Soils require the application of biochar to improve degradation. The objective of this study was to evaluate the long-term effects of a field experiment on soil organic matter (SOM), soil organic carbon (SOC), and soil moisture content in Karagwe, Tanzania. Seven years ago, a field experiment was conducted using a Latin rectangle design with four replications. The treatments included carbonization and sanitation (CaSa) and carbonization and standard compost (CaSt), which were compared to control Andosols (CoA). A total of 96 soil samples were collected to determine SOM, SOC, and soil moisture content. The data were analyzed using one-way analysis of variance. The results showed that soil samples from the CaSa-treated soil had an increase in SOM content of 17.3%, an increase in stored SOC content of 10.0%, and an increase in soil moisture content of 6.3%. Compared with those in CoA, the CaSt-treated soil showed increases in SOM, SOC, and soil moisture of 14.4%, 8.4%, and 4.0%, respectively. Therefore, all treatments improved soil properties, with CaSa proving more effective in enhancing SOM, SOC, and soil moisture content compared to CaSt and CoA. In conclusion, CaSa is recommended for its sustainable ability to enhance Karagwean soil over time.