reactions worked well. Using BLA along with compaction provides a sustainable and effective way to reduce the plasticity value of the soil and automatically increase the strength of soils with extremely high plasticity. These results show that BLA could be a green and practical option for soil stabilization using a large number of local plant materials. © 2026 Faculty of Engineering, Universitas Indonesia. All rights reserved.