Land degradation as a result of land-use practices is a major environmental concern to sustainable agricultural production in Tanzania. The effect of clearing and long-term cultivation on physical, chemical, and biological characteristics is described in two representative soil formations from eastern Tanzania: Mkindo (Eutric Fluvisol) in the Mvomero district and Mafiga (Ferric Lixisol) in the Morogoro district. The results have shown that in the Mkindo site, 10 years of continuous rice cultivation has led to severe changes in most characteristics of the soil. Significant effects of cul- tivation coincide with those considered to favor clay dispersion and crusting phenomena, including changes in soil reaction, clay content, and mineralogy as well as generalized desaturation of the exchange complex, increasing sodicity, and severe losses of soil organic matter (SOM). In contrast, at the Mafiga site, 30 years of a less disturbing cultivation system, including periodic fallows, have also modified some soil characteristics but to a lower extent than at Mkindo. Decreased soil colloidal properties at the Mkindo site and lower stability against biological degra- dation, reflected by carbon (C) – release curves, than the Mafiga site could be causally connected to clay illuviation processes leading to accumulation of calcium (Ca) and magnesium (Mg) but mainly caused by changes in SOM characteristics such as losses of humic (HA) and fulvic acid (FA) and accumulation of humin.