Declining soil fertility is a major constraint to coffee productivity in the Southern Highlands of Tanzania. This study assessed the impact of carbonized rice hull (CRH) amendments on soil properties in coffee-based systems of Mbozi district. Soil samples from four villages (Nsenga, Iyula, Nambinzo, Halungu) were collected at two depths (0 - 30 cm, 30 - 60 cm) before and after CRH application. Parameters measured included pH, electrical conductivity, moisture, organic carbon, macronutrients (N, P, S), exchangeable bases (K, Mg, Ca), micronutrients (Fe, Mn, Zn), and cation exchange capacity (CEC). Results showed marked improvements in soil moisture (3.4 - 7.0% to 8.3 - 18.2%) and organic carbon (0.53 - 1.97% to 0.79 - 2.30%), with Iyula recording the highest gains. Phosphorus and zinc concentrations increased significantly, enhancing fertility essential for coffee growth. Although mild acidification occurred in some sites, CEC improved notably, especially in Iyula (up to 22.74 Cmol/kg), indicating stronger nutrient-holding capacity. Site-specific differences were evident: Iyula soils were near-neutral and fertile, while Halungu soils remained acidic with low CEC, requiring corrective measures. Overall, CRH demonstrated strong potential to restore soil health, improve nutrient availability, and enhance resilience of smallholder coffee systems. Findings highlight CRH as a climate-smart amendment, though site-specific strategies remain vital for optimizing outcomes.
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